﻿24 ME. W, HILL OX A DEEP [Feb. I908, 



Glacial stream/ which may have been the precursor and originator 

 of the valley now occupied by the Purwell and IppoUitts Brook. 



The present drainage of the Hiz, before it is joined by the Purwell, 

 seems to be fully accounted for by the probable configuration of the 

 escarpment in early pre-Glacial days, the water making its way by 

 a saddle or col between the two knolls, and having since then 

 deepened its channel. 



A glance at the Drift-Map published by the Geological Survey 

 would perhaps suggest the possibility of another channel buried 

 under the broad area of Boulder-Clay and gravel which lies imme- 

 diately south of Stevenage and extends to the north as far as 

 Letchworth and Wilbury Hill. But a narrow space of bare Chalk 

 at an elevation of 240 feet O.D., connecting large areas east and 

 west of it, precludes the occurrence of a channel farther north 

 than Letchworth. 



Few wells of any depth occur between this point and Stevenage. 

 At Little Wymondley, however, Mr. Coartenay, in sinking one for 

 his water-supply, encountered 80 feet of Drift, 35 feet of which 

 immediately overlying the Chalk was blue Boulder-Clay. Although 

 I have been unable to find other evidence, deep clay- and gravel-pits 

 show that there is a very considerable thickness of Drift north and 

 south of Stevenage. 



Again referring to the Drift-Map, it will be noticed that a stream 

 of gravel turns eastwards from the narrow valley in which the 

 channel described in these pages occurs, and passing through Ippol- 

 litts and Titmore Green joins the larger spread just north of 

 Stevenage. Three or four wells in the centre of this stream touch 

 the Chalk 70 to 80 feet from the surface, so that a ridge of this 

 Tock must continue here about 200 feet above sea-level. Although 

 there is no evidence on the Stevenage side of the Gap, I am 

 inclined to think that an old channel formed by a drainage-system 

 which once flowed into that of the Thames is buried here beneath 

 the gravels and Boulder-Clay. If such be the case, its only course 

 must be through the narrow passage at Bragbury End. 



Discussioif. 



Mr. Clement Eeid said that he had just visited the area in 

 company with the Author, and had been impressed with the size 

 and importance of the buried valley. It was probably pre-Glacial : 

 but none of these Drift-filled channels had as yet yielded anything 

 closely resembling marsh-deposits or alluvium, nor had they yielded 

 fossils. The lowest beds, just above the solid rock, deserved close 

 examination. 



As an alternative hypothesis, he suggested that the valley might 

 be sub-Glacial, not pre-Glacial, the drainage flowing southward 

 towards the Thames, instead of northward towards the Ouse. The 



^ Proc. Roy. See. vol. Ixi (1897) p. 40; and Proc. Geoh Assoc, vol. xiv 

 (1895-96) p. 415. 



