The erratic rocks of the Upper Cretaceous Chalk of England: how did they get there, ice transport or other means?

Abstract

Rare erratic clasts – extraneous rock types – occur in the Upper Cretaceous Chalk, including a local basal facies, the Cambridge Greensand. The underlying Upper Albian Gault Clay and the Hunstanton Red Chalk Formations have also yielded erratics. The discovery of these erratics, their description and the development of hypotheses to explain their origins and significance are reviewed. They became the subject of scientific interest with the interpretation of a particularly large example “The Purley Boulder” by Godwin-Austen (1858) as having been transported to its depositional site in the Chalk Sea by drifting coastal ice. Thin section petrography (1930–1951) extended knowledge of their diverse provenance. At the same time the Chalk Sea had become interpreted as warm, so drifting ice was considered out of context, and the preferred agents of transport were entanglement in the roots of drifting trees, as holdfasts of floating marine algae, or as stomach stones of marine reptiles or large fish. Reconsideration of their occurrence, variable nature and sedimentary setting suggests that there are three zones in the English Chalk where erratics may be less rare (1) near the base of the Cenomanian in the Cambridge area, (2) the Upper Cenomanian–Middle Turonian in Surrey, and (3) the Upper Coniacian and Lower Santonian of Kent. The assemblage from each level and their sedimentary setting is subtly different. Present evidence suggests that the erratics found in the Upper Albian–Lower Cenomanian and the Upper Cenomanian–Middle Turonian zones represent shallow water and shoreline rocks that were transported into the Chalk Sea by coastal ice (fast-ice) that enclosed coastal marine sediments as it froze. The Upper Coniacian and Lower Santonian erratics from  Rochester and Gravesend in Kent are gastroliths.