Scotland. Peter Friend
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Название: Scotland
Автор: Peter Friend
Издательство: HarperCollins
Жанр: Природа и животные
isbn: 9780007465989
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A closer look at Late Devensian ice-flow directions
The broad ice-flow pattern shown in Figure 53 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, 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). The upland tributaries of these rivers are akin to mountain torrents, becoming wandering gravel-bed rivers СКАЧАТЬ