404 G. W. Bulman — Glacial Geology. 



It may have retreated far enough to allow of the deposition of 

 the Erie clay in Ohio (between lat. 38° and 42°) near its extreme 

 southern limit, the silting up of the lake, and the growth of forest 

 upon it. 



If the land then sank, it need not be supposed that the southern 

 fringe of the retreating ice was too far off to allow it to send ice- 

 bergs floating over the submerged land, and depositing the " Ice-berg 

 Drift " on the " Forest-bed." 



The phenomena of the American "inter-glacial beds," in fact, 

 seem to mark the retreat — perhaps intermittent and with prolonged 

 pauses — of the ice, rather than the occurrence of a warm climate 

 between two periods of cold. 



And this is the opinion to which Dr. Wright inclines after pro- 

 longed and careful study of the glacial deposits and actual glaciers 

 of North America, although it is in opposition to that expressed by 

 other American geologists. In his " Ice Age in North America," he 

 expresses himself thus : 



" A thorough study of the condition and distribution of the buried 

 forest beds bears strongly, as I cannot but think, against the com- 

 plete separation of glacial epochs in North America. In addition to 

 the facts about to be enumerated, it is a significant circumstance that 

 the buried vegetable deposits under consideration do not mark 

 a w^arm climate, but a climate much colder than the present — such 

 a vegetation, in fact, as would naturally flourish near the ice margin. 

 The buried forests of Southern Ohio have a striking resemblance 

 to those we described in Glacier Bay, Alaska. Peat and hardy 

 coniferous trees are predominant."^ 



And again, 



" Usually, as has been remarked, these buried deposits of peat and 

 wood have been assumed to imply the existence of two distinct 

 glacial periods. But, from what has been said above, it would 

 appear that the facts point rather to shorter periods of advance and 

 recession of the ice-front, analogous to those which are now in 

 progress in the Alpine glaciers, as heretofore noted." '^ 



At the same time Dr. Wright admits difficulties in the case of 

 some of the more northern deposits : 



" It must be confessed, however, that some of the facts concerning 

 vegetal deposits still further north, especially in the valley of Lake 

 Agassiz, are more difficult to explain upon the theory of a single 

 glacial epoch." ^ 



And finally summing up the matter he writes, 



" Such are, in brief, the considerations which seem to make it 

 proper to hesitate before recognizing the theory of two distinct 

 glacial epochs in America as an established doctrine to be taught. 

 Most of the facts adduced to support the theory of distinct epochs 

 are capable of explanation on the theory of but one epoch with the 

 natural oscillations accompanying the retreat of so vast an ice-front."* 



The intercalated beds on the Norfolk coast are instructive. They 



1 Ih. p. 482. 2 Ih. p. 490. => lb. p. 495. * Ih. p. 600. 



