The Dartmouth Beds of Bigbury Bay, south Devon

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Abstract
The oldest datable Devonian rocks of south Devon are the Dartmouth Beds. They have been examined in the coastal sections between Plymouth and Bigbury Bay and in Slapton (=Start) Bay, where the following successive units are proposed: Warren Sandstones, Yealm Formation, Scobbiscombe Sandstones, and Wembury Siltstones. The Warren Sandstones are similar to the Dittonian rocks of Wales and the Welsh borderlands. The Yealm Formation contains considerable amounts of pyroclastic rocks. The succeeding Scobbiscombe Sandstones occur only at Beacon Point and along the strike inland, and are regarded as a local fluvial grit deposit. Rapidly alternating sandstones and siltstones dominate the Wembury Siltstones succession. Plant remains, vertebrates, and molluscs occur, abundantly in places, but are generally poorly preserved. Most common are pteraspid fragments ( Pteraspis cornubica ), and it seems that at least two species are present. A correlation with the Dittonian and Breconian stages is highly probable. The rocks constitute a continental depositional initial phase in this part of the Cornubian section of the geosyncline. No base is seen, but the phase may be analogous to similar red beds in the Lower Devonian of western Europe, notably Germany, where they precede the mass of marine geosynclinal sediments. The material in south Devon may have come from either the north or the south but there is no conclusive evidence of the southern land-mass postulated by some authors. During the Variscan earth–movements the rocks were compressed, cleaved, and folded to give a shallow anticlinorial structure. Strike-faulting has been detected only on a minor scale but larger displacements along faults striking nnw–sse may be present. The early impression that the Dartmouth Beds comprise many thousands of feet of sediments is erroneous. No evidence is produced to show that the Dartmouth Beds and the Meadfoot Beds are at least in part coeval, but the Geological Survey’s (i.e., W. A. E. Ussher’s) basic succession and outcrop-pattern are confirmed.

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