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FIG 42. Relative sea-level curve for the upper River Forth at Arnprior.
Our second example of relative sea-level change comes from the upper River Forth and is quite different. It shows that there has been a fall of relative sea level of about 50 m over the past 15,000 years, so that former coastline features are now visible well above the present-day coast (Fig. 42 (#ulink_61b601bf-5753-5504-937e-c927f5e20c19), red circles). This type of curve is common in Scotland and, given the ~120 m worldwide rise in sea level shown in Figure 40 (#ulink_98c98f73-a241-5a85-9d85-77ecebb0d443), it is clear that the crust of the Forth region must have been subjected to significant uplift (~170 m) in order to produce the curve shown in Figure 42 (#ulink_61b601bf-5753-5504-937e-c927f5e20c19). This uplift is largely the result of isostatic rebound due to unloading of the crust as the ice retreated.
In some curves, as with the upper Forth, oscillations in the curve represent changes in the rates at which the two mechanisms of change were operating. Such changes may leave characteristic coastline features in the landscape, which will be considered in the Area descriptions.
At larger scales, it is useful to consider average rates of crustal movement over a given time period, which can then be plotted as contour maps. The contours shown on Figure 43 (#ulink_c7d3716f-a14d-5537-9ca0-f022ad054044) are based on estimates of local elevation changes averaged over roughly the last 5,000 years, attempting also to allow for the effects of eustatic sea-level variations. Additional support for this approach comes from data from tide-gauge studies, collected over the last 200 years, which show some consistency with this pattern.
Although our two local studies in the Thames region and the upper Forth (Figs 41 (#ulink_98352187-f11b-538f-90c7-d1eb7b85a983), 42 (#ulink_61b601bf-5753-5504-937e-c927f5e20c19)) involve a rather longer timescale than the regional analysis (Fig. 43 (#ulink_c7d3716f-a14d-5537-9ca0-f022ad054044)), all three studies highlight the clear contrast between crustal movements in southeastern England and those in western and central Scotland. These variations have been produced by differences in ice-sheet thickness and extent during the last (Devensian) glacial.
The distinctive rise of the land of Scotland relative to sea level lends itself to another approach to the study of sea-level change. Figure 44 (#ulink_8e1f8a88-c56c-588e-8bdb-bd05a4a3d355) shows a plot of the elevations and gradients of various old shoreline features that are now above present-day sea level across Scotland and northern England. These old shoreline features have clearly been uplifted, and those further inland have generally risen more than those near to the present-day coast, so that they now form a dome-like structure. This dome is broadly centred on Rannoch Moor, which was one of Scotland’s main ice centres during the last glaciation. We can therefore usefully identify a Rannoch Rebound Dome as an active feature of local Earth movement, resulting from unloading of the crust of western Scotland in response as the Devensian ice melted.
FIG 43. General trends of crustal movement, relative to sea level, averaged over the last 5000 years.
FIG 44. The elevations and gradients of various old shoreline features along a horseshoe-shaped traverse across northern Britain, suggesting the Rannoch Rebound Dome.
CHAPTER 6
Area 1: Galloway
AREA 1 EXTENDS FROM AYR in the northwest down to Dumfries in the southeast (Fig. 45 (#ulink_198b0e89-2d40-58d2-8ddb-d43a9f9d99df)). It lies mainly within the western half of the Southern Uplands, a terrain of rolling hills, bounded to the west by the Firth of Clyde and its numerous sandy bays. The Southern Uplands Fault crosses the northern half of the Area, separating the Southern Uplands from the generally lower-lying ground of the Midland Valley, with its volcanic hills and important coal reserves (Fig. 46 (#ulink_09dd45a9-6c18-5f65-a6b5-d38fb3378ef5)).
People have inhabited this Area for thousands of years, and it was an important gateway between England and Ireland. There are many remains of human occupation dating from prehistoric times to the present day, ranging from Mesolithic fish traps to medieval burghs and castles. Another interesting feature of Area 1 is the unusual place names, particularly in the Southern Uplands themselves, where Old Norse, Gaelic and Celtic influences can be seen. Examples are the Rig of the Jarkness and the Dungeon of Buchan.
FIG 45. Location map for Area 1.
FIG 46. Natural and man-made features of Area 1.
STORIES FROM THE BEDROCK
Geologically speaking, Area 1 lies mainly within the Southern Uplands terrane, sandwiched between the Iapetus Suture in the south and the Southern Uplands Fault in the north (see Fig. 20 (#ulink_92e7c4dd-e904-5c91-a5eb-a77514394804), Chapter 4 (#ub72c883b-08de-5367-91ab-f3d6ef483997)). The bedrock of the Southern Uplands (Figs 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e), 48 (#ulink_5cf891da-8d88-5bdd-8ef5-ca222d5bb5f5)) is mostly altered Ordovician and Silurian sedimentary rocks, deposited between 490 and 420 million years ago on the floor of the Iapetus Ocean (see Chapters 1–5 (#u661596f2-c933-517a-abc9-8c82715c3114)). The sediments which make up this bedrock were swept off a nearby continental shelf and down the continental slope in turbid (muddy, cloudy) currents – underwater avalanches, more dense than the surrounding sea water – that were probably earthquake-triggered. On reaching the flat ocean floor, the entrained sediments in each avalanche gradually settled – coarse sands first, followed by fine sand, and then the much slower deposition of clay and mud. In this way, each turbid current resulted in a graded bed, from coarse-grained at the bottom to fine-grained at the top, and as the process repeated itself many times, a thick sequence of such beds built up. Small amounts of limestone were also deposited, along with volcanic material such as pale ash layers. Graptolites – small, now-extinct marine animals – are common in the fine-grained sediments of the Southern Uplands. Their rapid evolutionary changes of form mean they have become very useful time markers for determining the relative ages of different sedimentary beds, especially when combined with studies of the folds and faults, in reconstructing the origins of the Southern Uplands.
FIG 47. Simplified geology and hill-shaded topography for Area 1.
FIG 48. Timeline of bedrock and surface-layer events in Area 1.
Prior to the Caledonian mountain building, the crustal foundations of Scotland and England were separated by the Iapetus Ocean. Around 490 million years ago, this ocean began to be destroyed by subduction: oceanic crust moved down into the mantle beneath the Grampian Highlands, and then beneath the Midland Valley (see Chapter 4). A small fragment of this oceanic crust escaped subduction, being instead thrust up onto the margin of Scotland, to be ‘welded’ onto the Midland Valley by around 470 million years ago. Today, this small but intensively studied area of complexly interfolded rock units outcrops around Ballantrae, the so-called the Ballantrae Complex. Rocks characteristic of the deep sea and oceanic crust are found – sediments such as black shale and chert, basalt lavas with pillow structures, ash, sheets of dykes and upper mantle rocks. The latter originated at depths of up to 40 km in the Earth’s crust, and are today coarse-grained (mafic) gabbros and serpentinite.
The main deformation of the Southern Uplands terrane occurred during the later stages of the Caledonian mountain building, between the mid-Ordovician and the early Devonian. As oceanic crust continued to be subducted, the sediments which today make up the Southern Uplands were scraped off the ocean floor along a series of thrust faults and stacked up in a pile against the edge of the Midland Valley (Fig. 28 (#ulink_97ce30e1-e360-557b-93ae-e0a8135abc77)). During this deformation, the sediments became tightly folded and weakly metamorphosed: fine-grained mudstone and siltstone became slate, while cement within sandstones recrystallised to produce a tough, hard rock (greywacke). Today, bedding in the Southern Uplands is aligned in a general northeast/southwest direction and dips very steeply to the southeast, and northeast/southwest faults divide the region into numerous fault blocks.
