Quaternary of the Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The present landscape of southern Scotland owes its origins to tectonic and lithological controls on drainage. The Quaternary Period spans the last 2.6 Ma and is subdivided into an earlier epoch, the Pleistocene, during which there were repeated alternations of glaciation and warm phases, and the later, postglacial Holocene Epoch. The long-term evolution of the landscape of the Southern Uplands prior to the last glaciation is not well known. However, the presence at outcrop of igneous intrusions of Paleocene age testifies to a thicker sequence of older rocks at the time of intrusion than is preserved at present. In addition to the remnants of the Carboniferous succession, which originally would have been far more extensive geographically, it is likely that the region was also the site of Cretaceous sedimentation. None of the Upper Cretaceous (Chalk) strata that were probably once present now survive, and it seems likely that from the Neogene onwards the Southern Uplands region has remained an area of relatively high ground undergoing rapid erosion. As the modern drainage system has developed, it has cut down through the postulated Chalk cover and the Upper Palaeozoic strata into the Lower Palaeozoic ‘basement’, locally exhuming valleys that originally formed during Late Palaeozoic times. Preferential erosion cutting back these pre-existing features appears to have resulted in capture of the headwaters of several of the major rivers, with accompanying adjustment of watersheds.
Many of today’s rivers such as the Cree, Dee, Nith and Annan flow nearly normal to the marked north-east to south-west structural ‘grain’ of the Southern Uplands’ Lower Palaeozoic rocks (P912310). The river valleys owe their origin, at least in part, to the presence of the north and north-westerly aligned, late Silurian faults that were reactivated to determine the geometry of the small half-grabens infilled with Upper Palaeozoic, and perhaps originally some Mesozoic strata. Of the main rivers in the Southern Uplands, the Nith has a composite history comprising a mature river valley in its lower reaches south of Thornhill through the Permian basins of Thornhill and Dumfries, a rock-cut gorge in its middle section north of Carronbridge, but a relatively wide alluvial valley farther upstream to the north, through the Sanquhar Coalfield and southern Midland Valley. Its tributary, the Carron Water, cuts through the soft Permian sandstones of the northern part of the Thornhill Basin, but owes its origins to capture of headwaters at the head of the spectacular ‘V’ shaped Dalveen Pass. To the east, the Potrail Water is a small misfit stream that flows through a mature valley of low gradient to join the River Clyde in its upper reaches. The latter rises as one of two streams known as the Clydes Burn at Beattock Summit, just east of the M74, whilst another adjacent Clydes Burn has been captured as a headstream of the southward flowing Evan Water. At Biggar, the River Clyde flows north out of the Southern Uplands but during ice-sheet deglaciation it formed a major tributary, via Broughton, of the easterly flowing River Tweed. The Tweed rises to the north of the Devil’s Beef Tub, a deeply eroded south-facing bowl lying 8 km north of Moffat. The Beef Tub forms the source of the River Annan which flows southwards through the partly fault-bounded Permian Moffat Basin, to be joined south of Moffat by the Evan Water and the Moffat Water. The latter drains the steep-sided Moffatdale valley, defined by a major north-east to south-west, Caledonian fault. Modification of the drainage system occurred during deglaciation, with many rivers following routes originally carved out by glacial meltwater. Some rivers were deflected from their preglacial courses, which are now concealed valleys filled with glacigenic material.
Across the summits and higher slopes of the rounded hills of the Southern Uplands, typified by the Moffat Hills, the underlying sandstone and mudstone generally have only a thin cover of superficial deposits. Similarly, the Galloway Mountains, with their granite and granodiorite plutonic centres surrounded by the aureoles of resistant hornfelsed Lower Palaeozoic rocks that form Merrick (P774191) and the Rhinns of Kells, have only a veneer of superficial material. Locally the granite surfaces are exposed as smooth, glaciated rock pavements on which are perched glacial ‘erratic’ granite boulders (P774193). In contrast, in the south-eastern part of the region, the Cheviot Hills are mostly covered with a relatively thick blanket of paraglacial and residual deposits. As described in Chapter 8, the andesitic volcanic rocks of the Cheviot Hills have been pervasively and deeply weathered, probably during prolonged subaerial exposure under the warm, humid conditions of the Cenozoic era and/or one or more of the Pleistocene Interglacial stages.
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