Q6 James Geikie — On Changes of Climate. 



The witlidrawal of the great ice-sheet was marked, as in Switzer- 

 land, by the accumulation of immense piles of moraine rubbish, 

 which are partially re-arranged or "modified," as Professor Hitch- 

 cock has it, " by the action of water." ^ It is thus difficult some- 

 times to distinguish true moraine drift from the re-assorted marine 

 drift, — the one, in short, seems to shade into the other. This, how- 

 ever, does not mean that the deposition of the moraines and their 

 re-assortment by the sea took place at one and the same time. The 

 ice may have melted away from all the low grounds of New England 

 and shrunk back to the valleys among the White Mountains before 

 subsidence of the land began. 



In reading descrijjtions of the mounds of gravel and sand which 

 cover large tracts of country in the New England and the North- 

 western States and also in Canada, one cannot fail to notice how 

 closely all the appearances coincide with those we are familiar with 

 in this country. Professor Hitchcock, describing the drifts of New 

 England, says, they " form ridges and hills of almost every possible 

 shape. It is not common to find straight ridges for a considerable 

 distance. But the most common and most remarkable aspect assumed 

 by these elevations is that of a collection of tortuous ridges and 

 rounded and even conical hills with correspondent depressions be- 

 tween them."^ This description would apply word for word to 

 some of the larger areas of Kames in Scotland. The American 

 mounds and cones are almost invariably composed of well water- 

 worn materials, usually gravel and sand ; and they are, moreover, 

 not infrequently false-bedded. Occasionally boulders are found in- 

 side these mounds, but this is certainly quite exceptional, and such 

 included stones are usually more or less rounded. Now and again 

 a mound appears to be composed of coarse shingle and rounded 

 boulders. But when boulders occur in mounds of sand and fine 

 gravel, they seem to be confined chiefly to the upper parts of the 

 deposits.^ 



Immense numbers of large erratics cumber the surface of the 

 ground in many parts of New England, the North-western States, 

 Canada, and Labrador, and are scattered over the tops and slopes 

 of the mounds and ridges of sand and gravel. Even much further 

 north the same phenomena are so striking as to arrest the attention 

 of the traveller who is not strictly a geologist. I was much in- 

 terested some years ago in reading the accounts given of the " barren 

 grounds" of North America by various writers who had visited these 

 inhospitable regions. Sandhills and huge erratics appear to be as 

 common there as in the countries further south. Captain Back, who 

 followed the course of the Great Fish Eiver (Back's Eiver) down to 

 the Arctic Sea, gives a very graphic account of the isolated cones and 



1 Smithsonian Contributions. Illustrations of Surface Geology, etc. 



2 Trans, of the Assoc, of Amer. Geol. and Natur. 1840-1842, p. 191. 



3 See Eeport on the Geology of the Lake Superior Land District, p. 235. Also 

 Geology of New York, part iii., p. 121, where Lardner Vanuxem says, " "With some 

 exceptions they (erratics) are generally found upon the surface, frequently upon the 

 tops of hills or on their sides, appearing in almost all their localities as if but recently 

 dropped," etc. 



