Midland Valley of Scotland

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Outline of the geology

The region has the structure of an ancient rift valley with the parallel Highland Boundary and Southern Upland faults forming the limits to the area. The downfaulted strip between the two faults is floored by rocks mainly of Devonian and Carboniferous age. Comparatively small inliers of Lower Palaeozoic rocks occur on the south side of the region and rocks of Permian age occur in central Ayrshire. Sedimentary rocks of Devonian and Carboniferous age underlie about 36 and 38 per cent respectively of the area of the Midland Valley. Igneous rocks, mainly of Devonian and Carboniferous age form about 21 per cent of the area.

The Pentland Hills looking North from Scald Law, a Silurian inlier. Photo by Bob McIntosh

The oldest exposed rocks are the Ordovician and Silurian sandstones,mudstones and conglomerates which occur as inliers in the Lesmahagow area, the Pentland Hills and in south Ayrshire. An upward passage from marine strata to terrestrial fluviatile rocks in the Silurian is followed by the semi-arid fluviatile clastic sediments of the Lower Devonian.

Great thicknesses of red and grey sandstones and conglomerates with contemporaneous piles of lava were deposited during the Lower Devonian, particularly in the north-eastern part of the area. These rocks are well exposed on the coast from the Tay estuary north to Stonehaven, and the lavas form the Ochil and Sidlaw Hills.

Following deposition of the Lower Devonian rocks there was a period of uplift, folding, faulting and erosion. Important fault movements occurred on the boundary faults in Middle Devonian times and probably during the Lower Devonian. No sediments of Middle Devonian age are known in the Midland Valley.

Red sandstones and siltstones of Upper Devonian age were laid down on a peneplaned surface in the eastern and southern parts of the area, but in the west coarser sediments, the result of more vigorous erosion in an area of greater relief, accumulated in considerable thicknesses. Prolonged periods of caliche-type soil formation resulted in the development of corns tones particularly in the upper part of the division.

In the Carboniferous Period there was a climatic change to humid equatorial conditions and a large fluvio-deltaic complex of continental dimensions developed in which large quantities of sand and mud were deposited at or near sea level. Occasional flooding by the sea caused deposition of thin limestones and calcareous mudstones, and luxuriant forest growth on emergent delta-top surfaces ultimately became coal seams. The Highlands and Southern Uplands remained, in part at least, above the level of deposition. Igneous activity occurred more or less throughout the period at one locality or another and large quantities of basalt lava were extruded, particularly in the lower part of the succession.

Differential movement during the Carboniferous on fractures in the basement caused notable variation in thickness within the region, probably exerted control on the location of the igneous rocks and accounts for local unconformities and non-sequences.

Coals, ironstones, limestones and oil-shales, which have been extensively worked, formed the basis of the industrialisation of the Midland Valley during the nineteenth and first part of the twentieth centuries.

In central Ayrshire, Permian strata overlie the Carboniferous rocks and indicate that the climate had reverted to arid conditions. The strata consist of basalt lavas with intercalated sediments overlain by red wind-deposited sandstones. The dry climate of the Permian is believed to be responsible for the deep oxidation and reddening of the upper part of the Carboniferous and there is an hiatus in the sedimentary sequence between the Carboniferous and the Permian.

The Permian sandstones are the youngest sedimentary rocks present in the Midland Valley, but Triassic strata occur in Arran and floor much of the Firth of Clyde between Arran and Northern Ireland. Fragments of Lias and Cretaceous strata within the central complex in Arran and the Lias and Cretaceous outcrops in Northern Ireland suggest that Mesozoic rocks may at one time have encroached upon parts of the Midland Valley. Triassic and Cretaceous rocks also occur in the Forth Approaches.

Dykes associated with the Tertiary volcanic centres of Mull and Arran are intruded into the strata of the Midland Valley and are the most recent solid rocks in the area. Erosion during the Tertiary and glaciation during the Quaternary combined to create the present landscape.