By the start of the Devonian (around 415 million years ago), the major deformation of the Southern Uplands had ceased and Scotland and England were welded along the Iapetus Suture. It was around this time that the major granite bodies of the Southern Uplands (Fig. 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e)) were emplaced: partial melting at the base of the thickened crust produced liquid magma, which then rose up into the upper crust where it slowly solidified to form coarse-grained igneous bodies (plutons). As the overlying rocks were subsequently removed by erosion, three major plutons were revealed in the Southern Uplands. The most northerly of these is the hourglass-shaped Loch Doon intrusion, said to be one of the finest examples in Scotland of a concentrically zoned pluton: the interior of the body is silica-rich (felsic) granite, separated from the outer silica-poor grey granodiorite which makes up most of the body by a transition zone. Similar well-developed concentric zonation is seen in the eastern half of the Criffel–Dalbeattie body on the south coast, although overall this body is much less compositionally evolved (i.e. it has a lower silica content) than the other Southern Uplands granites. It is also the most deformed: originally oval, its western part has been distorted southwards by complex faulting. Porphyrite dykes and sills commonly surround the main intrusion (e.g. at Black Stockarton Moor), made up of large crystals embedded in a fine, glassy groundmass. Between the two, the roughly oval Fleet pluton was intruded around 390 million years ago (Devonian) into a broad ductile shear zone, making it the youngest reliably dated Caledonian pluton in mainland Scotland (Fig. 49 (#ulink_87059e73-9e67-58ac-9415-3f87fed46f49)). It is also the most evolved of the Southern Uplands intrusions, consisting entirely of granite, and is the only intrusion whose magmas were sourced wholly from the melting of metamorphosed sediments (rather than igneous rocks). Because of both its young age and its evolved composition, this pluton has more in common with the Lake District and Northern Ireland granites than with those of Scotland, and it has been suggested that these areas shared a magma source.
FIG 49. Cairnsmore of Fleet, 711 m (10 km east of Newton Stewart), viewed from the southeast. This mountain landscape has been created by erosion of the Fleet granite intrusion. (© Lorne Gill, Scottish Natural Heritage)
As these hot granite bodies were emplaced, their heat baked the surrounding rocks, creating an encircling metamorphosed zone (an aureole) 1 km or, in the case of the Criffel–Dalbeattie intrusion, even 2 km wide. These aureoles are often rich in mineral veins, deposited by hot circulating fluids released by the crystallising granite. Gold, silver, copper, lead and zinc are common, particularly around the Fleet intrusion, and over 60 copper and iron-rich carbonate veins have been located northwest of the Criffel–Dalbeattie pluton.
Volcanic vents active during the early Devonian are also present in the area, although they are generally poorly preserved. An exception is the large vent at Shoulder o’ Craig, 17 km southwest of Castle Douglas, on the Dee estuary. The headland here is principally made up of a vent-filling intrusion breccia, which consists of Silurian sandstone and siltstone clasts within a basalt (mafic) matrix. Both vent rock and country rock are cut by very potassium-rich dykes, indicating a magma source deep within the mantle. These dykes often have irregular shapes, and one dyke in the area is known as the ‘Loch Ness Monster’ due to its particularly bizarre outcrop pattern. On a regional scale, this area presents a bit of a conundrum, as volcanic vents, mantle-derived dykes and granite plutons, i.e. igneous rocks from all depths within the crust, were intruded around the same time (between around 415 and 400 million years ago, earliest Devonian), and are now seen at the same level of erosion.
Further north, the late Silurian and early Devonian was the time when a series of basins first began to develop in what would become the Midland Valley, as crustal tension caused movement on the Highland Boundary and Southern Uplands faults. At this time (around 420 to 400 million years ago), Scotland lay in the interior of a large continent some 20 degrees south of the equator, and in this environment the new Caledonian mountains were eroded rapidly because soil-binding plant cover had not yet evolved. Rivers and streams washed the sediment into the developing Midland Valley basins, forming coarse conglomerates, red sandstones and mudstones, collectively called the Lower Old Red Sandstone. Volcanic rocks (associated with crustal extension) are common in the upper 600 m of the Lower Old Red Sandstone, where lava sheets (predominantly andesite) are intercalated with river and lake sediments, mostly sandstones. Today, principal outcrops include a 400 m-thick lava pile underlying the Carrick Hills and a 600 m-thick lava pile in the Dalmellington area (20 and 30 km east of Girvan, respectively).
The Carrick Hills lava pile is particularly well exposed along the coast around Dunure (10 km southwest of Ayr). This coastal section has been studied for over a century in an attempt to unravel the complex relationships between the lava and intervening sediment; the upper and lower surfaces of andesite (mafic) sheets are often very irregular, with bulbous, finger-like protrusions that extend upwards and downwards into the sediment, or have become detached completely, forming zones of lava pillows. In places, lava engulfs patches of sediment; elsewhere, the lava is surrounded by sediment. The andesite sheets are generally well jointed, and these joints are often filled with hardened sandstone. Despite these contorted relationships, lamination in the sandstones is generally intact, save for a small zone near the contact. Such irregular contacts are thought to result from the sills being intruded into wet, unconsolidated sediment; as hot magma was emplaced, it vaporised water at the magma–sediment contact, fluidising the sediment in a narrow zone next to the contact. This vapour and its entrained sediment then flowed away along the hot contact surface, offering very little resistance to the magma and allowing bulbous protrusions to form. Likewise, the liquid magma could not push directly against the wet host sediment, and so this sediment remains largely undeformed, except at the contact zone. After intrusion, large amounts of water vapour were trapped in sediment enclaves and at contact zones. As the andesite then cooled, it contracted and cracked, often resulting in a sudden decrease of pressure in the sediment. This led to explosive boiling of the water, fluidising the sediment and blasting it along the fractures and cooling joints. Vesicles (cavities formed by gas bubbles) are also very common in the lavas, generally now infilled by minerals such as quartz, agate or chalcedony precipitated by circulating groundwaters.
By the middle Devonian (400 to 385 million years ago), further earth movements resulted in uplift and erosion of much of the sediment laid down in early Devonian times, and some of the underlying Ordovician and Silurian. The main granite bodies probably became exposed at the surface during this time, as evidenced by the clasts of Criffel–Dalbeattie granite found in Upper Old Red Sandstone deposits in Area 2 to the east. These late Devonian deposits are rare in Area 1, only outcropping near Dalmellington in a thin strip north of the Southern Uplands Fault.
The Caledonian Mountains had been largely eroded by the start of the Carboniferous, around 360 million years ago, although the Southern Uplands still formed a considerable upland area. Throughout the following 60 million years of the Carboniferous, deposition occurred mostly in the lowlands of the Midland Valley and the Solway Firth basins in marine or coastal-plain environments. Sea levels varied, resulting in the deposition of limestones, sandstones, mudstones and coal, often arranged in ‘cycles’ of varying layers, as shallow seas and river estuaries gave way repeatedly to swampy forests. Towards the end of the deposition of the Lower Carboniferous, the Southern Uplands had been sufficiently lowered by erosion to be breached by the sea along what is today Nithsdale, and the Midland Valley and Solway Firth basins were linked. Coal deposits were laid down under swampy conditions in the Carboniferous, and are today found around Sanquhar and Thornhill and in the larger Ayr Basin. These sedimentary basins were defined by numerous northwest-trending normal faults. The Carboniferous was also a time of renewed igneous activity, after the quiet of the mid- and late Devonian. This activity was associated with faulting and basin formation, and continued intermittently for some 100 million years until mid-Permian times. Today, lavas, volcanic plugs and sills from this time underlie much of the high ground in the Midland Valley. Hot fluids associated with this igneous activity resulted in mineral veins forming, and in many cases these have been economically important for the region. Gold, silver and lead have been mined for centuries from the well-known mining district around the Lowther Hills and Leadhills (20 km north of Thornhill, Fig. 46 (#ulink_09dd45a9-6c18-5f65-a6b5-d38fb3378ef5)). Leadhills has been designated a Site of Special Scientific Interest (SSSI) because of the variety of rare lead minerals present. Lead smelting in the Leadhills area has left its mark on the countryside, in the form of old tips, abandoned machinery and poisoned vegetation.
