Published online by Cambridge University Press: 01 May 2009
The Upper Palaeozoic rocks at Wickwar pass eastwards below Mesozoic strata, but about half a mile farther to the east the pre-Triassic surface is laid bare in places in the floor of the Little Avon River for a distance of 1,300 yards between Stmt Bridge and a point some 360 yards below Horse Bridge. The rocks found here are of Silurian age. Exposures are small and discontinuous since the stream has covered the valley floor with its own deposits and only at intervals does it flow on, or cut laterally into the solid rock. Moreover, the base of the Trias, which is nowhere many feet above the thalweg, sinks twice below stream level, dividing the Silurian outcrop into three inliers. The first two, conveniently styled the northern and southern Sturt Bridge Inliers, are separated by only 40 yards of Triassic deposits, but between the more northerly of these and the Horse Bridge Inlier 330 yards of Triassic rocks intervene.