Tuesday 1st May 1990:
The Warminster Bypass – Soil Descriptions
Three basic types of soil were encountered along the Warminster Bypass, and these are described below (Gault Clay is also present in the vicinity of the River Wylye, but was only encountered during piling for bridge foundations):-
Chalk
Chalk was encountered in the earthworks for the two western cuttings (Norridge Wood and Barton) and in Norton Cut. It was also present at Hazel Underpass, Sutton and Cotley Hill Bridges. The chalk is described as highly to moderately weathered, light greyish-brown, clayey, weak to moderately weak, becoming with depth slightly weathered (occasionally fresh) clayey moderately weak to moderately strong. Highly weathered chalk comprises fragments within a firm silty clay matrix.
Greensand
The Greensand is a silty, frequently very silty, sometimes clayey or very clayey, fine and medium sand, occasionally cemented to a weak to moderately weak sandstone. Bands of fractured, very strong chert and sandstone occur within the upper parts of the greensand beds, whilst very weak siltstone occurs in the lower parts. The clayey parts of the greensand appear random and occasionally occur in pockets; soft silty clay layers sometimes occur in thin (less than 0.5m thick) layers below chert bands.
Head/Alluvial Deposits
Head deposits are principally found overlying areas of chalk, where they comprise firm to stiff clays with flint and chalk gravel. Head deposits are also to be found on the floors and sides of valleys, although in these cases they are similar to the underlying greensand (though looser and possibly more segregated) and are less discernible.
Alluvial deposits comprise very soft to firm organic, occasionally peaty, silty clays, and are to be found mainly at the river crossings, the floor of the Cannimore Valley, and to the north of the Bypass in Buffage Wood.
Background Information
The rocks over which the Warminster Bypass runs are of two main types: the Upper Greensand and the overlying Lower Chalk. These sediments were laid down in a sea that covered much of northern Europe about 90 million years ago, and they contain the fossilised remnants of animals that lived in those waters.
Upper (Younger) Middle Chalk
Cretaceous Lower Chalk (inc. Warminster beds)
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Lower (Older) Upper Greensand
Cretaceous Gault Clay
During the course of building the Bypass, some of these fossil remains have been unearthed.
First published in Warminster & District Archive magazine, No.4, published Tuesday 1st May 1990. Archive gratefully acknowledges the co-operation and assistance of Scott Wilson Kirkpatrick & Partners (Consulting Engineers to the Warminster Bypass) in the preparation of this article.