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Department of Geology, The University, Leicester LE1 7RH, UK
In the Wigmore Rolls area of the central Welsh Borderland, westward-directed mass movements (sliding and slumping) on the Silurian Welsh basin slope removed at least 275 m of the Wenlock and Ludlow (Homerian to upper Ludfordian) succession of mudstones and calcilutites. The resulting slide scar was more than 2.5 km wide. Lower Leintwardine Formation (lower Ludfordian) flaggy siltstones rest with marked unconformity in this erosional hollow. Continuing instability is shown by a basal coarse boulder bed up to 10 m thick, channellized in places, that resulted from a debris flow coming to rest downslope, and by widespread slumping during the accumulation of the Lower Leintwardine Formation siltstones. Instability at this time outside the Wigmore Rolls area is confirmed by the formation of submarine canyon heads, channelling and hardgrounds. Comparable mass movements, leaving slide scars and channels, on modern basin slopes are becoming better known, but ancient examples such as those described here are rare. The slide scar and channels were filled by the end of Early Leintwardine times, and sandier siltstones of the Upper Leintwardine, Lower Whitcliffe and Upper Whitcliffe formations prograded uniformly over the whole area in shallower water. During the Acadian orogeny, the area was folded and faulted into a southwesterly continuation of the Ludlow anticline.
