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Newsletter No. 202

The Black Country Geological Society

August 2010

Field Meeting Report

Saturday 24th April: Field Visit to the Cotswolds - Cleeve Hill and Cleeve Common. Led by Dave Owen, Gloucestershire Geoconservation Trust.

This field visit followed the Rural Geology Trail Guide for Cleeve Common, which covers 2.5kms visiting numerous quarries and exposures of Middle Jurassic Aston and Birdlip Limestone Formations. We met Dave Owen at around 10:30am at the start point, a very busy car park, Wickfield Quarry, at Cleeve Hill golf club where he introduced the local geology and the trail guide.

Wickfield Quarry (Site 1), exposes the Lower Freestone or Cleeve Cloud Member (Birdlip Limestone Formation), a yellow, orange, white and grey oolitic limestone deposited under tropical shallow marine/shelf conditions where current and wave activity washed ooliths back and forth. Ooliths and the larger related pisoliths are concentric spherical grains of precipitated calcium carbonate around shell fragments, sand grains or, in the case of pisoliths, algae. Deposition, exposure, hardground formation and further deposition gives the limestone its yellow and orange colouration, whilst bleaching makes it white, and exposed surfaces weather hard and grey. Unlike the general shallow, southeastly regional dip of the Cotswolds these beds dip into

Cross bedding at Cleeve Cloud (Site 8, below)

the hillside as part of a rotational landslip block at the edge of the Cotswolds escarpment. Landslips are common around the periphery of the Cotswolds, as shown on BGS maps, They result from the erosion of older and softer underlying Lower Jurassic strata, activating old faults and lines of weakness. The Lower Freestone, so named because of an absence of fossils, is easily cut and has been quarried and mined since Roman times. Quarrying reached its height in the mid 19th Century and the rock commonly used as a building and ornamental stone across Gloucestershire and Worcestershire, where yellow village buildings are common, was mined at nearby Winchcombe. It was also used to build much of Cheltenham.

From Wickfield Quarry the trail climbed southwards to the grass and gorse covered summit of Cleeve Hill, which is pock marked by quarries including Site 2, Grass Grown Quarries. Here the top of the Lower Freestone is exposed and unconformably overlain by the Harford Member (upper Birdlip Limestone Formation) an orange, calcareous sand indicating slight environmental changes possibly from fluvial deposition or shallowing shallow marine conditions. The unconformity results from the missing Scottsquar Member, and is marked by erosion surfaces and bioturbation from worms within the underlying Lower Freestone. Vertical soil filled grykes cut across bedding. These result from water erosion and infilling during glacial times followed by calcium carbonate cementation and crystallisation.

Next, the trail headed southwest across Cleeve Common to Site 3 looking westwards over a deeply incised glacial meltwater valley with Postlip Quarry on the far side. This was also worked for Lower Freestone. The northwest Cotswold escarpment was covered by ice caps during the last Ice Age, with meltwater draining east and westwards eroding deeply incised valleys. West of the Cotswolds the meltwater accumulated behind a wall of ice which blocked the Bristol Channel, and flooded the southern Severn Valley forming a body of water called Lake Harrison. The flood water eventually breached a weak point just north of the Cotswolds allowing it to drain eastwards into the Moreton Gap near Moreton-in-Marsh, where the Vale of Moreton lies. Erosion by ice and water has removed the Cretaceous and Jurassic strata across the region. Current thinking suggests that these strata once continued from the Cotswolds westwards across the Severn Valley, over the Malverns and into Wales. Daylighting springs within the incised valley indicate where the underlying Lower Jurassic strata have also been cut by the glacial meltwater. Cleeve Hill and Common have been common grazing land for centuries. This, along with the golf course, helps conserve the area which is a designated SSSI because of its biodiversity. ►

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