14 SURFACE GEOLOGY. 



channels were deposited materials gathered by the streams from the 

 melting glacier. By the low water of winter layers of sand would be 

 formed, and by the strong currents of summer layers of gravel, often very 

 coarse, which would be very irregularly bedded, here sand and there 

 gravel accumulating, and without much order interstratified with each 

 other. Sometimes the melting may have been so rapid that the entire 

 section of a kame may show only the deposition of a single summer, 

 which would then be very coarse gravel without layers of sand. When 

 the bordering and separating ice-walls disappeared, these deposits re- 

 mained in the long ridges of the kames, with steep slopes and irregularly 

 arched stratification. Very irregular short ridges, mounds, and enclosed 

 hollows resulted from deposition among irregular masses of ice. 



The glacial rivers which we have described appear to have flowed 

 in channels upon the surface of the ice, and the formation of the kames 

 took place at or near their mouths, extending along the valleys as fast as 

 the ice-front retreated. Large angular boulders are sometimes, but not 

 frequently, found in the kames, or upon their surface. They appear to 

 have been transported by floating ice. Their rare occurrence forbids the 

 supposition that these deposits were formed in channels beneath the ice- 

 sheet, from which many such blocks would have fallen upon the kames. 



The necessity of referring the formation of the gravel ridges to glacial 

 rivers became apparent during the exploration and study of our modified 

 drift in 1875; and in August, 1876, this was announced in a paper "On 

 the Origin of Kames or Eskers in New Hampshire."* In the revised 

 edition of Geikie's Great Ice Age, published in London in the winter of 

 \Z'j6-'j'j, this distinguished glacialist retracts his former opinion that the 

 kames were heaped up by marine currents, and attributes their formation 

 to sub-glacial rivers.f This may be the true explanation in some cases, 

 for such rivers probably existed through the glacial period; but more 

 commonly it would appear, as already shown, that the kames were depos- 

 ited at the final melting of the ice-sheet in channels formed upon the 

 surface of the ice. 



* Proceedings of American Association/or the Advancement of Science, vol. 25. 



t Great Ice Age, second edition, revised, pp. 217, 239, 243, 469, 478, etc. By page 414 it appears that this theory 

 was first proposed by Mr. D. Hummel of the geological survey of Sweden, in 1874; and on page 415 allusion is 

 made to a recent paper by Dr. N. O. Hoist, also of Sweden, in which the kames have been explained in the 

 same manner as in this chapter. 



