Scotland. Peter Friend
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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). 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