G. W. Bulman — Glacial Geology. 341 



In describing the middle glacial beds, Mr. Whitaker writes : 

 " Besides these, however, thei'e is sometimes a layer of true Boulder 

 Clay in the gravel, that is, a clay that contains irregularly rounded 

 fragments or boulders of various rocks, the surfaces of which are 

 furrowed and scratched in the way that is peculiar to stones that 

 have been dragged along by masses of ice over a floor of rock. 

 This clay being like that which occurs in force above the gravel, 

 serves to link the two deposits together, and to show that the 

 glacial conditions which seemed to have reigned supreme during 

 the great mass of Boulder Clay existed also during the deposition 

 of the lower bed, though to a less extent." ^ 



This layer of Boulder clay in the gravel can hardly be considered 

 as the product of a glacial epoch, followed and preceded by a mild 

 epoch when gravel was laid down. 



And there is another waj^ in which true river sands and gravels 

 may be intercalated in glacial beds without the intervention of a 

 warm interglacial period. They may be preglacial river deposits 

 pushed on by the advancing ice over boulder clay already laid down. 



Such preglacial river sands and gravels must have been abundant, 

 and it is usually believed that a part of the work of the ice was to 

 sweep these down the valley. What more likely than that some 

 of these deposits, thus pushed on, would be stranded in some quieter 

 spot and at times left on the surface of already deposited till ? 



In writing of the glaciation of South Lancashire, Mr. A. 

 Strahan speaks of the Boulder clay as " sometimes rudely strati- 

 fied," although " usually devoid of such lamination as is shown by 

 clays deposited freely in water." ^ 



He thus further describes the alternation of sheets of Boulder clay 

 with sand and gravel in certain districts : 



" In every case where the drift attains any thickness, as in a 

 preglacial river-valley, or the maritime plain of North Flintshire 

 and Denbighshire, it is found to consist of alternations of sheets of 

 Boulder-clay with sand and gravel, the beds running sometimes for 

 a mile or two without interruption." ^ 



Here, again, we cannot suppose each sheet of Boulder clay to 

 represent a glacial epoch, and each separating gravel a mild period ; 

 we are rather bound to conclude that the glacial conditions which 

 produce Boulder clay are able also to produce sand and gravel ; 

 that the alternations here seen are due to merely temporary alterations 

 of conditions, as between summer and winter or small oscillations in 

 the extent of the ice. 



The section at Blackpool is, perhaps, one of the most favourable 

 to the hypothesis of a mild interglacial epoch. There it is said there 

 are two Boulder clays, separated by a sei'ies of sands and gravels. 



This series of beds is described in detail with a section by Mr. T. 

 Mellard Eeade.* 



But although it is true there is in this coast section a lower bed 



^ Geologv of London, Geol. Surv. Mem. pp. 52, 53. 



2 Q.J.G.S. vol. xlii. pp. 373-4. 3 Qp. cit. p. 383. 



* Q.J.G.S. vol. xxxix. p. 83. 



