Geology of Gloucestershire - Severn Vale

Severn Vale

To the east is the wide fertile Severn Vale, floored by Triassic 'New Red' sandstones and marls of the Mercia Mudstones group (formerly known as the 'Keuper'), and Jurassic lias clays further east. The Triassic deposits were formed in a Sahara -like desert when the British Isles lay about 15 degrees north of the equator, whereas the clays represent deep-water sediments. The landscape here is flattish, with the only feature of note a rather weak low scarp which meanders across the vale from SW to NE marking the Triassic/Jurassic border. This is superbly illustrated at the ‘Garden Cliff’, Westbury-on-Severn (see picture), where the river Severn has sliced a convenient ‘cut-away’ section of this transition from the red Triassic marls, through the thin Penarth Group (formerly 'Rhaetic') strata, to the lias clays and limestones of the lower Jurassic.

Traditional building stone in the Severn Vale is scarce, necessitating brick or half-timbered construction in the main, although along the scarp mentioned above harder limestone bands in the Lower Lias (known as 'Blue Lias') have been used in vernacular building.

Towards the east side of the Severn Vale the Lias clays are overlain by the sands and limestones of the Middle Lias - well displayed on the slopes of Robinswood Hill, a Jurassic outlier overlooking the city of Gloucester. Further east, forming a rather unstable base to the Cotswold escarpment, are the sands and clays of the Upper Lias.

Superficial deposits are widespread. Floodplain alluvium accompanies the course of the Severn itself as well as tributaries such as the Leadon, Chelt, Frome and Cam. Ancient terraces of former, more elevated floodplains mark the position of the major rivers through various glacial and inter-glacial periods in the last Ice Age. A sheet of mainly Jurassic limestone fan gravel probably covered most of the vale in the past but has since been eroded away leaving isolated deposits, most notably the Cheltenham Sand, which forms a well-draining light soil in the Cheltenham-Gloucester region.