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Charity no.252290

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          GEOLOGY AND TOPOGRAPHY

 

The present boundaries of Frocester enclose about 1800 acres (728ha) in the form of a triangle three miles (4.8km) long with a base some 3/4 of a mile (1.2km) wide on the south-eastern side. This runs along the top of the steep 750ft (230m) high escarpment, but most of the parish lies between the 40ft (12m) and 200ft (60m) contours on a gentle north-westward facing slope towards the River Severn. The land surface is dissected by small streams which flow from the hill and combine to form the northern boundary.


The parish lies almost entirely on Lower Lias Clay largely covered by an isolated area of the third River Severn Terrace and a tiny, fragmented part of the fourth. These are solifluction deposits of coarse unsorted gravels interspersed with pockets of fine sand which survive in thicknesses of up to 5ft (1.5m) in eroded hollows of the underlying clay. They fill rapidly after heavy rain, and act as localised, shallow underground reservoirs with naturally-impeded drainage so that, if inadequately farmed, the flatter areas tend to revert to poor quality rushy pasture. The fourth terrace overlies clay of the Middle Lias, which contains outcropping, thinly-bedded irregular bands of mudstone nodules.


In general the topsoil, which overlies a yellower subsoil over the gravel, is an easily worked, fertile, slightly acidic brown loam, although brashy deposits can be brought to the surface by the plough. However, there are areas, sometimes in the same field, of stiffer, intractable soil over the exposures of brown or blue/grey clay. Early farming is likely to have been confined to the naturally better-drained slightly higher ground, but during the mediaeval period almost the entire parish was cultivated in ridge and furrow, now mostly levelled. Today's farmland is mainly temporary grass for milk production, with some arable cropping.


On the lower hill slopes, ancient land-surfaces are deeply buried under colluvial material derived from the Dyrham Silts and Cotswold Sands further up, and there are minor deposits of calcareous tufa. An outlying ridge of the hill slope is capped by Middle Lias Marlstone at about 300ft (90m). Above this level the silts and sands lie under a thick cap of limestone of the Lower Inferior Oolite. On one area of the upper, steeper parts of the escarpment there are traces of prehistoric fields, but most of the hillside has been managed as woodland since at least the early mediaeval period, when it was more extensive than it is today. Latterly the slopes have grown oak and beech, while on the more fertile soils of the Vale elm predominated, until almost wiped out recently by Dutch elm disease. It was interspersed with fast-growing ash and oak, many of the trees retaining evidence of pollarding. Material found during the course of the excavation showed that all these natural resources, along with others, were certainly being exploited during the Roman period.