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 



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