QUATERNARY AGE. — GLACIAL PERIOD. 539 



formed ice. As the ice moved, the trees would have been rooted up 

 or broken off and mixed up, and partly ground up, with other debris, 

 and, afterward, if- not wasted by decomposition, deposited with the 

 Drift, — some portions, perhaps, in beds of vegetable material, and 

 others as scattered logs, stems, and roots. Land and fresh-water shells 

 also would have been gathered up for transport and distribution. 



The excavation of valleys was part of the work of the ice-period. 

 The valleys of the continent owe their depth to erosion by the streams 

 flowing in them. Now this excavation has been carried much deeper 

 in very many cases than could have been done with the continent at its 

 present level. Dr. Newberry states that all the river valleys of Ohio 

 are examples of this ; that the valley of Beaver River is excavated to 

 a depth of 150 feet below the present river level ; that of Tuscarawas 

 River, at Dover, 175 feet; that of the Ohio River, much deeper, 100 

 feet of boring near Cincinnati not reaching the bottom of the alluvium. 

 Such facts are evidence of erosion at some period when the continent 

 was more elevated than now, and are attributed by many to the agen- 

 cies of the Glacial era. The remarks on fiords on page 533 are in 

 further illustration of this subject. 



The excavation of lake hasins also has been attributed to glacial 

 action. In the case of many lakes in Alpine regions, the origin is due 

 to the filling of the narrow outlet of a deeply excavated valley, by 

 Drift. But, in some cases, especially when the rocks underneath the 

 glacier were soft and easily abraded, the ice may have gouged deeply 

 into the underlying deposits, and then have had this excavating action 

 stopped by a barrier of harder rock in front ; and thus a lake-basin 

 may have resulted. 



IV. Icebergs. 



While the glacier theory affords the best and fullest explanation of 

 the phenomena, over the general surface of the continents, and en- 

 counters the fewest difficulties, icebergs have aided beyond doubt in 

 producing the results along the borders of the continents, across ocean- 

 channels like the German Ocean and the Baltic, and, before the final 

 disappearance (as explained in the account of the Champlain period), 

 over the region of the Great Lakes of North America. Their effects 

 are well exhibited along the coast of Labrador. 



V. General Observations. 



1. Geography — The Glacial period a period of high-latitude eleva- 

 tion, and hence of deep valley-excavation. — Elevations of land do not 

 leave accessible records like subsidences. Still, there is evidence on 

 this point deserving consideration. 



(1.) The existence of an epoch of unusual cold in the early Qua- 



