Southern Uplands

Between the England-Scotland border to the south and the Southern Upland Fault to the north, lie the Southern Uplands. The area is a discrete tectonic block with a distinct geological history that spans almost 500 million years.
Like the rocks of Northern England, there are rocks here that tell the tale of the convergence and collision of the ancient Laurentian, Avalonian, Baltican, Armorican and Gondwana tectonic plates and the compressive, transtentional and extensional forces that resulted.

Bedrock in the Southern Uplands

Map of the Southern Uplands geology

Map based on the much more detailed online British Geological Survey Map

Key

Key to bedrock in the Southern Uplands

The oldest rocks in the Southern Uplands are those of the Crawford group that were scraped off the bed of the diminishing Iapetus Ocean and laid down here on the Laurentian plate in the Lower Ordovician period (about 480 million years ago). Currently, there are two recognised formations: the Raven Gill formation and the Kirkton formation. The first is a succession of basaltic pillow lavas (chemically proven to have erupted at different times from both mid-ocean ridge and island-arc zones), blue-grey cherts and brown mudstone. The second is a sequence of red and grey cherts that contain broken pieces of pillow lava, and red and green mudstones with chert nodules. The two formations outcrop in the Abington area.
Next in line came the Moffat shale group deposited here in the mid-Ordovician to mid-Silurian periods (460 to 430 million years ago). These shales had been originally laid down as sediment over the great expanse of the Iapetus Ocean. In the 1870s, Charles Lapworth studied the fossilised remains of graptolites, long-extinct marine fauna of the Ordovician and Silurian times, in these shales in Moffatdale and in the Girvan district. His observation of their systematic development over time not only enabled him to establish a zonal scheme for the deposition of the shales throughout the region, but also a system of biostratigraphy that continues to have international application today.

By far the most common rock types in the region are the greywacke, siltstone and shale sequences that were also beginning to be laid down in the mid-Ordovician during the continued subduction of the oceanic plate under Laurentia.
Over the millions of years taken for the closure of the Iapetus Ocean, vast quantities of marine sediments, together with the sand, mud and small rock fragments that cascaded in turbidity currents down the steep slopes of Laurentia’s continental shelf onto the ocean bed, were sliced off from the subducting plate and thrust onto the Laurentian plate beneath previously stripped-off slices.

Sandbox modelling of the formation of an accretionary prism in a subduction zone
The steep monoclinal structures and ‘faults’ produced in the later stages this model are closer to the current models of the Southern Uplands Accretionary Prism.

The resulting mass of sediments, known as the Southern Uplands Accretionary Complex, is regarded by many to be the best-preserved ancient accretionary complex in the world. It occupies some 10,000 km2 of Southern Scotland as well as about 6,000 km2 Ireland.
The striking bands seen on geological maps of the region graphically display the division of the terrain into discrete units, each bounded by north-east to south-west running, parallel-strike faults.
Emplacement in the accretionary prism, faulting, and rotation have resulted in units overall younging from northwest to southeast. However, within each unit, the younging of deposits is reversed – the earlier beds to be deposited being to the north and the later to the southeast.

Model of section through the Southern Uplands Accretionary Complex

Formation of the Southern Uplands Accretionary Complex
This model after Stone & Merriman (see references below) explains the two younging patterns observed in the terrain. The subjecting oceanic plate acts like a conveyor belt that transports sediments to the toe of the prism. Sediments are forced wedge -like under the system pushing older units up and rotating them towards the vertical. Most, if not all of the complex would have been underwater until the later stages of its formation and material would have been stripped from its steep surfaces and transported and deposited at its base by turbidity currents, and from there, recycled back into the prism.

Following further rotation and uplift by compressive forces, a swarm of calc-alkaline felsic and lamprophyre dykes then cut the accretionary complex in advance of the emplacement, early in the Devonian period (c420-390 Ma), of large granitic plutons at Criffel, Cairnsmoor of Fleet and Loch Doon. These plutons are related to the granitic plutons of North-Western England and also to the slightly later Cheviot pluton to the east – but with interesting and not fully explainable, differences.
The section on Igneous rocks in the Southern Uplands and Ballantrae Area begins to explore this more fully.

