Vol. 63.^ OKIGIN OF CEKTAIN CANON-LIKE VALLEYS. 509 



Lateral ice-streams seem to have been forced up the pre-Glacial 

 valleys before mentioned near Hitchin and Newport ; it was by 

 one or other of these routes, I think, that Chalky Boulder-Clay 

 containing Oxfordian debris was carried to St. Albans and 

 Einchley. 1 The Boulder-Clay which covers the dip-slopes of the 

 Chalk-escarpment could not have been deposited by an ice-stream 

 moving along its south-eastern margin, at right angles to the 

 general direction of the gradient. It is clear, on the contrary, 

 that the ice lay in sufficient thickness over the Fenland between 

 Cambridge and Hitchin to overflow the whole of the East Anglian 

 ridge ; having done so, it travelled downhill into Essex. 



The Chalky Boulder-Clay of the Ouse basin is stiff and dark, its 

 matrix being prevalently Oxfordian, it contains, however, much 

 Oolitic detritus ; that which sheets the south-eastern dip-slopes 

 is, as a rule, of the chalky type, especially where the ice overflowed 

 the higher parts of the escarpment, although its connexion with 

 the Eenland Drift is shown by the widespread, yet less abundant 

 presence in it of Oolitic material. Mr. H. B. Woodward found, 

 moreover, at Barkway, near the summit of the Chalk-ridge, 

 phosphatic nodules from the so-called ' coprolite-bed ' at its base. 

 He attributes the well-known disturbance of the strata at this 

 place to the pressure of ice overflowing the escarpment, moving 

 along shear-planes at a higher level than that which occupied 

 the lower ground. 2 Where the ice moved south-eastwards from 

 the Eenland or the Ouse basin through the transverse valleys 

 of the Chalk-hills it carried with it more Jurassic material. Near 

 Withersfield, 15 miles east-south-east of Cambridge, for example, 

 the Boulder-Clay is as characteristically Kimeridgiau as it is in 

 Suffolk. 



Similar conditions occur in Lincolnshire. East of the Wolds, 

 the Boulder-Clay, of North-Sea origin, is brown or purple; but when 

 the watershed is passed it puts on abruptly, to the west of the 

 latter, as before stated, an intensely chalky character. In both 

 regions the ice which overflowed the Chalk-hills must have been 

 comparatively clean, as Prof. Chamberlin says that the upper part 

 of the Greenland ice-sheet is. 3 A few Scandinavian or doleritic 

 erratics were carried over to the west of the W T olds from the 

 North Sea, as Jurassic detritus was occasionally carried from the 

 Eenland into Essex. 



Although the ice, mounting the slopes of the Chalk-hills, brought 

 with it no appreciable amount of Oxfordian Boulder-Clay in the 

 one case, or North- Sea Boulder-Clay in the other, as soon as it crossed 

 the watershed it began to grind into and erode the Chalk-Bock, 



1 Ice moved from the Fenland towards Leckhampstead, I believe, by the 

 direct route, and not by way of Dunstable. 



2 Quart. Journ. G-eol. Soc. vol. lix (1903) pp. 367-71. 



s Bull. Gteol. Soc. Am. vol. vi (1895) p. 205; see also P. F. Kendall, in 

 G. F. Wright's ' Man & the Glacial Period' Internat. Sci. Ser. 2nd ed. (1893) 

 p. 166. 



