CHAP. XIII.] PLEISTOCENE EPOCH. 281 



I do not propose, however, to venture on any detailed 

 application of the hypothesis, but only to note that when 

 the land had risen again to within 100 feet of its present 

 level in Scotland, there seems to have been a pause, during 

 which the latest marine beds containing Arctic shells were 

 formed, while glaciers still occupied the Highland valleys. 

 Lastly, it may be observed that the action of these last 

 glaciers has hardly been sufficiently studied. Mr. Jamieson 

 has discussed some of their effects l in the way of sweeping 

 out the traces of the marine occupation of the country ; 

 but it is not unlikely that the present patchy distribution 

 of Boulder-clay in the higher parts of certain valleys is due 

 to their action, the clay being left where these compara- 

 tively feeble glaciers had not sufficient power to remove it. 

 Again, the arrangement of Boulder-clay into " drums," 

 " sow-backs," and similar ridges is more likely to be the 

 work of retreating than of advancing ice. 



Mr. J. Gr. Goodchild has suggested another explanation 

 of the formation of Boulder- clay, which has much to 

 recommend it. 2 He has evidently felt the difficulty of 

 conceiving the possibility of material accumulating under 

 an advancing ice- sheet, and sees that Professor J. Greikie's 

 theory is incapable of explaining either the internal struc- 

 ture or the external configuration of Drift mounds. He 

 relies on the fact that both glacier-ice and polar-ice are 

 always more or less dirty, and on J. D. Forbes' observa- 

 tion that the lower layers of glacier-ice work up toward 

 the surface as the whole moves onward. He naturally 

 infers that the rock detritus does not remain at the 

 bottom of the moving ice, but that the whole mass becomes 

 charged with the products of erosion, both in the form of 

 mud and of stones and boulders. So long as the ice is in- 



1 " Quart. Journ. Geol. Soc./' vol. xxx. p. 320. 



2 Ibid. vol. xxxi. p. 55 ; " Geol. Mag.," Dec. 2, vol. i. p. 496, and 

 " Trans. Cumb. and West. Assoc.," 1887, No. xi. p. 111. 



