A Silurian Inlier in the Eastern Mendips

Abstract

I. I ntroduction . T he Mendip Hills consist of four periclinal upfolds of Carboniferous Limestone, arranged en échelon from north-west to south-east. Each perieline includes a core of Old Red Sandstone, and the south-eastern or Beacon-Hill pericline further shows a series of igneous rocks. It is with these that the present paper deals. The existence of igneous rocks in the Eastern Mendips was first noted by Charles Moore, who described them as ‘a basaltic dyke of considerable thickness emerging from beneath the Old Red Sandstone at East End near Stoke Lane.’ He considered that, from the general physical character of the Mendips, it was not improbable that the dyke might be co-extensive with their range. He not only attributed the upheaval of the whole Mendip range to the intrusion of this igneous mass, but also considered that it was responsible for the remarkable inverted character of the Carboniferous beds at Luckington, where the Coal-Measures are worked under the Carboniferous Limestone. John Morris refers to the rock at Stoke Lane, as ‘a dyke of considerable thickness, emerging from beneath the Old Red Sandstone, occurring as bosses in the field, but, traced for some distance over the district, it is conglomeratic in places, and pronounced by Mr. D. Forbes to be dolerite.’ The igneous rocks are not shown in Sanders's map of the Bristol Coalfield (published in 1864), but appear in tile map of the Geological Survey (1884) as a series of isolated patches extending from Downhead on the east, to Beacon Plantation, southwest