By the end of the Carboniferous, Scotland had drifted northwards from the equator and the climate changed from tropical to arid. Throughout the Permian (between 300 and 250 million years ago), Scotland had a desert climate in which the red sandstones and conglomerates of the New Red Sandstone were deposited, often on top of Carboniferous rocks as sedimentary basins continued to subside. Today, significant outcrops of Permian sediments are found between Loch Ryan and Luce Bay (near Stranraer), in the southern and central parts of Nithsdale and east of Ayr.
During the Mesozoic, sea levels were at times up to 300 m higher than today, and shallow-water sediments are likely to have been deposited at least in the Midland Valley. However, no Mesozoic rocks are preserved today, showing that, overall, the last 250 million years have been a time of net erosion in Area 1, as in much of Scotland.
FIG 50. South end of Ailsa Craig. The highest point of the island is 338 m above sea level. The term ‘Paddy’s Milestone’ has been applied, because the island is a marker by sea between the Clyde ports and those of Ireland. The paddle steamer Waverley is close to the shore, which is fringed by a raised beach marking the recent uplift of the island relative to sea level. (© David Law)
The youngest bedrock in this Area underlies the small but remarkable island of Ailsa Craig, some 15 km northwest of Girvan (Fig. 50 (#ulink_aa7379e2-a84e-5957-8cf4-007e3146be66)). The island is the deeply eroded remains of a volcanic plug, emplaced at the start of the Tertiary (around 60 million years ago) into gently dipping Permo-Triassic rocks. The intrusion is a fine-grained granite, whose unusual minerals give the rock a characteristic bluish colour. Columnar jointing is very prominent around the island, as are quarries from which the rock has been extracted to manufacture the famous polished curling stones (or ‘ailsas’).
MAKING THE LANDSCAPE
In early Tertiary times, sea-floor spreading in the North Atlantic was accompanied not only by the eruption of lavas in the Tertiary Volcanic Province (including the intrusion of the Ailsa Craig microgranite), but by widespread uplift across much of the Scottish mainland. The Southern Uplands and Highlands were once again uplifted, while the Midland Valley, lying on the periphery of these two blocks, became relatively lowered. The uplift, and the more modest episodic uplift events of the later Tertiary, were accompanied by vigorous denudation, often concentrated along lines of geological weakness such as faults and softer sedimentary units. In the generally warm, wet climate of the Tertiary, the intervening phases of tectonic stability were times of deep bedrock weathering that enhanced the pre-existing relief, widening valley floors and basins and resulting in the development or extension of erosion surfaces. In this way, the main landscape features seen today were initiated during the Tertiary: an erosion surface between 400 and 600 m in elevation developed across the Southern Uplands, dissected by numerous river valleys. The final form of the Southern Uplands owes much to glacial erosion, but the Tertiary erosion surface is still apparent as the smooth, rounded hills tend to be at uniform heights at approximately this elevation. The projecting hills of the Southern Uplands tend to be underlain by more resistant material, which would have formed topographic features during the Tertiary before being moulded by glaciers. Examples are the higher hills of the Lowther Hills or Leadhills (in places over 750 m high), which tend to be made of tougher and more resistant quartzites and thick beds of grit, whereas the thinner greywackes and shales have been weathered into gentler rolling hills. Further west, the highest hills of the Southern Uplands are found around the Loch Doon granite, although they are not underlain by the granite itself. This will be examined later, when looking at the effects of glacial erosion on the landscape.
FIG 51. Elevation map of Area 1, showing the main river valleys and upland areas.
The rolling hills of the Southern Uplands are interrupted by the broad valleys of the rivers Cree, Dee and Nith, which flow roughly southeast off the high ground into the Solway Firth (Fig. 51 (#ulink_0038c7b5-86de-59b0-86a7-9ea4660af799)). Another prominent area of low ground oriented roughly northwest to southeast has been flooded by Loch Ryan and Luce Bay, and therefore separates the Rhins peninsula from the mainland. It is obvious in Figures 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e) and 51 (#ulink_0038c7b5-86de-59b0-86a7-9ea4660af799) that the river valleys of the Cree and Dee are aligned roughly parallel to large northwest/southeast-trending faults, and it seems likely therefore that the more easily weathered rocks in the fault zone provided a relatively easy pathway for river erosion, probably as early as the Tertiary and certainly more recently. It is also clear from Figure 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e) that Luce Bay–Loch Ryan and Nithsdale are in part underlain by Devonian to Permian sedimentary rocks. These rocks are softer than the surrounding Ordovician and Silurian rocks, and have been more extensively weathered to form the low ground seen today. In effect, the Nith is once again flowing down what would have been a valley at least as far back as Carboniferous times, when a sedimentary basin became established running at right angles to the northeast/southwest-trending major faults, such as the Southern Uplands faults and the general folding of bedrock.
FIG 52. Hill-shade map of southwestern part of Area 1. The Southern Uplands Fault shows up well, as do other erosional and depositional features due to glaciation. Note the drumlins north and west of Patna Hill.
The fault which is most obvious in the landscape is the large Southern Uplands Fault. River and stream valleys have been preferentially eroded along the fault over much of its length. The fault is particularly prominent at its southwestern end (Fig. 52 (#ulink_7b1f5143-141b-5238-bf11-4293cff1d9a7)), where it splits into two (the southern Glen App Fault and northern Stinchar Valley Fault: Fig. 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e)). Preferential weathering along the Glen App Fault has resulted in the remarkably steep-sided, linear valley of Glen App, whilst the more curved line of the Stinchar Valley Fault has been excavated and now underlies Stinchar Valley. In broader terms, the Southern Uplands Fault separates the generally higher, hillier ground of the Southern Uplands from the lower-lying, flatter ground of the Midland Valley. This change in topography is not, however, generally clear-cut across the fault, as Carboniferous and Permian sedimentary rocks infiltrate into the Southern Uplands along the Nith valley, as described above, whilst igneous rocks are relatively common just north of the Southern Uplands Fault within the Midland Valley and, as described later, have often resisted erosion to form hills comparable to those found just south of the fault.
Glacial landscape development
Whilst the broad outlines of the present Scottish landscape had probably been established by the end of the Tertiary, its detailed configuration owes much to events of the Quaternary period. During the last million years, ice sheets have repeatedly expanded to cover much of Scotland, including Area 1. These ice sheets flowed radially outwards from centres in the Highlands and Southern Uplands and were powerful agents of erosion and deposition, moulding the uplands, scraping sediments from the lowlands and locally depositing great thicknesses of boulder clay (till).
The most recent glacial episode, the Devensian, reached its coldest about 25,000 to 20,000 years ago, when an ice sheet centred on the Western Highlands and Southern Uplands had expanded to cover most of Scotland and all of Area 1. The broad pattern of ice flow during this time is shown in Figure 53 (#ulink_4e497bf7-251f-5d19-940c-bea67c775b74): the thickest ice was centred on the Southern Uplands, and it flowed radially outwards from an ice divide that extended from Merrick in the west to the Lowther Hills in the east. Ice flowing northwards into the Midland Valley came up against southwards-flowing Highland ice, forcing ice to flow east and west across the low ground of central Scotland.
Landscape modification by glacial erosion
Glacial erosion has played an important role in creating the final shape of the landscape seen today. Most of Area 1 was extensively ice-scoured throughout the course of the Pleistocene glaciations, and the land surface present at the end of the Tertiary became heavily modified. Glacial erosion in this Area is most obvious in the uplands, which have been extensively ice-scoured. The mountains around the Loch Doon and Carsphairn igneous intrusions have an, albeit very rounded, Alpine form, with corries, rounded arêtes and intervening glacial troughs. These large-scale landforms were produced over the course of multiple glaciations, in particular by local valley and cirque glaciers during early and late phases of glaciation. The intensity of glacial erosion, at least during the Devensian glaciation, decreased eastwards towards the Lowther Hills, where more localised erosion took place: powerful ice streams continued to deepen the main valleys, but the intervening ridges and plateau were relatively unmodified. The corries that are relatively common in this southwestern part of the Southern Uplands are largely absent from the northeastern Southern Uplands. This could reflect a difference in climate from west to east, with the glaciers of the warmer, wetter southwest flowing much more vigorously, and hence eroding more than those of the colder, drier northeast, where the ice was frozen directly onto the rock and was therefore unable to scour deeply.
