Quaternary lithostratigraphy 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 British onshore glacigenic deposits of Quaternary age are subdivided into the Albion Glacigenic Group (deposits of the Anglian Glaciation) and the Caledonia Glacigenic Group (deposits of the Devensian Glaciations). Although the south of Scotland was glaciated during the Anglian Stage there are no known deposits in this district, and any evidence of age from sections exposing two or more tills is equivocal (see below). The Caledonia Glacigenic Group embraces all tills, gravels, sands, silts and clays that form surface deposits within the limits of the Devensian ice-sheet. These deposits commonly have distinct morphological expression and are equivalent to the ‘Newer Drift’ of previous classifications. Subgroups within the Caledonia Glacigenic Group (P912369; P912389) are based on the provenance of the constituent till, which strongly reflects the composition of both the underlying bedrock, and that of the particular suite of rocks that the ice crossed before the subgroup till was deposited. The composition of morainic, glaciofluvial and glaciolacustrine deposits is similarly influenced, though to a lesser extent.
Tills of the Southern Uplands Glacigenic Subgroup are typically yellowish or greyish brown diamictons with clasts derived mostly from the Southern Uplands mountains: lithologies present include ubiquitous Lower Palaeozoic clastic lithologies, whilst tills derived partially from the Galloway plutons contain varying proportions of granite and granodiorite.
The Irish Sea Coast Glacigenic Subgroup includes tills, glaciofluvial and glaciolacustrine deposits derived in part from the Permo-Triassic basins of southern Scotland and the Vale of Eden together with lithologies from the floor of the Solway Firth and Irish Sea (and locally in south-west Scotland from the Firth of Clyde). These deposits occur typically along the coastal fringes of the Solway Firth and around the Rhins of Galloway. The tills are typically vivid reddish brown or grey, containing clasts of red and yellow Permian sandstone and dark grey Lower Palaeozoic wacke-type sandstone. Along coastal regions of the inner Solway Firth, the clast assemblage also includes varying proportions of Upper Palaeozoic sandstone, siltstone and limestone, and granite and granodiorite from the Galloway plutons. Grey tills containing marine shell fragments are present on the western side of the Rhins of Galloway and around Loch Ryan. This material is associated with distinctive erratics of arfvedsonite-microgranite derived from Ailsa Craig, and so probably originated from the floor of the Firth of Clyde.
The Borders Glacigenic Subgroup includes a suite of brownish grey to reddish brown till and glaciofluvial deposits that contain clasts derived predominantly from the Devonian to Carboniferous rocks of Berwickshire (yellow, pink, grey and white sandstones, mudstone and limestone, basalt), and the Lower Palaeozoic and Devonian rocks of southern Scotland and the Cheviot Hills (wacke sandstone and mudstone, granite, andesite, red sandstone) The subgroup was deposited by ice sourced mainly in the Scottish Borders that flowed eastwards through the Merse of Berwickshire.
The Cheviot Glacigenic Subgroup comprises sandy diamictons, sand, gravel, silt and clay distributed over the Cheviot massif. The deposits contain clasts derived predominantly from volcanic (basalt, andesite) and intrusive (granite) rocks of the Cheviot Hills. Varying proportions of Lower Palaeozoic wacke sandstone and siltstone are also present.
Non-glacigenic deposits are classified under two groups (P912389). The British Coastal Deposits Group includes all raised marine, marine and coastal deposits. Most of these are currently classified as morpho-lithogenetic units and are unnamed, but the Loch Lomond Stadial to Holocene, Carse Clay deposits of the Inner Solway Firth area are assigned to the Newbie Silt Member of the Carse Clay Formation (defined from the estuaries of the rivers Forth and Tay).
The Britannia Catchments Group includes all fluvial and mass movement silt, sand and gravel, together with organic peat. Stratigraphically important late glacial to Holocene organic beds of the Solway district are referred to the Blelham Peat Formation, originally defined in west Cumbria. The south of Scotland spans three catchments (P912310) and the fluvial deposits (alluvial and river terrace silts, sands and gravels) are accordingly assigned to three catchment subgroups. The Solway Catchments Subgroup includes the Solway Esk Valley Formation together with fluvial deposits of the valleys of the rivers Cree, Fleet, Dee, Nith and Annan. The Tweed Valley Formation of the Tweed Catchments Subgroup includes the deposits of the River Tweed and its tributaries. The Clyde Catchments Subgroup includes deposits of the Clyde Valley Formation and of other, smaller river valleys draining northward from the Southern Uplands.
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