Yol. 59.] IN THE CHALK NEAR ROYSTON. 371 



as pointed out by Mr. Crosby, would be rent asunder from the more 

 solid unfrozen beds beneath. The detritus was moved with the 

 ice, as he and others have explained, 1 as an englacial drift, and not 

 as a ground-moraine. It was set free as a ground-moraine by the 

 melting-back of the ice, during temporary, or after final, retreat; 

 an accumulation thus liable to be overridden and pressed into the 

 exceedingly-tough stony clay which is the usual character of the 

 Chalky Boulder-Clay. Inequalities of the ground led to overthrust 

 in the mass of the ice, and detritus that had been basal was thus 

 carried to higher levels. The rock-fragments, such as fractured 

 flints and lumps of chalk, were rubbed together along shear-planes 

 in the ice, causing the chalk-lumps and other rocks to be scored and 

 flattened as we find them. Much of the Boulder- Clay being blue 

 in colour, was torn from unweathered clays, the soil having been 

 removed perhaps prior to the advance of the ice, but this not in all 

 cases. 



That the ice advanced from a northerly direction and impinged 

 against the Chalk-hills, may be conceded without doing any violence 

 to the imagination. That higher portions of debris-laden ice over- 

 rode lower portions that were arrested by inequalities in the ground, 

 may be assumed. Much ice was doubtless for long stopped by the 

 higher Chalk-scarp ; and to the thrust or long-continued pressure of 

 ice along shear-planes at higher levels against the crest of the scarp, 

 we may attribute that belt of disturbance which occurs at eleva- 

 tions of 387 and 400 feet on the east, and 535 feet on the west. 



The debris-laden ice was eventually pushed far and wide beyond 

 the limits of the Chalk-scarp, planing off the higher portions of the 

 Upper Chalk and carrying material as far as the Thames Yalley 

 near Hornchurch. 



Penning and others have commented on the distribution of the 

 Middle Glacial gravels and sands which underlie the Boulder- Clay 

 along the dip-slope of the Chalk south of the crest, and are over- 

 lapped towards the summit. There these gravels occur up to a 

 height of 300 feet or more, and quite 200 feet above the bottom of 

 the valley to the north, where no Middle Glacial Drift is recognized. 

 Finding no evidence of its former presence and subsequent erosion 

 in this area, Penning was led to conclude that the Middle Glacial 

 Drift was confined to the seaward side of the Chalk-ranges. 2 

 Locally, there appears much in favour of this view ; and we might 

 even regard the Glacial gravels within the London Basin as the 

 modified Drift or moraine of the earliest invasion of the ice, and 

 before it extended on to the Tertiary tracts of Essex and Hert- 

 fordshire. 



There are, however, some patches of gravel in the broad valley 

 of Wardington Bottom, to which Penning drew special attention. 3 



1 See also J. G. Good child, Geol. Mag. 1874, p. 501. 



2 Quart. Journ. Geol. Soc. vol. xxxii (1876) p. 197. Eeferences are there 

 given (p. 201) to papers by S. V. Wood, Jun. 



3 Ibid. pp. 199-200. See also Jukes-Browne, ' Post-Tertiary Deposits of 

 Cambridgeshire ' [Sedgwick Prize Essay for 1876] 1878, p. 46. 



Q. J. G. S. No. 235. 2 d 



