Barnstaple. Report from the Geology Section

Some thoughts on the geology of Barnstaple

Barnstaple is a geological enigma, lying between the documented (though poorly understood) geology of Exmoor and the well understood (but indifferently documented) history of the Culm Basin. Both are sedimentary basins, with beds of sand, silt and mud accumulating under water. Again, as in much of South West England, the geology is one dimensional, being excellently exposed and hence well described along the coastline, but inland: cows and grass.

Lacking coastal outcrop (and with many cows), the Taw-Torridge is thus best described in terms of projections of the cliff sections to the north (Baggy Point) and to the south (Westward Ho!). Within this gap, we move from the Devonian Exmoor to the Carboniferous Culm, from sands to mudstones, from drained sandy to wet heavy clay soils, or quantitatively from 400 million years to 300 million years. (Geologists always move forward in time, describing the oldest rocks first.) And as a further complication, faults (fractures – imagine scientists whose explanations include explicit faults!), have removed some of the evidence.

Anyway, we need to set the scene (easily understood if you’ve been attending to Channel 4). Some 400 million years ago, Barnstaple sat on the edge of the northern continent plate (Laurussia), with the southern continent plate (Gondwana) rapidly approaching. Barnstaple was in deep water!

Laurussia to the north comprised the Old Red Sandstone continent, drained by massive rivers, the sand, silt and mud from which built out southwards from a coastline in the Bristol Channel to form the thick sediments of Exmoor. Barnstaple, still in deep, quiet water, accumulated the Pilton shales (360 million years) and then the Coddon Hill Cherts (340 million years).

But the Gondwana plate, with its Variscan mountains, was fast approaching from the south, its rivers piling sediment into the deep water basin. This southerly invasion, recorded by the muds of the Crackington and the sands of the Bude formations, filled the basin leading to shallowing water. Finally, the mountains arrived, crumpling (folds) and fracturing and stacking massive slices of rock (thrust faults) to place Barnstaple under about 8km (5 miles) of strata. It is this high pressure (>12,000 psi) and high temperature (~220°C) burial event which defines the strength and structure of Barnstaple’s bedrock. As with many mountains, the 8km of rock were rapidly removed by erosion (250 million years) to expose roughly the land surface we see today.

After this exciting start to its geological history, the next 250 million years lacked entertainment, with Barnstaple hanging around on its stable plate, largely oblivious of the surrounding dramas, such as the opening of the Atlantic (200–100 million years) to the west and the plate collisions leading to the Pyrenees and Alps (60–10 million years) to the south. A ripple of excitement was experienced when glaciation formed the Fremington clays (200,000 years). And, as they say, the rest is History!

Chris Cornford

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