Three Tectonic Problems of the Lulworth District: Studies on the Middle Limb of the Purbeck Fold

Abstract

Of all the folds thrown up across the Chalk plains of North-West Europe during the Alpine orogeny, the most interesting and the most perfectly exposed is the Purbeck anticline. In the cliffs of the Dorset coast the sea has laid bare all parts of the structure, from the core or crestal region, through the vertical and overturned middle limb and the “foresyncline” (Busk 1929, p. 29), into the foreland, where the Upper Cretaceous rocks are seen reposing with sharp unconformity upon a previously folded and eroded Jurassic foundation. The general and some special aspects of the fold and its associated faults have been discussed in a previous paper (Arkell 1936). The present paper results from a study of the middle limb, where it is dissected by the sea cliffs of the Lulworth district. These cliffs have long been renowned for their beauty, but their marvellous tectonic features, both great and small, have been neglected, with the result that their value to students of tectonics has not been fully utilized. The first three parts of the paper attempt a solution of the three most outstanding problems : contortions in the Purbeck Beds, the Purbeck Broken Beds, and the abnormal attenuation of the strata towards Durdle Door. The fourth part offers an interpretative essay and synthesis. The area dealt with is included in sheet 342 of the Geological Survey one-inch map and sheet 141 of the Ordnance Survey one-inch map. The contortions in the Purbeck Beds at Stair Hole are illustrated in