FIG 53. Generalised map of ice flow during the Devensian.
It is often the case in Scotland that, where granite intrusions are present in the bedrock, they are visible in the landscape because they form topographic highs. The relationship does not, however, seem to be so clear-cut in Galloway, where the different granite bodies have responded differently to both Tertiary and glacial erosion. The Cairnsmore of Fleet and Dalbeattie bodies do seem to be loosely associated with topographic highs: round, smoothed hills in the case of the Fleet body, and some of the only elevated ground on the south coast in the case of the Dalbeattie body (Figs 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e), 51 (#ulink_0038c7b5-86de-59b0-86a7-9ea4660af799)). However, in both cases only part of the intrusion seems to have resisted erosion to form elevated ground: the western half of the Fleet body and the southeastern margin of the Dalbeattie. Elsewhere, the elevation of the land is not discernibly different to that underlain by the surrounding Palaeozoic metamorphics. However, a contrast is seen in the ‘texture’ of the land, with those areas underlain by granite or granodiorite having a much more ‘smoothed’ appearance, probably reflecting the more uniform nature of the rock, and its response to erosion, than the surrounding folded Silurian rocks. Likewise, the Carsphairn intrusion underlies the large hill of Cairnsmore of Carsphairn, with its knock-and-lochan topography and craggy faces. Again, however, there is no change in topography at the contact between the pluton and the surrounding rock, and the area north and east is almost equally as mountainous.
Something very different is associated with the Loch Doon granite. This area formed the centre of accumulation for the local icecap during the Devensian glaciation, and likely earlier, with glaciers moving outwards onto lower ground with an approximately radial pattern of flow. As such, it has been subject to intense glacial erosion, both under an extensive icecap during the glacial maximum and by local valley glaciers during early and late phases of glaciation. It is obvious from Figure 52 (#ulink_7b1f5143-141b-5238-bf11-4293cff1d9a7) that the Loch Doon pluton itself has resisted this erosion much less than the baked Ordovician sediments which surround it. These tough hornfels (baked sediments) today underlie the distinctive elevated ridge of peaks which almost completely surrounds the Loch Doon pluton. These hills include the Rhins of Kells, with Corserine (814 m) on the eastern flank and Merrick on the western flank (Fig. 51 (#ulink_0038c7b5-86de-59b0-86a7-9ea4660af799)), which at 843 m elevation is the highest point in Scotland south of the Highlands. The view from the summit of Merrick is exceptional, from Ben Cruachan northeast of Oban, across to the Paps of Jura and then south across the Isle of Man and the Lake District to Snowdonia. The granodiorite which makes up much of the Loch Doon pluton has a tendency to form the low boggy ground between these hills, averaging around 300 m elevation. Glacial rock basins have been gouged out of this granodiorite, and today they are flooded to form the numerous small lochs which are seen within the boundaries of the intrusion, such as Loch Enoch. Meanwhile, the granite which forms the centre of the intrusion is obviously more resistant than the granodiorite, and underlies a ridge of high ground including the hills of Craiglee, Hoodens and Mulwarchar (692 m).
Throughout these uplands, bare, scoured rock is relatively common, particularly around the Loch Doon and Carsphairn area, although the jagged peaks and cliffs common in the Highlands are mostly absent. Where present, this bare rock provides an interesting contrast to the otherwise rolling moorland. One particular craggy outcrop southwest of Loch Enoch has been named the ‘Grey Man of Merrick’, because of its resemblance to a man’s face when seen from the side. Another interesting feature is the so-called ‘Devil’s Bowling Green’ on Craignaw, a remarkably flat, smooth glaciated rock surface strewn with rounded boulders.
Elsewhere in Area 1, the ground is much lower-lying, and the effects of glacial erosion are not so obvious. This ground, already low at the end of the Tertiary, has been further lowered and smoothed by the passage of ice. Where hills are present, they tend to have been rounded by ice scouring, and often have a streamlined shape. An excellent example is the island of Ailsa Craig (Fig. 50 (#ulink_aa7379e2-a84e-5957-8cf4-007e3146be66)). Although the island has been considerably modified by subsequent marine action (which produced its precipitous cliffs, discussed below), its overall shape is round, but elongated from north to south, reflecting southerly ice flow. The granite has clearly resisted erosion much more effectively than the soft Permo-Triassic sandstones into which it was intruded. Ailsa Craig was positioned in the path of many different ice streams during the Devensian glaciation, and these ice streams carried blocks of Ailsa Craig microgranite in the direction they were flowing. As the microgranite has a very distinctive composition, these blocks are easy to identify, and they have proved very useful in tracing flow directions of the last ice sheet across Britain. Indeed, the Ailsa Craig boulder train is one of the most famous and largest in the British Isles, extending south across the Irish Sea and parts of England and Wales as far as Pembroke, and westwards to Ireland.
North of the Southern Uplands Fault, the relatively soft sedimentary bedrock of Devonian to Permian age has generally been heavily weathered and eroded by the passage of ice, forming a low-lying landscape. However, the sedimentary units are punctuated by horizons of lava and numerous plugs of fine-grained igneous rock, only the largest of which are shown in Figure 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e). As a result, the generally low-lying landscape is frequently interrupted by rounded hills, underlain by the more resistant units. This is well illustrated in the Straiton area (20 km northeast of Girvan), where the craggy hill tops of Bennan Hill and Craig Hill are underlain by more resistant Devonian lavas, with Devonian sediments underlying the gentler slopes of the surrounding area. Likewise, Mochrum Hill (Fig. 52 (#ulink_7b1f5143-141b-5238-bf11-4293cff1d9a7)), near Maybole, is underlain by the eroded remains of a large Devonian volcanic vent, around 1 km in diameter. The vent is filled with agglomerate (coarse angular blocks of volcanic material), which has resisted erosion to form the prominent, rounded hill, whilst the surrounding Lower Old Red Sandstone is much softer and lower-lying. The sandstone in this area is feldspar-rich, and has weathered to produce particularly fine arable soils. Younger volcanic rocks are also common, such as the Permian, agglomerate-filled volcanic neck underlying Patna Hill, just northeast of Patna. There are more than 20 such vents in the Patna–Dalmellington area, many of which are responsible for small topographic features. As these intrusions are often basaltic (mafic), they have weathered to produce nutrient-rich soils, the so-called ‘Green Hills’ of Ayrshire.
In places, prominent hard bands within the sedimentary rocks are also associated with rounded, glacially scoured hills. An example is the ‘Big Hill of the Baing’ southeast of Straiton (20 km northeast of Girvan), an elongated, faulted ridge of Ordovician boulder conglomerate. More extensive outcrops of this conglomerate occur in the Girvan–Ballantrae area, where, along with the Ballantrae Complex, they underlie higher, hillier ground than the softer rocks further south.
Landscape modification by glacial deposition
Much of Area 1 is relatively low-lying, and here the effects of glacial erosion are more subtle than in the high ground of the Southern Uplands: the ground level was lowered, pre-existing Tertiary valleys were deepened and the low hills were moulded and streamlined. Equally important in the formation of today’s landscape in these lowland areas was glacial deposition: on deglaciation, great thicknesses of till were deposited and today glacial till, sand and gravel mantles much of the lowlands. These deposits have a range of surface forms, including eskers, kames, outwash terraces and, in particular, drumlins.
