﻿Vol. 64.] CHANNEL OF DRIFT AT HITCHIN. 15 



(Drift series) there is a narrow width of sand and gravel along its 

 bottom. Between the last boring and the next the features of 

 the valley alter, and deserve special notice. Proceeding down it 

 towards Hitchin, the highest ridge of the Chalk-escarpment on 

 the left trends rapidly to the west, and the ground descends to 

 the lower level of the Middle-Ghalk plain. 



The western flank of the valley of the Ippollitts Brook now 

 ceases to be bare Chalk, and a gradually-widening spread of gravel 

 and loam, which well-sections show to be at least 50 to 60 feet 

 thick, comes in. This, forming a broad and fairly-level plateau 

 nearly a mile wide, thins away westwards, and the Chalk again 

 appears at the surface as a low ridge nowhere below 260 feet O.D., 

 which runs in a north-westerly direction from the escarpment to 

 Charlton, and forms the watershed between the valleys of the 

 Ippollitts Brook and the Hiz. At Charlton, however, this ridge, 

 like that which separates the Purwell stream from the Ippollitts 

 Brook, is breached by the Hiz ; and, although the natural features 

 are complicated by a lateral valley of the Hiz drainage-system, the 

 ridge can nevertheless be followed on the other side of the valley 

 of the Hiz, where it rises to a well-marked knoll at Fox Holes 

 or Mount Pleasant, a little over 300 feet above the level of the 

 sea. 



On the eastern side, except for a narrow lateral valley at Little 

 Almshoe, the ground rises immediately in a well-marked acclivity 

 nearly to 300 feet O.D. ; and although this is covered in places 

 with a very considerable thickness of Drift, a Chalk-ridge can be 

 followed to a little north of Ippollitts. Here the ground sinks 

 where the ridge is breached by the Ippollitts Brook, but, rapidly 

 rising again, the Chalk can be followed to Highbury, where it rises 

 as a knoll just over 300 feet above Ordnance-datum. 



Here, then, we seem to have evidence of the gradual widening of 

 an old valley in the Chalk-escarpment, its confines being outlined by 

 ridges of Chalk to the east and to the west, while its embouchure on 

 to the Lower-Chalk plain from that of the Middle Chalk is marked 

 by prominent headlands rising to 300 feet above the present sea- 

 level. It will be seen in the sequel that the deep channel in the 

 Drift runs between them. The whole of this old valley is now 

 filled in with Drift of great thickness ; and there seems to be good 

 reason for thinking that it once nearly filled up the space between 

 the two headlands, with perhaps a saddle or low col between them. 

 By this the drainage-system of the Hiz probably made its way, cutting 

 the deep trench which it now occupies obliquely to the direction of 

 the old buried valley. 



At the present time the southern side of the Hiz valley is steep, 

 rising from 200 to 300 feet O.D. Between the ridge which marks 

 the old valley on the west and the headland on the east there is no 

 sign of Chalk, and the hill which overlooks Hitchin from this direc- 

 tion is a mass of Drift banked against the eastern knoll. On the 

 northern bank of the Hiz the ground rises sharply also ; the first part 

 is gravel and sand, but there then occurs a platform of bare Chalk 



