140 PROBABLE CAUSES OF ACCUMULATION. CHAP. Tin. 



lying strata do not participate, a subject to which I shall have 

 occasion again to allude in the sequel, I will state in this 

 place that such contortions, whether explicable or not, ai-e 

 very characteristic of glacial formations. They have also no 

 necessary connection with the transportation of large blocks 

 of stone, and they therefore afford, as Mr. Prestwich remarks, 

 independent proof of ice-action in the post-pliocene gi"avel of 

 the Somme. 



Let us, then, suppose that at the time when flint hatchets 

 were imbedded in great numbers in the ancient gravel which 

 now forms the terrace of St. Acheul, the main river and its 

 tributaries were annually frozen over for several months in 

 winter. In that case, the primitive people may, as Mr. 

 Prestwich hints, have resembled in their mode of life those 

 American Indians who now inhabit the country between 

 Hudson's Bay and the Polar Sea. The habits of those Indians 

 have been well desci-ibed by Hearne, who spent some years 

 among them. As often as deer and other game become 

 scarce on the land, they betake themselves to fishing in the 

 rivers; and for this purpose, and also to obtain water for 

 drinking, they are in the constant practice of cutting round 

 holes in the ice, a foot or more in diameter, through which 

 they throw baited hooks or nets. Often they pitch their tents 

 on the ice, and then cut such holes through it, using ice- 

 chisels of metal when they can get copper or iron, but when 

 not, employing tools of flint or hornstone. 



The great accumulation of gravel at St. Acheul has taken 

 place in part of the valley where the tributary streams, 

 the Noye and the Arve, now join the Somme. These tribu- 

 taries, as well as the main river, must have been running at 

 the height first of a hundred feet, and afterwards at various 

 lower levels above the present valley-plain, in those earlier 

 times when the flint tools of the antique type were buried 

 in successive river-beds. I have said at various levels, be- 