By the end of the Devonian and into the Carboniferous period (359 – 299 Ma), tectonic movements had placed what was to become the British Isles within a supercontinent and lying between 30o and 20o south in the mid-latitude low rainfall belt. The Scottish Uplands was, therefore, wholly terrestrial and situated in arid conditions that led to the erosion of higher ground and the deposition of red sand and other sediments into transtentionally created basins. These deposits were transported by wind and huge river systems to form the Old Red Sandstone deposits which currently overlie the greywackes in some localities of the eastern Scottish Uplands.

in the Late Carboniferous, extensional and transtentional tectonic forces thinned the crust and stimulated renewed igneous activity across the region. In the south east, adjacent to the northern limit of the Cheviot andesite lava flows, a cluster of volcanoes erupted alkali lavas over the sandstones, and associated sills and dykes were intruded into the sandstones and the greywackes. In the same period, further west, the Birrenswark Formations olivine-rich basalts were erupted .

View across the Scottish Borders to the Cheviot Hills

The Scottish Borders with the Cheviot Hills in the distance
The view from Peniel Heugh, one of the mafic intrusions of the Lower Carboniferous period.

The Pangaean supercontinent continued to move northwards carrying the region into and beyond equatorial latitudes where wet tropical conditions supported tropical rainforest. Land subsidence produced shallow lagoons. The tropical conditions supported lush Carboniferous plant growth that led to the deposition of coal deposits and warm shallow waters gave rise to marine life and so to limestones along with siltstones and mudstones.

As the Late Carboniferous transitioned into the Permian period (299 – 252 Ma), effects of the final tectonic movements that had brought about the Pangaean supercontinent’s completion became evident in the region. Immense compressive forces were building mountains in South-western England and Wales building and permitting plutonic emplacement. Here in the Southern Uplands, Variscan compression had less effect but nevertheless was responsible for some faulting, folding and uplift in the Carboniferous sedimentary strata. The material that was eroded in this process became the fluvial and aeolian Permian sandstones and conglomerates we can see today in the Thornhill area.

Early in the Jurassic period (201 -145 Ma), much of the Southern Uplands was submerged and marine sediments must have been widespread. However, uplift in the mid-Jurassic and early Cretaceous period (145 – 66Ma) promoted erosion that created a widespread unconformity across the region before rising sea levels led to renewed submersion and the deposition of sediments in the late Cretaceous.

At the outset of the Palaeogene period (66 – 23 Ma), the great continent of Pangaea started to rift apart to form the Atlantic Ocean. It is now believed that this may have been triggered by a strong mantle plume or hot spot. From 60 Ma to 55 Ma, what is now the Western seaboard of Scotland from Arran to Skye, and the North of Ireland became a centre of intense volcanic activity erupting vast amounts of basaltic magma and intruding plutons, sills and dykes far and wide into surrounding rocks. The Mull volcano in particular drove a number of these tholeiite basalt dykes across the region from north-west to south-east into Northern England – the Cleveland and Acklington dykes being the most prominent.

Palaeogene dyke exposed with elements of the obducted Ordovicisan Ballantrae Complex (sepantinised ultramafic oceanic mantle along with the remains of island arc and ocean crust)

Palaeogene dyke exposed with elements of the obducted Ordovicisan Ballantrae Complex
The major elements of the complex are serpentinised ultramafic oceanic mantle andthe remains of island arc with ocean crust.

Continued uplift into the Neogene period (23 – 2.6Ma) promoted more and deeper erosion so that up to 2km of material was removed in places. Most of the Jurassic and Cretaceous sequences disappeared as did some of the Carboniferous and Devonian rocks revealing the Palaeozoic basement rocks.

The cyclic deposition and melting of ice in the Quaternary, from 2.6Ma until 12,000 years ago led to intense erosion and redistribution of the rock material. At one stage ice covered the whole of Scotland with the main accumulations in the Southern Uplands being north of Glentrool and in the Tweedsmuir Hills and Moffat Hills. Glacial valleys can be seen at Glentrool, Loch Dee and the Grey Mare’s Tail. Later, as the climate warmed, glacial meltwaters deposited expansive sand and gravel sheets in the valleys that were reworked by modern rivers into alluvial flood plains.

References

MacAdam A.D., Clarkson E.N.K., Stone, P. (editors) 1993 .Scottish Borders geology: an excursion guide. Edinburgh, Scottish Academic Press.

Stone, P. (editor) 1996.Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.

Stone, P, McMillan, A. A., Floyd, J D, Barnes, R.P, and Phillips, E. R.British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey, 2012

Stone, P. 2014 The Southern Uplands Terrane in Scotland : a notional controversy revisited. Scottish Journal of Geology,
http://nora.nerc.ac.uk/id/eprint/508842

Hubbard, J. 2016 Compressional Sandbox Experiments. https://www.youtube.com/channel/UCH_a8W1ptiIDU7ZsiA9oJSA

BGS online Geology of Britain http://mapapps.bgs.ac.uk/geologyofbritain/home.html

No vestige of a beginning, – no prospect of an end

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