Drumlin swarms are important landscape features throughout the lowlands of this Area, tending to broadly correspond with the arrows on Figure 53 (#ulink_4e497bf7-251f-5d19-940c-bea67c775b74). They mantle much of the Rhins of Galloway, the Machars, the Glenluce, Ballantrae and Girvan districts and Nithsdale. They also make up much of the land surface of the Midland Valley, being responsible for the rather intriguing, ‘hummocky’ texture that is so characteristic when viewed from the air, or on a simple hill-shade map (Fig. 52 (#ulink_7b1f5143-141b-5238-bf11-4293cff1d9a7)). The drumlin swarms in these areas produce a distinctive landscape of low hills, typically around 30 m high and 300 m long, all oriented in the same direction and with similar shapes – blunt at one end and tapered at the other, rather like an egg. This streamlined shape is produced by deposition at the base of a flowing glacier: drumlins often have a core of rock or glacial till, and as sediment-laden ice flows over these obstructions, material is deposited downstream of the core, where it is relatively sheltered from ice erosion. As this process repeats itself, a streamlined mound is gradually produced, with a tapered end pointing downstream and a blunt end pointing upstream. One is aware of the whaleback shape of the drumlins that make up these swarms from the ground, but an aerial view allows the best appreciation of their three-dimensional streamlined form. Excellent examples are seen, for example, around Newton Stewart and in the New Galloway district. Smaller swarms are also present in the uplands.
The broad Carsphairn Valley cuts across some of the highest ground of the Southern Uplands, with the Loch Doon hills to the southwest and the Cairnsmore hills to the northeast. Reconstructions of former ice-flow directions in the valley indicate that, during the Late Glacial Maximum, a northeast/southwest ice divide was located across its central part, passing from Cairnsgarroch summit through Craig of Knockgray to the Cairnsmore Hills. The thickest and most extensive till deposits present in the Carsphairn Valley are found around the area of this ice divide, which seems somewhat contradictory. Horizontal ice flow is minimal at ice divides, and the till cannot therefore have been deposited when the divide existed, so the source and age of this till is an interesting question. The answer seems to be that the till was deposited during or before the glacial maximum, during the growth of the Late Devensian ice sheet. At the start of the Late Devensian glaciation, ice would have initially accumulated in the corries and trough heads northeast and southwest of the Carsphairn Valley. As the glaciation advanced, these glaciers expanded and finally converged in the valley bottom, and as their flow was impeded till would have been deposited. During the subsequent glacial maximum, the preservation of this till beneath great thicknesses of ice is likely due to its location under the ice divide, as although the ice sheet expanded and thickened, the slow rates of ice movement meant the ice had little erosive power here.
This till, deposited during the growth of the Late Devensian ice sheet and preserved under the ice during the glacial maximum, was then remoulded during a late stage of glaciation into a set of interesting landforms – rogen, or ribbed, moraine, which consists of sinuous, 20 m-high, elongated ridges that run perpendicular to the valley axis. The mechanism by which these till ridges formed, perpendicular to the down-valley direction of Devensian ice flow, is another interesting point. A likely scenario is that the rogen moraines represent ridges of sediment produced by thrusting (by compression) or fracturing (by extension) at the base of the ice sheet. For this to happen, the flow speed of the lower part of the ice must have varied downstream: a sudden speeding-up would produce fracturing by extension; a sudden slowing-down, such as upstream of an obstacle, would produce thrusting by compression. This would have been most likely to happen during a late stage of ice-sheet deglaciation, when faster, more concentrated flow occurred within the main valleys. The most recent episode recorded by this till involved the drumlinisation of the rogen moraine, as the original landforms became elongated down-valley to varying degrees.
Important amounts of sediment were also deposited by sediment-charged meltwaters flowing out from retreating glaciers, referred to as glaciofluvial deposits, and present in a variety of forms. Sediment may accumulate in channels, ponds and lakes trapped between lobes of glacier ice or between a glacier and the valley side. Where such sediment has a ridge or mound form, it is termed a kame; where it is a flat-topped mound, it is termed a kame terrace, and is likely to have been deposited in a lake. When sub-glacial meltwater drains through tunnels, the sub-glacial stream may deposit sediment as a surface stream would, but confined to the tunnel. The result is a long sinuous ridge of gravel, termed an esker. Outwash plains often build up downstream of the melting glacier, large plains of poorly sorted, stratified sediment deposited by braided streams. Sequences of terraces are often seen in these plains, formed by river incision. Kettle holes are also common features, formed by blocks of ice that become buried in outwash sediment, and then melt to leave behind a depression. Many of these kettles have been infilled with sediments, particularly peat, during the post-glacial times, but some are still visible today as small isolated lakes or deep water-filled depressions in boggy areas that were once the low-lying outwash plains.
A famous example of ‘kame-and-kettle topography’ is found in Nithsdale, north of Dumfries. Particularly on the eastern side of the valley, there are many short, linear glaciofluvial ridges separated by depressions and hollows. The relative relief between ridge crest and depression is usually between 8 and 25 m, and the ridges are relatively short, with very few being over 500 m long. The extensive gravel pit at Kilblane, for example, is developed in three such kame ridges. The coarse sediment that makes up these ridges, and other linear kame ridges in this part of Nithsdale, was probably deposited in meltwater channels that flowed between ice-cored ridges parallel to the ice margin. As the ice ridges melted, the sediment-filled channels became inverted to produce the kames seen today. Kame terraces are also seen on both sides of the Nith, with the best developed just east of Duncow (8 km north of Dumfries) at an altitude of around 55 m. Further north, in the mid-part of the Nith valley (south of Thornhill), a similar kame-and-kettle topography is seen. The glaciofluvial deposits of Nithsdale account for the rather large number of sand and gravel pits seen just north of Dumfries, now often flooded. Glaciofluvial deposits are relatively common elsewhere in Areas 1 and 2, such as in many of the valleys on the south side of the Southern Uplands and in the area around Stranraer.
The mapping of glaciofluvial deposits in the Nith Valley has allowed the reconstruction of the pattern of glaciofluvial drainage which developed during a late stage of deglaciation. The result shows a narrow marginal zone of ice-cored ridges and troughs in the north, feeding meltwater and sediments to the ice front north of Dumfries. This ice front is marked by a terminal moraine across the Nith valley, which is crossed by the River Nith in a gorge in Dumfries. Further southeast, the drainage fed outwash systems in the Lochar Water and Nith valleys. Today, this outwash plain underlies much of the uniformly flat surface southwest of Dumfries.
A closer look at Late Devensian ice-flow directions
The broad ice-flow pattern shown in Figure 53 (#ulink_4e497bf7-251f-5d19-940c-bea67c775b74) is useful, but presents a highly generalised picture; in reality, ice-flow directions over the course of the Late Devensian were somewhat more complicated. The large number of streamlined glacial deposits found throughout Area 1, particularly in the lowlands, has allowed a much more detailed reconstruction of Devensian ice flow. A recent study looked in detail at the glacial features present in the western part of Area 1, in particular at drumlins, erratic trains and glacial striae. It was found that several generations of these features can often be seen superimposed on one another, recording multiple passages of ice from different ice centres. These changing flow directions are summarised in Figure 54 (#ulink_02913ac2-f5e9-515a-a5aa-d79b9c21ae51), and record the changing relative strengths of the Southern Uplands and Highlands ice centres.
Some of the earliest features in the western Southern Uplands indicate that ice from the Highlands was initially dominant during the Late Devensian, when it streamed southwards from the Firth of Clyde and crossed the Glenluce lowlands, producing north/south lineations. This Highland ice was then replaced over much of Area 1 by Southern Uplands ice, as shown by a southwest-oriented flow set running across Glenluce and the southern Rhins. A similar story is recorded by till deposits in the southern Midland Valley, around the margins of the Southern Uplands. For example, a vertical section cut by the River Nith at Nith Bridge, just south of Cumnock, reveals three tills deposited during the Late Devensian and separated by glaciofluvial sands and gravels. These tills have been carefully studied, and the bottom two were both found to have been deposited by Highland ice, which probably flowed across central Ayrshire from the Firth of Clyde area. The topmost till, by contrast, was deposited by ice originating in the Southern Uplands. There were, therefore, at least two distinctive phases of ice movement across central Ayrshire, with an initial advance of Highland ice being succeeded by Southern Uplands ice. The evidence at Nith Bridge matches similar evidence found across the southern Midland Valley, and the story indicated by streamlined landforms further southwest. It seems, therefore, that Highland ice initially expanded to encroach on the Southern Uplands, and that it was only as glaciation progressed that Southern Uplands ice became more dominant in Area 1. Further south, another major ice centre was established in the Lake District, and converging drift lineations at the tip of the Machars peninsula mark the confluence of this and Southern Uplands ice.
