Horsts and Grabens:  A field trip to Cleeve Hill , Gloucester with Nick Cridlaw.  8th Oct 2023

by Jane Allum

Photo by Jane Allum

Towards the end of the Palaeozoic (c.290m years ago), the north south trending Severn Basin began to form between what is now the Malverns, the Birmingham area and the Gloucestershire Oxfordshire border. Subsidence of the basin continued as sediments were deposited  through the Triassic and early Jurassic. At that time the UK was at the same sort of latitude as Morocco is today, with a warm tropical sea depositional setting. It was not lost on the participants that the almost tropical weather experienced on this trip was very apt for examining rocks laid down as sediment in a warm tropical environment.

Specifically what developed in the tropical seas were limestones containing ooids, peloids, intraclasts and oncoliths that were cemented by a lime water (Sparite) created as shells eroded, or a lime mud (Micrite) produced as calcareous green algae disintegrated.

In addition to everything else that makes Cleeve Hill a GeoPark we observed the geomorphology which gave the appearance of giant steps down into Valley. These have been formed by landslips caused by rotational slip of the bedrock. This commonly occurs when a competent permeable bed overlies an impermeable mudrock such as the underlying Whitby Mudstone Formation.

One day is not enough to see everything in this fantastic park and many thanks to Nick who lead the trip and a group of very knowledgeable professional and amateur geologists. Cleeve Hill, Gloucestershire – Wikipedia