Brief geologic history of Hartland Quay, Devon
I wanted to talk about the chevron folds at Hartland Quay in north Devon. They’re some of the best known in the world.
The beds themselves were formed as part of the Crackington Formation during the mid to late Carboniferous (~325 Mya) and exhibit alternating sandstone and siltstone layers, typical of turbidite sequences, formed deep underwater when vast amounts of arenaceous sediment are suddenly moved downslope underwater, covering up the silt beds below.
The feature is a peneplain, or the level remains of a mountain formed over millions of years of erosion. It was formed during an event known as the Variscan Orogeny between the late Devonian and early Permian, which caused the formation of Pangaea during the collision of Laurasia and Gondwana. It closed the Rheic Ocean, where the ocean basin was folded into the pattern seen in the images below and formed a mountain belt known as the Variscan Orogen which extended roughly from the Severn Estuary in the west to mid Poland in the east.
As the continents continued to collide, existing crust thickened and therefore shortened, causing a tectonic basin — the Culm Basin — to form due to faulting. Rivers flowed into this basin, depositing sediment which eventually lithified into shales. Occasional earthquakes caused by the ongoing orogeny caused the downslope movement of coarser sediment (as mentioned above). The Culm Basin too eventually closed due to the continental collision, causing the beds in it to buckle and fold into the dramatic chevrons visible today.
References
Davison, I., Jeffcoate, A. and Oing, H. (2004) ‘Geometry of chevron folding shortening and estimates at Hartland Quay, North Cornwall, UK, and some regional implications for Culm Basin development’. Geoscience in south-west England. Vol. 11, pp. 42–50. Available here. (Accessed 31 July 2022)
Fichter, L. (9 January 2013) ‘What is the Wilson cycle?’ Geology Page. Available here. (Accessed 31 July 2022)
Schulmann, K., Martínez Catalán J.R., Lardeaux, J.M., Janoušek, V. and Oggiano, G. (29 April 2014) ‘The Variscan orogeny: extent, timescale and the formation of the European crust’. Geological Society. Vol. 405, pp. 1–6. Available here. (Accessed 31 July 2022)
The Geological Society (unknown date). ‘Hartland Quay, Devon’ The Geological Society. Available here. (Accessed 31 July 2022)
The Geological Society (unknown date). ‘Variscan Orogeny’ The Geological Society. Available here. (Accessed 31 July 2022)
Wikipedia. “Extensional tectonics”. Last modified 13 May 2022. Available here. (Accessed 31 July 2022).
Work initially published on 31 July 2022 by Asriel Wilde, at their blog Asriel Talks on Tumblr.
