﻿Vol. 64.] CHANNEL Oi<' DRIFT AT HITCHIN'. 11 



heads of valleys —one at the northern end of Stevenage, where the 

 ground is only 315 feet above the sea ; and one south of Norton 

 Green, where the divide between the two systems is only about 

 305 feet above Ordnance-datum. 



It has long been known that this gap is largely filled with Drift- 

 deposits which are partly at any rate of Glacial age, and the surface- 

 area of these deposits is shown on the Geological-Survey Map. Some 

 of these Drifts near Hitchin were briefly described in a report of an 

 excursion to that place in 1896 ; and Dr. A. E. Salter then pointed 

 out their position at the 



' northern entrance of one of the few gaps which penetrate the Chalk hills to 

 the north and north-west of London.' ^ 



. No one, however, has .yet given a complete account of them, so 

 that the depth to which they extend is at present unknown, and 

 the relations of the existing surface-features to those of the pre- 

 Glacial contours could only be a matter of speculation. Until some 

 information was obtained with regard to the varying depth of the 

 Chalk-floor upon which the Drifts rest, it was impossible to say 

 whether the ancient (buried) valley extended northwards from 

 Langley and Stevenage, or southwards to some point outside the 

 present Chalk-escarpment. 



The western flank of the valley just defined is almost entirely 

 bare Chalk down to, sometimes below, the 300-foot contour-line ; 

 but the highest ground on the summit of the escarpment is covered 

 with Clay-with-Flints, Along the summit occur, near the edge 

 of the slope, five xmtches of Boulder-Clay, all of insignificant 

 dimensions, but suflicient to lead to the belief that the high ground 

 as well as the low was once covered by that material. 



The lower slopes of the eastern flank are also bare Chalk, but 

 from Wilbury Hill, south-eastwards as far as Little Wyraondley, 

 on the plateau of the Middle Chalk, is a large area of Boulder-Clay 

 which descends in some places below the 300-foot contour-line. 

 The area of this clay is broken at Letchworth by a narrow outcrop 

 of Chalk 250 feet above Ordnance-datum ; at Willian, by a tongue of 

 gravel' half a mile wide; and again by alluvium at Little Wymondley, 

 where the Purwell valley turns to the eastward : but on the other 

 side of this valley nearer Stevenage, it seems to be continued by a 

 small patch. Besides the tongue of gravel at Willian, several other 

 areas of gravel occur lying on the slopes just below the edges of 

 the Boulder-Clay. 



xill the country enclosed within the angle of the Y, with the 

 exception of two knolls, to be mentioned hereafter, is below the 

 300-foot contour. The country between tlie headlands already 

 mentioned north of Hitchin, is fairly level and is about 2U0 feet 

 above the sea, with the exception of a shallow and narrow channel 

 cut by the rivers Purwell and Hiz. It is here covered with 

 Boulder-Clay ; but nearer Hitchin gravel and sand take its place on 



1 Proc. Geol. Assoc, vol. xiv (1895-96) p. 415. 



2 Wells sliow that this tongue of gravel is about GO feet thick. 



