Scotland. Peter Friend
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Название: Scotland
Автор: Peter Friend
Издательство: HarperCollins
Жанр: Природа и животные
isbn: 9780007465989
isbn:
In places, weathering has clearly been concentrated along lines of geological weakness, such as faults and softer sedimentary units. The remarkably linear valley of Moffat Water, for example, coincides with a large Caledonian fault and a thin band of softer Ordovician shale. Likewise, the major valley of Annandale appears to have been eroded in the softer Permian sedimentary rocks, whilst the thin band of Silurian rocks separating the Permian of Dumfries and Lochmaben is obviously more elevated. The main rivers in Area 2 are the Tweed, Annan, Esk, Ettrick, Yarrow and Teviot, and the sources of all but the last lie in the higher hills in the northwest of the Area. Many of these rivers flow along northeast/southwest-trending valleys, parallel to the underlying folded strata, the water having taken advantage of either faults or weaker bands within the succession. Another important valley is found in the southern part of the Area: Liddesdale runs approximately northeast to southwest, with Liddel Water defining the Scottish–English boundary for several kilometres before joining the Esk. The development of this valley, too, appears to have been influenced by the underlying geology, as its location coincides with an area of heavily faulted and soft Carboniferous sedimentary rocks.
FIG 61. Digital elevation model of Area 2 with the main river valleys shown, as well as main upland areas. Sub-areas discussed in later maps are indicated by rectangles with red borders.
Evidence that the Solway Firth area acted as a basin for sedimentation in Permian and Triassic times has been outlined above. It seems very likely that it was acting as a river valley during Tertiary times.
Landscape development during the Quaternary glacials
The final form of the landscapes in Area 2, as in the rest of Scotland, owes much to the action of ice and ice meltwater. During the last 2 million years, ice sheets have repeatedly expanded to cover much of Scotland. These ice sheets flowed radially outwards from centres in the Highlands, the Southern Uplands and the Lake District, and were powerful agents of erosion and deposition.
The most recent glacial stage reached its coldest in the Late Glacial Maximum about 20,000 years ago, at which time Area 2 was overrun by Southern Uplands ice flowing roughly eastwards across the Area from an ice divide located over the outer Solway Firth (Fig. 62). This flow is recorded by west/east-oriented drumlins along the low ground adjacent to the Solway Firth, and by a beautifully preserved array of ice-streamlined ridges along the Teviot valley. This flow direction must have been influenced by the presence of a strong ice dispersal centre in the Lake District that deflected the Southern Uplands ice to flow eastwards along the inner Solway Firth. Ice also accumulated in the Moffat Hills in the northwest of this Area, and a small but independent ice centre, powerful enough to withstand the pressure of the main ice mass, was present in the Cheviot Hills.
After the glacial maximum, the Lake District ice centre decreased in size and strength and Southern Uplands ice became dominant throughout the region. In the northeastern half of the Area, ice continued to flow roughly northeastwards down Teviotdale, but in the southwestern half, flow directions were reversed: drumlins and glacial striae indicate that ice flowed from the uplands around Moffat southwestwards down into the Solway Firth. As deglaciation continued, ice flow became valley-contained, and a final flow phase is recorded by north-northeast/south-southwest-oriented drumlins in the lowlands around the Solway Firth. Mapping of glacial deposits west of Annan and north of Gretna shows that ice then receded up the Solway Firth, as the Solway glacier retreated.
As mentioned above, the Cheviot Hills acted as a small but independent centre of ice accumulation and glacier dispersion during the Devensian glaciation. During this time, Cheviot ice flowed radially outwards from the centre, shielding the higher parts of the massif, particularly those above about 300 m, from the ice from the Solway Firth and Tweed basins. The lower peripheral hills, meanwhile, were overwhelmed and smoothed by this ice, and blocks of bedrock from the Tweed and Solway areas were deposited by the ice up to altitudes of around 300 m. Above this level in the central parts of the massif, soft, deeply rotten bedrock is relatively common and there are tors, erosional relics, over the border in England. These areas have clearly been glaciated, as they are often overlain by till, and yet they have survived erosion. Perhaps at the centre of Cheviot ice dispersal, the preservation favoured net deposition rather than erosion.
FIG 62. Ice flow in Area 2 at different times during the Devensian (LGM is Late Glacial Maximum).
The brief return to glacial conditions which occurred during the Loch Lomond Stadial (around 13,000 to 11,500 years ago; see Fig. 39) had only a limited effect in this Area. Glaciers returned only to the highest ground, such as around Broad Law, but even here the largest was only a few kilometres long. The position of these glaciers is clearly represented by terminal moraines – ridges of glacial till bulldozed by the Loch Lomond Stadial glaciers. Good examples are found, for example, at Loch Skene (Fig. 63), where a series of terminal moraines records fluctuations in glacier growth during retreat. The terminus of one ice lobe from this time has been located at the head of the Grey Mare’s Tail waterfall, at an altitude of around 450 m. Another terminal moraine forms a prominent ridge some 250 m long that runs parallel to the northeastern side of the loch (‘the Causey’). Indeed, Loch Skene owes its presence to these moraines, which effectively dam the loch outlet.
Glacial modification of the uplands
Glacial erosion has played an important role in creating the final shape of the landscape. Most of Area 2 was extensively ice-scoured throughout the course of the Quaternary glaciations, and the land surface present at the end of the Tertiary became heavily modified. In the uplands, moving ice lowered the valley bottoms and smoothed the mountains, creating the high broad hills seen today. This erosion was not as intense as that to which the western Southern Uplands were subjected, and whilst the main valleys were deepened by powerful ice streams, the intervening ridges and plateaus escaped deep scouring. In general, the uplands of this Area therefore lack the ‘Alpine’ form seen in the Highlands of Scotland: corries, arêtes and troughs are not as common nor, when present, as pronounced (Fig. 63
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