Scotland. Peter Friend

Чтение книги онлайн.

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, СКАЧАТЬ