The geology of the Corallian ridge (Upper Jurassic) between Gilling East and North Grimston, Howardian Hills, North Yorkshire

Abstract

SUMMARY The northern edge of the Howardian Hills, running for 25 km from Gilling East to North Grimston, North Yorkshire, comprises a near-continuous ridge of Oxfordian limestones and calcareous sandstones, here termed the Corallian ridge. These strata rise gently from the Vale of Pickering up to a steep scarp face which overlooks underlying Oxford Clay to the south. The limestones and sandstones belong to the Corallian Group, which in the Howardian Hills includes the Lower Calcareous Grit, Coralline Oolite and Upper Calcareous Grit formations. The Lower Calcareous Grit Formation comprises a succession of fine-grained, calcareous or spicular, siliceous sandstones, and forms a prominent feature. The Coralline Oolite Formation underlies gently rolling hills leading down to the Vale of Pickering, and is composed of a series of oolitic and fine-grained limestones with subordinate calcareous sandstones and sandy limestones. The Upper Calcareous Grit is represented here largely by calcareous strata, the North Grimston Cementstone, and is only exposed in faulted inliers to the south. The sequence was laid down in a tectonically active area, and movement of faults during deposition of the strata in the Mid Oxfordian led to marked changes in thickness and lithology across the area. Complications caused by these penecontemporaneous movements have meant that, though the outcrop has been investigated by numerous geologists over the last 150 years, the exact sequence of strata present and its correlation with the Corallian sequence on the north side of the Vale of Pickering have long presented problems. Much that was uncertain about the geology of the area has been resolved through detailed mapping of the area west of Malton.