The folded and faulted greywacke sandstone beds forming the spectacular cliffs at Back Bay represent ocean floor sediments of the 600-400 million-year-old Iapetus Ocean
The rocks at Back Bay formed some 435 million years ago during the Silurian period. They accumulated as sands and muds in massive channels on the floor of a long-lost ocean called Iapetus (named after the Greek Titan god). The sands and muds became buried by more sediments and after millions of years they solidified into greywacke – a hard grey sandstone made up of minerals and rock fragments – and siltstone beds. On the east side of Back Bay these rocks can be easily viewed in a spectacular cliff at the sandy beach. The beach is strewn with colourful boulders of sedimentary rocks including examples of greywacke and igneous rocks, such as granite erratics brought by glaciers from the Galloway Hills. The best time to view the cliff is at mid-to-low tide.
These ancient oceanic rocks form much of the Southern Uplands of Scotland and parts of Northern Ireland. Rocks of the same age and origin lie on the other side of the Atlantic Ocean in Newfoundland. The North Atlantic is a relatively new ocean which started opening up and splitting the continents of North America and Greenland from Northwest Europe some 200 million years ago.
Evidence for the closure of the Iapetus Ocean can be seen at Back Bay. The greywacke beds, up to a metre thick, along with thinner, finer-grained siltstone beds, represent the sediments derived from a geologically ancient continent which lay to the north of the Iapetus Ocean called ‘Laurentia’. Today, the remnants of this continent make up the ancient geological core of North America together with Greenland and North-West Scotland.
As Laurentia moved towards another continent to the south, ‘Avalonia’ (fragments of which are found in the Lake District, Wales, and Nova Scotia), the Iapetus began to shrink in size. It’s a curious coincidence that the oceanic rocks from opposite sides of Iapetus Ocean have been brought together at a position almost coincident with the political border between Scotland and England!
As the continents moved towards each other the ocean floor plate was consumed or subducted (forced to sink under gravity) under the leading edge of the Laurentian continental plate. During this process, slices of sedimentary rock, thousands of metres thick, were stripped off the descending plate and stacked up, each new slice pushed beneath its predecessor and sequentially accreted.
This stacking process continued until the final stages of ocean closure some 420 million years ago, and resulted in the succession (many kilometres thick) of folded and faulted sedimentary rocks that now form the Southern Uplands. The style of deformation of the rocks varied with time and the cliffs at Back Bay show examples of different types of fold, alongside evidence for more than one phase of folding – in other words, the rocks have been folded once and then refolded.
Text contributed by Andrew McMillan
Find out more
Directions: Take a walk across the sandy beach from the car park beyond St Medan Golf Course, one kilometre south of Monreith. Just off the single-track road to the car park is a bronze otter memorial to the author Gavin Maxwell.
Stone, P. 1996 Geology in south-west Scotland (Keyworth, Nottingham: British Geological Survey), 105-113 (Excursion 13 Whithorn: turbidite sequences and deformation in the Hawick Group by R P Barnes).
McMillan, A and Stone, P. 2008. Southwest Scotland – a landscape fashioned by geology (Perth: Scottish Natural Heritage)
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