The well-exposed sedimentary rocks on the present-day Solway coast tell the story of the ancient rivers and warm shallow seas which were present in this area some 340 million years ago
With the backdrop of Criffel (a rounded hill formed of 397-million-year-old granite), the attractive low coastal plain south of Kirkbean is underlain by rocks which formed some 340 million years ago during the early Carboniferous period. The sedimentary rocks (mainly sandstones, mudstones and limestones) are well exposed on the Southerness shore and between Powillimount and Borron Point.
The layers of Carboniferous rock form part a major succession of rocks along the northern margin of the Solway-Northumberland Basin. The basin was a subsiding trough, bounded by contemporary active faults, in which sediments mainly from rivers and seas accumulated (the basin is similar in development to the Midland Valley of Scotland). The Scottish part of the basin extends through eastern Dumfries-shire and parts of the Borders to the Berwickshire coast. The westernmost outcrops of conglomerates, yellow sandstones and grey mudstones can be visited at Castlehill Point, Rockcliffe where they are seen in faulted contact with the much older, tough, grey greywacke sandstones (together with reddened igneous rocks) of the Southern Uplands along the trace of the North Solway Fault.
At Southerness you can find sandstones (some with plant fossils), limestones and mudstones which were laid down in close to a seashore. At times, the sea would infiltrate this low-lying area of land creating a shallow water environment. Some of the mudstone beds are filled with fossilised shells and corals and there are also the remnants of at least three algal beds.
Close to the car park at the Powillimount shore, sandstones and thin limestones are folded in an arch-like and trough-like shape – an anticline and syncline structure – that dips gently towards the north-east. To the north you can find the Thirlstane Sandstone, in places up to 15 m thick. The sandstone layers become increasingly contorted as you walk towards the notable Thirlstane Arch and beyond. These sands, which may have formed in either a river channel or offshore under the sea, were disturbed before they solidified, most likely by contemporary earthquakes.
Continuing northwards along the shore beyond the Thirlstane Sandstone and across a faultline, you can find shallow marine limestones, mudstones and calcareous sandstones. On the foreshore is a huge erratic boulder of silver-grey granite from Criffel, brought down to the shore and dumped by a glacier. It is known locally as the ‘Devil Stone’ – tradition has it that the devil bit off this chunk of the hill and spat it out!
North of Arbigland Bay there are many fossil beds of limestone and mudstone full of brachiopods, gastropods (shells) and corals including spectacular colonies of Lithostrotion clavaticum. Some of the sandstones exhibit a range of trace fossils – the remnants of animals’ behaviour such as preserved burrows – indicating repeated reworking of the sediments by sediment feeders. There are excellent examples of sedimentary structures including cross-bedding and ripples.
Diversions from the geology are many and include Arbigland Gardens and Paul Jones’ Cottage (both open during the summer) where John Paul Jones (1747-92), founder of the American Navy, was brought up. At New Abbey, south of Dumfries lies the late 13th century Sweetheart Abbey built of local red sandstone from across the mouth of the River Nith. The abbey, which is managed by Historic Scotland, was founded by Lady Devorgilla of Galloway, in memory of her husband Lord John Balliol.
Text contributed by Andrew McMillan
Find out more
There is easy access onto the shore close to the old now disused lighthouse (constructed in 1749 it is the second oldest in Scotland) at the village of Southerness and from a car park at Powillimount, both localities being approached from roads off the scenic coastal road between Dumfries and Dalbeattie (A710). The shore sections are best seen and accessed at mid to low tide. On a clear day there are fine views across the Solway to the Lake District mountains.
Stone, P. 1996 Geology in south-west Scotland (Keyworth, Nottingham: British Geological Survey), 88-94 (Excursion 10 Southerness to Borron Point: Lower Carboniferous of the Solway Basin by A A McMillan).
Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, Phillips, E R. 2012. British Regional Geology: South of Scotland (Fourth Edition). (Keyworth, Nottingham: British Geological Survey.) . Especially chapter 6 Carboniferous pp. 111-154.
General geological description of the region: McMillan, A and Stone, P. 2008. Southwest Scotland – a landscape fashioned by geology (Perth: Scottish Natural Heritage)
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