At the coldest stage of the last glaciation, around 20,000 years ago, Highland ice one again played a role – an ice stream flowed out from the Highlands and along the western seaboard of Area 1. As the ice moved down the Firth of Clyde, it scraped marine deposits off the sea bed and re-deposited them further south. These shelly deposits are found, for example, on top of a 10 m-high shore platform around the Mull of Galloway. The Highlands ice sheet also brought glacial erratics of the distinctive Ailsa Craig microgranite and Arran granite southwards, found today throughout the Rhins peninsula.
FIG 54. More detailed examination of local ice-flow directions during the Devensian (LGM is the Late Glacial Maximum that occurred late in the Devensian). After Salt and Evans, 2004
Following the glacial maximum, climate began to warm, and both the Southern Uplands and Highland ice centres contracted. Some time after Highland ice had retreated from the western seaboard, a phase of local ice expansion interrupted the general waning, and Southern Uplands ice once again flowed across the western part of Area 1. For much of the western, lowland parts of this Area, this would be the last time they were ice-covered, and the ice sheet left behind the extensive drumlin swarms described above. This Southern Uplands ice flowed southwestwards across the Rhins of Galloway, bringing with it erratics of Loch Doon granite. It also flowed roughly westwards across the Ballantrae and Girvan districts. In the Ballantrae area, flow was somewhat valley-contained, showing that the ice sheet was much thinner than it had been during the glacial maximum. Further north around Girvan, the lineations are particularly notable for their cross-cutting relationships, probably produced by the slight shifting in the main ice-dispersal centre with time.
The most recent flow set was also produced during the waning of the Southern Uplands ice sheet, again during a minor re-advance. This time, valley glaciers radiated out from a Southern Uplands dispersal centre located around Merrick, down valleys such as Nithsdale, Glenluce and northwards into the Midland Valley. Again, erratic trains from distinct granite outcrops around the Galloway area provide useful trackers, along with moraine ridges and drumlins. A local surge of Highland ice down Loch Ryan also occurred at this time, and a prominent moraine at the head of Loch Ryan (the Stranraer moraine) marks the outer limit of this re-advance.
Pollen and beetle records indicate that temperatures in Area 1 may have risen to as warm as present by around 13,000 years ago, by which time southwest Scotland must have been completely deglaciated. Temperatures then fell sharply around 12,000 years ago, culminating in the Loch Lomond Stadial. This climatic deterioration was accompanied by the return of glaciers to parts of the Southern Uplands, although these glaciers were of very limited extent, generally being confined to the highest corries. Where glaciers developed, they bulldozed earlier till deposits into moraine ridges. A fine example of one such moraine is seen at Loch Dungeon, just southeast of Corserine. Steep cliffs of Silurian sediments rise from the southeast shore of the loch, and a subsidiary corrie of Corserine opens out on the northwest shore. A glacier emerged from this corrie, and its terminus is marked by a large terminal moraine to the west of the loch and by a shallow area within the loch itself.
Even in unglaciated upland areas, ice growth often caused extensive frost shattering. The scree and loose rock this produced is still visible today, particularly on summits and upper slopes, and has often been modified by subsequent flow to form a series of lobes and sheets. Elsewhere in the lowlands, the cold climate of the Loch Lomond Stadial made itself felt through the development of permafrost, as evidenced today by features such as ice wedge casts, seen most commonly in gravel pits. Evidence for periglacial disturbance and movement of the soil (solifluction) is also widespread on lowland slopes, usually affecting 1–2 m depth of soil, though in the valley floors of the Southern Uplands, great thicknesses of solifluction deposits have accumulated.
Post-glacial landscape development
At the end of the Loch Lomond Stadial, a temperature rise of around 7 °C occurred within just 700 years, marking the start of the current Flandrian (or Holocene) period. Although the effects of glaciation still dominate much of the landscape, in the 10,000 years since the disappearance of the last glaciers the land surface has been slowly adjusting to non-glacial conditions. These changes are particularly evident in areas of high relief, where glacial retreat exposed a bare rock landscape with over-steepened slopes. Soon after deglaciation, this landscape began adjusting to the new conditions, with rock falls, debris flows and reworking of glacial sediments. As the landscape re-equilibrated and soils and stabilising vegetation became established, it seems that these processes almost stopped, as shown by the vegetated, relict nature of most of the talus slopes, debris cones and alluvial fans in this Area.
Today, Area 1 is notable for its variety of river types and sizes, reflecting contrasts in relief and catchment size throughout the Area. Most of the main rivers originate in the high ground of the Southern Uplands and drain southwards, including the Nith, Cree and Dee (Fig. 51 (#ulink_0038c7b5-86de-59b0-86a7-9ea4660af799)). The upland tributaries of these rivers are akin to mountain torrents, becoming wandering gravel-bed rivers in their middle reaches as the relief becomes more subdued. Most of the major rivers have highly sinuous, meandering courses in their lower reaches, and drain into silty estuaries in the Solway Firth (Fig. 55 (#ulink_4cf1a00e-33ae-5a42-93e7-7b3926fd97d8)). The River Ayr, in the Midland Valley, has one of the most meandering courses in this Area, as it weaves across a flat lowland strewn with glacial deposits. Many of the lower reaches of the watercourses in this Area have been embanked to prevent flooding of adjacent land, and some of the smaller rivers show signs of having been straightened in the past.
The first vegetation to become established early in the Holocene was a juniper-dominated community, followed by birch and hazel around 9000 years ago, and then by oak and elm during the middle Holocene. Pine forest was present in the Galloway Hills, but was never the dominant species in this Area. From around 5000 years ago, human activity first began to have a significant impact on the landscape, primarily through forest clearance to make way for agriculture. There is evidence that woodland began to be progressively replaced by peat around 5000 years ago, and that by around 4200 years ago forest cover had essentially disappeared from the Area, replaced by blanket mire. This deforestation is thought to have led to enhanced soil erosion, with an increase in slope failure, debris flow activity and river incision. Peat has been the most widespread soil type in Area 1 since around 4000 years ago, in the form of blanket bog (including the internationally important Silver Flowe Bog in the low ground of the Loch Doon intrusion), or drier heather-covered slopes.
More recently, damming is another way in which humans have significantly altered parts of Area 1, flooding valleys to create reservoirs. The Galloway hydroelectric scheme was built between 1930 and 1936, and was the first of its kind in Scotland. Although small compared to some of the later Highland schemes, it is a model of unobtrusive and ecologically sensitive hydroelectric engineering, and is studied by engineers from around the world. Making use of water principally stored in Loch Doon, Clatteringshaws Loch and Loch Ken, the scheme includes eight dams, 12 km of tunnels, aqueducts and pipelines together with six power stations along 130 km of river. Whilst Loch Doon is on the site of a natural loch, damming has increased the water level by some 9 m, submerging various small islands. Before the Loch Doon dam was built, Loch Doon Castle, a thirteenth-century castle originally located on an island in the centre of the loch, was moved, stone by stone, to the adjacent bank where it now stands. Elsewhere, dam building flooded valleys, thereby significantly altering the landscape. Such reservoirs include Clatteringshaws Loch and Loch Ken on the Water of Ken. Loch Ken is now a major nature reserve and a breeding ground for many varieties of wild birds.
