Scotland

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). 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). 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). 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 and 51 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 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), where it splits into two (the southern Glen App Fault and northern Stinchar Valley Fault: Fig. 47). 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: 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, 51). 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 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), 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). 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. 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), 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. 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). 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.