﻿Vol. 64.] CHANNEL OP DKIFT AT HITCHIN. 23 



any water which may have found its way into it flowed towards 

 the north. 



It seems, on the whole, more prohable that the floor of the buried 

 valley sloped northwards below the Drifts which now occupy the 

 Hitchin and Stevenage Gap, and we should expect to find the valley 

 continuing northwards into and below that of the Ouse. 



With regard to the age of the channel, it must be older than the 

 Chalky Boulder- Clay which still partly fills it as far south as 

 Langley, which may have blocked it to the southward and have 

 given rise to the features now presented in the drainage on the 

 northern slope of the escarpment. 



It will be noticed that silty clay was met with at the bottom of 

 Mr. Kausom's, the Ickleford, and Holwell-Bury borings, the colour 

 of which is similar to that of the rocks over which the channel 

 passes. This might be taken as evidence of the gradual silting- 

 up of the river-bed due to the slow sinking of the land at the 

 advent of the Chalky Boulder-Clay, the second phase of the Glacial 

 Epoch, 



If the channel was formed before Glacial times, the main features 

 of the escarpment must have then existed, and, although the Boulder- 

 Clay may have spread over the country as a mantle in varying 

 degrees of thickness, those features were not obscured, and the 

 courses of the streams formed subsequently must have already been 

 outlined.- Torrential rains following the Glacial Epoch might 

 have swept away the lighter parts of the Boulder-Clay, and con- 

 verted the remainder into the gravel which now fills up the 

 inequalities of the old Chalk-surface and now lies between the 

 knolls at Hitchin. 



Though this explanation may in part account for the accumulation 

 of gravel in the Hitchin Valley, it is not wholly satisfactory. The 

 spread of the Boulder-Clay, which contains large blocks of ice- 

 scratched limestone on the summit of the eastern knoll, its presence 

 on the western knoll, and its occurrence interstratified among gravels 

 of lower elevations, show, I think, that ice must have played no 

 unimportant part in damming-up the old Chalk-valley at Hitchin. 

 Whether this ice was in the form of a glacier moving down from 

 the northward, or whether it was bay-ice formed during the last 

 phase of the Glacial Epoch, must remain an open question. Granting 

 that the main features of the escarpment were in existence before 

 the time of the Chalky Boulder-Clay, the position is one in which, 

 on the gradual emergence of land from the sea at the conclusion 

 of the Glacial Epoch, bay-ice might accumulate ; and, catching the 

 ground at the rise of the Middle-Chalk plain, it might remain 

 there and form with the debris which it carried an effective dam 

 in the pre-existing valley. It seems clear also that the drainage- 

 lines of the upper part of the Hitchin Valley have been turned 

 towards the east by the mass of Drift which lies immediately to 

 the south-east of Hitchin Town. There yet remain traces of a past 



^ A. J. Jukes-Browne, ' Stratigraphical Geology ' 1902, pp. 571-72. 