FIG 55. Tidal marshes showing typical, highly sinuous channels, where the wavelength of the channels cut in the muddy sediments reflects the tidal discharges involved. (© Patricia & Angus Macdonald/Aerographica/Scottish Natural Heritage)
Man has also altered the landscape by mining and quarrying activities, particularly prevalent in the Midland Valley where large opencast coal workings are still operational today in places, such as east of Patna. Recent clean-up efforts have greatly reduced the impact of colliery tips (bings) on the landscape, such as northeast of Girvan, where they have been landscaped and forested. Local stone has also been quarried for building stone, roadstone and crushed rock aggregate. An important source of building stone is the area around Mauchline, from which the attractive orange-red Permian sandstone has been extracted. Stone from this area has been widely used throughout the UK and Ireland, and even shipped to the USA. The granites have also been economically important for the region – the Glasgow and Liverpool docks, for example, were constructed using Criffel–Dalbeattie granite. Glacial sands, silts and gravels have also frequently been quarried, often leaving their mark on the landscape with flooded gravel pits.
Today, the Area is generally very wet and mild, as the North Atlantic Drift maintains higher temperatures than those found on the east coast. Indeed, plants normally associated with more southerly latitudes are found on the Rhins, along with dolphins and basking sharks off the coast. Much of the Southern Uplands in this Area lie within the Galloway Forest Park, managed by the Forestry Commission, and as well as rolling moorland, conifer plantations are common on the shallow, poor soils. The main river valleys (such as the Urr, Dee, Cree and Nith) provide a contrast to this rolling moorland, providing much of the good arable land of the Southern Uplands.
The formation of the coastline
As described in Chapter 5 (#u661596f2-c933-517a-abc9-8c82715c3114), sea level in much of Scotland has not been constant, but has risen and fallen according to the interplay between global sea level and the elevation of the land. Global sea level decreases during an ice age, as water is locked up within the ice, and rises again as this ice melts. Meanwhile, during glacials, the crust becomes depressed locally under ice sheets, sinking into the mantle, and slowly rebounds once this ice has melted. The sea level at the coast at any one time therefore depends on the interplay between these two effects, and in the past sea level in Area 1 has been both higher and lower than at present. When sea level has remained constant for long enough, shorelines formed – marked today by erosional features such as rock-cut platforms backed by cliffs, or by beach deposits. Where subsequent uplift has outstripped global sea-level rise, these shorelines now take the form of raised beaches, raised deltas, raised estuarine deposits (known in Scotland as carse), and raised rock platforms and cliffs. Good examples of these features are found in this Area. Shore platforms are relatively common on stretches of rocky coastline, and are particularly well developed, for example, between the Heads of Ayr and Turnberry and around Ballantrae. In the latter area, numerous small caves and gullies often delineate the foot of the cliff, and occasional raised sea stacks rest on the raised platform. Further south, a shore platform can be traced around most of the Mull of Galloway at approximately 10 m above present sea level, and deposits of glacial till on top of it show that it predates the Devensian glaciation. In places, several shorelines are present, such as some 8 km southwest of Girvan, where two old shorelines give the skyline a stepped profile. These shorelines occur as a series of benches, commonly cut into boulder clay. Mafic, dolerite dykes have been etched out by the sea during the formation of each of these shorelines, and today stand as raised sea stacks.
In the lower-lying coastal areas, past increases in sea level involved the flooding of sometimes large patches of land. For example, along the Ayrshire coast the late glacial sea was nearly 30 m higher than at present, and so the sea would have come several kilometres inland from its present position. Along the Solway coast in the south of the Area, large sections of the present shore are backed by raised estuarine deposits of silt and clay, which provide valuable records of this sea-level change over the last 15,000 years. Good examples occur flanking the heads of the Cree and Fleet estuaries, which in this area lie between 7 and 10 m above present sea level.
The sediments within cores taken from the carselands flanking the Cree estuary have been analysed and dated, and have proved very useful in reconstructing the sea-level history of the Solway Firth, as summarised in Figure 56 (#ulink_8bebc753-f365-59ed-bb54-d07aa50aaf50). These deposits show that sea level rose to cover this area by around 9600 years ago, followed by a rise to the so-called Main Post-glacial Shoreline by 6500 years ago, when sea levels were between 7 and 10 m above present and some 9 m of estuarine deposits were laid down in the Cree area. Sea level then fell from the uppermost carse surface to its present level as the land continued to rebound, whilst worldwide sea volume changed very little.
The large-scale shape of the coastline is controlled by a number of factors, many of which are in turn related to one another. Rising sea levels cause valleys to become flooded, forming bays and islands, whilst the location of these valleys often reflects rock type (i.e. hardness) and structure (both the presence of jointing within a rock unit and lines of weakness, such as faults). Climate and tidal energy also play an important role, controlling wave energy environments and terrestrial processes, such as sediment supply. The amount of sediment supply to a coast of course depends on the availability of that sediment, and in this regard glaciation has been very important. At the end of the Devensian, great volumes of glacial debris were deposited on the continental shelf. As sea level recovered to present levels, this sediment was reworked and moved towards the shore, to form the basis of our present beach and sand-dune systems. Today, where sediment supply is abundant, the coastline is currently advancing seawards, whilst in areas where sediment supply is in decline, the coastline is usually retreating through erosion. Humans, also, can have an impact – for example through building, quarrying, constructing sea defences and trampling of stabilising vegetation.
FIG 56. Sea-level curve for the Solway Firth. (Data from Smith et al. 2003, Transactions of the Royal Society of Edinburgh: Earth Sciences, 93, 301–31)
The number of large bays is one of the more obvious features of the coastal strip of Area 1. The main ones are Loch Ryan and Luce Bay, which together define the Rhins of Galloway, and Wigtown Bay further east. Numerous smaller bays are present along the south coast, including the Water of Fleet and Kirkcudbright Bay on the southeast side of Wigtown Bay. In general, these bays are located in the lower-lying ground of this Area, and therefore do not reflect a large difference in rock strength between headland and bay. Instead, they seem to roughly coincide with the large northwest/southeast faults shown in Figure 47 (#ulink_5f5e697c-fb37-5248-acae-75ecf871e10e), and with the outlets of major rivers. The exception is the Loch Ryan–Luce Bay pair, which may have been the site of a Tertiary river, now partially flooded by the sea.
Igneous rocks underlie some of the more prominent headlands on the west coast of Area 1, such as the Carboniferous vent rocks that make up the Heads of Ayr, a prominent headland some 6 km southwest of Ayr. The vent has been intruded into a colourful mix of Early Carboniferous sediments, including limestone, grey-green shales and red and green sandstones. Ash erupted from the Heads of Ayr vent is thought to underlie the knoll on which Greenan Castle has been built, around a kilometre up the shore towards Ayr. Where Silurian and Ordovician sedimentary rocks have been cut by cliff sections, the result is often rather impressive because of the exposed folding within the bedrock. Particularly spectacular ‘textbook’ examples of rock folds and other structures are found on the Machars peninsula, for example at Back Bay, just south of Monreith on the west coast, and around the Isle of Whithorn in the southeast. The fold structures at Back Bay not only illustrate two fold generations with a second set of folds superimposed on a first, but represent one of the most dramatic large-scale exposures of major re-folded folds in the UK.
Perhaps some of the most dramatic coastal scenery in Area 1 is found on the island of Ailsa Craig (Fig. 50 (#ulink_aa7379e2-a84e-5957-8cf4-007e3146be66)). Despite being only 1.2 km wide, the island is nearly surrounded by 340 m-high cliffs. The height of these cliffs reflects the exposed nature of the island to marine erosion, the strength and resistance of the bedrock to this erosion, and also the topographic high left behind here after the retreat of the last glaciers. For the most part, the foot of these cliffs is now between 5 and 10 m above sea level, and so marks a raised shoreline. A distinctive triangular raised beach is located on the eastern side of the island, fringed by storm ridges and an associated spit. These landform features highlight the importance of prevailing wind and storm direction, as they were caused by the westerly winds and waves since post-glacial times: sediment is deposited on the sheltered side of the island. The importance of exposure is also seen on the Rhins of Galloway, where the exposed western coast is generally rugged with steep cliffs and occasional inlets, in contrast to the calmer eastern coast with its sandy beaches.
