Head, including fan gravel, Quaternary, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Head, including Fan Gravel
The widespread Head deposits are a varied group of locally derived, unsorted or poorly sorted materials accumulated by a process of downslope sludging (solifluction), the mobilisation being due to an excess of porewater. Their composition varies according to the upslope parent material, for example mainly silty sand is derived from the Upper Greensand or Upper Lias sands, loamy limestone gravel from Jurassic limestones, and clayey loam or loamy clay from various clay formations. This process appears to have been most active under periglacial conditions during cold periods in the Pleistocene. As larger quantities of water became incorporated into the rock mass, partial sorting occurred and there is a complete gradation downslope into fan gravels and normal waterlain fluviatile deposits. The majority of these deposits date from Devensian times.
Within the present region the most striking manifestations of these processes are the sheets of limestone gravel, both sorted and unsorted, that front the Cotswold scarp between Dursley and Mickleton, and which surround Bredon Hill. They contain a variable proportion of quartz sand believed to be of aeolian (windblown) origin. These sheets grade downwards into the Main and Kidderminster terraces of the River Severn and the First and Second terraces of the River Avon. They are mostly 1 to 2 m thick but may locally attain 4 m. They have yielded Arctic tundra faunas including mammoth, woolly rhinoceros, musk ox, reindeer and lemming. In the Carrent Brook main terrace on the south side of Bredon Hill, radiocarbon dates of 27 000 to 28 000 years confirm a mid-Devensian age for that deposit.
In the neighbourhood of Cheltenham, there are thick deposits of quartzose sand with occasional Jurassic pebble debris, known as the Cheltenham Sand. It is best developed in the valley of the Chelt where, under much of Cheltenham it is 6 m thick and may locally exceed 15 m. Although the constituents are similar to the local fan gravels, their proportions are quite different and it appears to have originated as an aeolian ‘cover sand’ blown in from the Midlands and redeposited by water action late in the Devensian.
Head and fan gravel deposits front the Mendips, Broadfield Down and elsewhere, where they are typically associated with the lower reaches of gorges and valleys cut in the hills. Close to the Cotswold scarp, masses of unbedded oolitic limestone gravel probably represent ancient screes, whilst the Pleistocene breccias of the Weston–Clevedon district, composed of angular Carboniferous Limestone in a matrix of loamy sand, are probably of the same nature.