J. Lamplugli — Marine Shells in Boulder-clay. 515 



must be considered as portions of this old sea-bottora whicb have 

 escaped destruction, and I am inclined to think that the Bridlington 

 Crag beds include conditions, which render it probable that its great 

 thickness and amassed appearance may have been due to the accumu- 

 lating power of a huge mass of ice, which, grounding (and not, as 

 with the smaller bergs, merely grating) on a soft bottom, would 

 slowly continue its forward course for some distance, forced irresist- 

 ibly onward by its immense momentum and the pressure of the ice 

 behind, and might push before it a constantly increasing mass of 

 sand and shells which might attain considerable proportions ere the 

 whole came finally to rest. A mass thus gathered would have its 

 greatest thickness in the centre, and would thin out on either hand, 

 and this agrees with the earlj' descriptions of the ' Bridlington Crag ' 

 as seen under Fort Hall. This view would also account for the 

 numerous single and broken shells in the ' Crag,' as well as for those 

 in which the valves still remain united, a& the latter molluscs might 

 remain alive during the whole forward movement, afterwards re- 

 suming their natural attitude and becoming imbedded in that 

 position. I would also suggest this as a partial explanation of the 

 admixture of species from different life-zones, as noticed by Dr. J. G. 

 Jeffreys.^ 



Whilst slowly melting, the ice would form an impassable barrier 

 to the advance of the masses which followed, thus protecting the 

 sand and shells at its landward edge from- undergoing the crushing 

 process, an office which might be performed by its transported 

 burden after the melting of the parent mass. And, indeed, some 

 such explanation is needed to account for the presence of these 

 patches of almost incoherent sand, of which the main body has 

 evidently been swept away. 



Then must have followed a period of, at any rate local, quiescence, 

 during which the snuff-coloured laminated clay was deposited, the 

 product of still muddy waters, probably of a considerable depth. 

 These conditions may have been owing to some obstacle preventing 

 the access of ice to the neighbourhood. 



This was evidently again followed by a return of the ice, this 

 time probably the true glacial stream from the north-west, bearing 

 with it huge masses from the granitic mountains of Cumberland, 

 and from the Carboniferous strata of the West Eiding, with the 

 remains of a few land animals^ (chiefly, perhaps solely, of the 

 Mammoth). 



As has already been observed, the ' Purple ' seems to be the only 

 clay present on the higher graunds of the coast, the ' Basement ' 

 beds not occurring very far above the sea-level, and it is therefore 

 probable that during the formation of the ' Purple ' clay, the lower 

 beds were swept away from elevated positions and carried forward, 

 to be re-deposited as a part of the mass then forming. It is to this 

 cause that I would refer the occasional presence of waterworn 



1 Phillips's Geology of Yorkshire, 3rd ed. p. 277. 



- I have in my possession part of a Mammoth's tusk from this division of the clay. 