South of the Southern Uplands Fault, the coast is generally rocky, with low cliffs and only small local beaches or small stretches of shingle. Cliffs are particularly well developed along the exposed western coast of the Rhins, reaching a maximum of 120 m in height near Dunman. This cliff line is largely inactive now, having been raised clear of wave action by crustal uplift. On the north coast of the Solway Firth in particular, the rocky coastline is punctuated by a number of large bays, generally river estuaries. The more sheltered conditions within these bays and the ample sediment supply have allowed wide expanses of sand-flat, mud-flat and salt-marsh to accumulate, and together the flats and marshes of the Solway Firth provide one of the largest continuous areas of intertidal habitat in Britain. Remnants of formerly more extensive lowland peat bogs are developed on raised estuarine sediments. These include the nationally important Lochar Moss and Moss of Cree.
Beaches are generally more common and extensive in this Area than in the Highlands of Scotland. The main reason for this is the relative abundance of sediment, primarily glacial sediment laid down on the near-shore shelf during the Devensian glaciation, and then driven onshore during the Flandrian sea-level rise, where it became stranded as sea levels fell again across the region. The relatively mild winds and waves experienced by the lowlands then meant that this abundant sediment source has remained fairly stable, and many of the beaches in this Area are still accreting today, rather than eroding. Old dune deposits are relatively common on the coastal strip south of Troon on the north edge of the Area, and sands were formerly worked from pits in these dune sands northwest of Monkton. Now, most of the dune deposits on the coast around Ayr are covered by golf courses.
Large expanses of tidal sand-flats are found along the southern coast of Area 1, and at low tide many kilometres of sand are exposed, such as at Mersehead Sands (20 km south of Dumfries). Further inland, the reworking of raised beach deposits and other sandy sediments by the wind has, in some coastal locations, produced extensive spreads of sand dunes, now for the most part anchored by coarse grass or forestry. The largest beach-dune system in southwest Scotland is found at the head of Luce Bay, home to a complex array of dune-related landforms, and still actively accreting today. The entire peninsula of the Rhins acts as a huge breakwater from the currents of the North Channel, creating the relatively calm waters of Loch Ryan and Luce Bay.
Salt-marshes are typically developed on low, raised beaches of sand or shingle and display a complex topography of pans, creeks and terraces. The salt-marshes of the Cree estuary in Wigtown Bay are particularly well developed, sandwiched between extensive sand-flats seawards and reed-swamps and emerged estuarine deposits (carse) landwards. The extensive carse deposits of the Cree estuary show that sedimentation has prevailed here over most of the last 10,000 years, despite changing sea levels. The estuary is well sheltered by Burrow Head to the south, and this has produced a largely unidirectional wave climate in which sediment is brought into the bay, with little subsequent removal. It appears that this system still operates, since many of the sand-flat and salt-marsh systems are accreting today. The presence of Sellafield-derived radionuclides attached to the sediment in the Cree and Water of Fleet sandbanks confirms the Outer Solway as a major sediment source, whilst important amounts of mud within the Cree mouth itself suggest a more fluvial source, further enhanced by active reworking of sediment from the carse deposits.
CHAPTER 7
Area 2: Southern Borders
AREA 2 COVERS THE CENTRAL PART of the Southern Uplands, sandwiched between Areas 1 (Galloway) and 5 (Edinburgh) (Fig. 57 (#ulink_47424c5b-7d09-59f4-b9f5-ee0eac35342b)). It straddles the southern part of the English–Scottish border, extending from Annandale and Moffatdale in the west to the Cheviot Hills in the east (Fig. 58 (#ulink_5dc1ce89-5134-5a8f-a2e3-be76ef0fd668)). The discussion provided here is restricted to features on the Scottish side of the border.
In the northern half of the Area, the River Teviot drains the relatively low-lying ground of Teviotdale, which in turn feeds into the larger Tweed Basin of Area 5 (Chapter 2 (#uc0e147ab-0db8-5cd3-a46c-c03cd0831740), Fig. 7 (#ulink_7473398e-e912-5841-b9c0-27bb75749122)). Teviotdale is bounded to the east by the Cheviot Hills and to the west by the Moffat Hills of the Southern Uplands. These rounded hills reach heights of over 800 m, and form a broad region of elevated ground which dominates the landscape in the northwestern part of this Area. The Solway Firth with its adjacent low-lying plains is the main feature of the southwestern corner of Area 2, along with the broad valleys of the Annan and Esk, which penetrate northwards into the hills of the Southern Uplands.
FIG 57. Location map for Area 2.
FIG 58. Natural and man-made features of Area 2.
STORIES FROM THE BEDROCK
Area 2 is principally made up of hard Ordovician and Silurian sediments, which are also the oldest rocks in the Area (Figs 59 (#ulink_52db03e9-d893-5ca4-a784-de798678b239), 60 (#ulink_c79c2c08-ea50-54c2-abd6-817f62198f72)). These sandstones (greywackes), siltstones and shales were deposited between around 490 and 420 million years ago on the floor of the extensive Iapetus Ocean that, during this time, formed an Atlantic-scale ocean with Scotland and England on opposite margins. Around 490 million years ago, this ocean began to become smaller, as oceanic crust became subducted beneath the Grampian Highlands, and the Ordovician and Silurian sediments were scraped off the ocean floor and stacked up in a pile against the Midland Valley (Fig. 28 (#ulink_97ce30e1-e360-557b-93ae-e0a8135abc77)). This deformation created a fold system across the uplands of this Area, with the fold axes trending northeast to southwest. Superimposed on this larger structure are countless minor parallel folds, which give a ‘grain’ to the oldest bedrock of the Area. During these movements, the sediments were altered to brittle sandstones and slates.
FIG 59. Simplified geology and hill-shaded topography for Area 2.
By the Devonian (around 410 million years ago), the major uplift and deformation of the Southern Uplands had ceased and the crustal materials of what are now Scotland and England became ‘welded’ along the Iapetus Suture. The mountainous ground of the Southern Uplands then began to be eroded, and large rivers flowed across the Area, depositing sandstones, siltstones and occasional conglomerates on lower-lying plains fringing the mountains. This Area lay in the interior of a large continent, and sediments accumulated on river flood plains where oxidising conditions led to the deposits being stained with red iron oxide. These Devonian sediments were deposited on the eroded surface of older, deformed Silurian and Ordovician rocks. This surface was far from uniform – in places, it had a relief of elevation of over 100 m. As a result, the thickness of the Devonian sediments varies from place to place, in part reflecting the fact that these river sediments were filling hollows in the topography. These deposits, collectively termed the Old Red Sandstone, today underlie a sizeable area just west of the Cheviot Hills.
FIG 60.Timeline of bedrock and surface-layer events in Area 2.
The early Devonian was also a time of igneous activity, and Devonian igneous rocks are often associated with interesting landscape features in Area 2. The largest area underlain by Devonian igneous bedrock is the Cheviot Hills in the northeast. Here, great thicknesses of sub-horizontal lava flows (predominantly mafic andesite) rest unconformably on Silurian marine sediments. This lava was extruded from an igneous centre that today straddles the border between Scotland and England. The centre experienced several stages of igneous activity: an initially explosive volcano was followed by a large out-pouring of lava, which today, after erosion, covers around 600 km2 of the Earth’s surface. This large area of lava was then intruded at depth by felsic granite, which after millions of years of erosion has been exhumed and today underlies the highest part of the Cheviot Hills, over the border in England. Elsewhere in the Area, much smaller Devonian volcanic vents and intrusions are relatively common.
