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
