During the formation of the Gault the Lower Greensand was
extensively eroded, as well as the various Jurassic rocks. The Gault
was spread over their worn surfaces, resting in places in the Vale of
Aylesbury on Lower Greensand, Purbeck and Portland Beds and Kime-
ridge Clay ; and between Wing and Soulbury on Corallian and Oxford
Clay.
GAULT
The Gault, a stiff dark blue and pale calcareous clay with concretions of carbonate of lime or ' race,' occupies a considerable tract, and is more calcareous in the northern part of the area. It occurs at Wing, Cubling- ton, Mentmore, and thence from Cheddington it comes to the surface over the Vale of Aylesbury by Stoke Mandeville and Ilmer to Towersey.
The Lower Gault, which is characterized by Ammonites interruptus and A. lautus, together with Belemnites minimus, attains a thickness of from 140 to 150 feet. About 20 or 30 feet from the base there is a band of phosphatic nodules, and other nodules are found at the junction with the Upper Gault. The Upper Gault characterized by Ammonites rostratus is 70 or 80 feet thick.
The lower band of phosphatic nodules was formerly worked at Towersey, and between Ford and Moreton, south-east of Dinton, and near Bishopstone. The seam, which is but 3 or 4 inches thick, is made up of buff and black nodules, comprising coprolites and phosphatized shells of the Gault fossils A. rostratus, A. varicosus, Inoceramus sulcatus, etc. The upper band of nodules, about 1 8 inches thick, was at one time worked at Puttenham, Cheddington and Slapton.[1]
The soil of the Gault is naturally thin, so that the land is often heavy, cold and tenacious, and best adapted for pasture. From infor- mation communicated by the Rev. F. W. Ragg, it appears that as late as the fifteenth century, before the district was drained, there were many swampy tracts and two or three lakelets in the vale north of Marsworth. The soil is however much modified in places by the scattered Drifts and by downwashes from the neighbouring hills of Upper Greensand, Lower Greensand and Portland Beds. Hence it is that the celebrated Vale of Aylesbury, ' the pastoral garden of the county,' which extends from Mentmore and Cheddington to Ilmer and Waddesdon, while mainly a clay country of Gault and Kimeridge Clay, has a soil improved by the waste of the bordering and outlying hills, as well as by the superficial drifts; and thus it ranks high as grazing and dairy land.[2] The Gault is utilized in many places for brickmaking.
UPPER GREENSAND
This formation, which enters largely into the scenery of many southern counties, is thin and impersistent in Buckinghamshire, being in fact largely replaced by the Gault clay. It comprises greenish (glau- conitic) sands and marls, with layers of fine-grained clayey calcareous
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