Page:VCH Hertfordshire 1.djvu/40

 A HISTORY OF HERTFORDSHIRE undergone. It is present in our area only beneath the surface, the whole of its outcrop being outside the county, trending in a south- westerly direction from the neighbourhood of Potton, through Shefford, to Leighton Buzzard. In a well-boring at Long Marston, six miles south of the latter place, it was met with about 10 feet in thickness, after 215 feet of Gault clay had been passed through. It thins out to nothing in the south-east, being absent where the Gault was passed through at Ware and Turnford. The Lower Greensand is the highest bed of the Lower Cretaceous Series. Hitherto only the rocks which do not come to the surface in the county have been considered. The oldest formation which does so is of Upper Cretaceous age. This is the Gault, the earliest of that age, being next in succession to the Lower Greensand. Before its deposition there was a considerable disturbance of the strata previously deposited, result- ing here in a subsidence which even brought beneath the sea the Palaeo- zoic ridge that for ages had formed a barrier across our county between the seas on the north and on the south. While on the north-west the Gault reposes on the Lower Greensand, on the south-east it rests directly on Silurian and Devonian rocks. At Cheshunt it is 153 feet thick, at Ware 166 feet, and it increases in thickness towards the north and west, being 180 feet thick at Radwell near Baldock and at Hinxworth, about 200 feet at Ashwell, 210 feet at Hitchin, and 215 feet at Long Marston. It consists of calcareous marls and dark bluish-grey clays, with concre- tionary and phosphatic nodules. Owing to its soft and easily-weathered character it forms a plain and sometimes a depression at the foot of the Chalk escarpment, partly along, but chiefly beyond, the north-western margin of Hertfordshire. It enters the county from Cambridgeshire at the extreme north, in the Cam district, between the River Rhee and the Ruddry Brook ; continuing just within the margin of the county, it passes into the Ivel district ; it is again seen near Radwell north of Baldock, and again north-west of Pirton. The Thame district is in great part on it, and here it occupies the spur of the county beyond the Mars- worth, Startups End, and Tringford reservoirs, the Wilstone reservoir being the only one which is actually on the Gault. Although in great part a stiff impermeable clay, the soil upon it is extremely fertile, having been rendered so by a covering of drift from the Chalk. This Gault plain has, indeed, long been known as a fine corn-grow- ing district. The greater part of it not under arable culture is well wooded with oaks, a characteristic feature of the formation, as the name ' Oak-tree Clay,' which has been given to it as well as to the clays of Kimeridge and Wealden age, indicates. The Gault suffered much from denudation before the deposition of the next bed upon it, and its surface is very irregular. Towards the north-east it thins out greatly through the upper beds having been eroded. While in that direction it is immediately followed by the Chalk Marl, the lower beds of which are even sometimes wanting, towards the south-west the Upper Greensand is present ; but by whatever 6