372 ME. H. B. WOODWARD ON DISTURBANCES [Aug. 1903, 



They 



' occur at an elevation of 20 to 60 feet, or thereabouts, above the level of the 

 river, and consist of gravels and sands with intercalated masses of loam and 

 clay, the latter having somewhat the appearance of Boulder-Clay, or, rather, 

 of a wash from Boulder-Clay in its immediate vicinity.' 



He thought that 



■ all the phenomena might occur as well, either in an esker or in deposits left by 

 a river of tolerable magnitude.' 



He concluded that the deposits marked the course of an ancient 

 river, while admitting that they might have been 



' formed daring the removal by denudation of the Boulder-Clay which once 

 filled the valley.' 



We can readily believe in this derivation of the material, even if 

 we hesitate to accept the view that the deposits mark the actual 

 course of an ancient river. 1 The melting of the ice which occupied 

 this valley, the lower portion of which had been for long banked 

 up against the scarp, was doubtless attended by powerful currents 

 which distributed the debris, and even tore away a good deal of 

 Chalk from the scarps. 



We may thus consider that the outlines of the combes, to which 

 attention has already been directed, were largely sculptured at this 

 period ; and we might go some way with Sedgwick when he re- 

 marked in 1861 that these lesser features in the Chalk 



' do not appear to have been produced by a long-continued and slow process of 

 erosion ; but rather to have been cleanly swept out by rapidly-descending 

 water-floods.' 2 



Postscript. 



Mr. A. J. Jukes-Browne writes to me as follows : — 



' What you tell me about that curious flexure along the Royston Downs is 

 very interesting. Penning was much puzzled by it, as is clear from what he 

 wrote on p. 67 of the Memoir on the Neighbourhood of Cambridge. I only 

 saw the sections once, and in company with him. It is of course quite 

 possible for Boulder-Clay to be forced into and between beds of Chalk in situ. 

 Do I understand that you attribute the whole disturbance, for a distance of 

 more than 5 miles, to the impact of ice ? It seems a large order ! ' 



Discussion. 



The Rev. E. Hill welcomed the chronicle of facts, but thought that 

 rather a large superstructure was built upon them. Disturbances 

 were described under Boulder-Clay, but in many cases absence 

 of disturbance could be seen, and equally required explanation. 

 Trimingham was quoted as a parallel : there Boulder- Clay over 

 Chalk looks undisturbed, while sands over the Clay are contorted. 



1 Some terrestrial and freshwater mollusca were found in gravels, regarded as 

 equivalent, in the area to the north of that now under consideration. See 

 ' G-eology of the Neighbourhood of Cambridge ' Mem. Geol. Surv. (1881 ) pp. 82, &c. 



2 ' A Lecture on the Strata near Cambridge & the Fens of the Bedford Level 5 

 8vo [privately printed] p. 37. 



