376 CLIMATIC CONDITIONS OF THE GLACIAL EPOCH. 



and also on a pretty careful perusal of most of that which has been published 

 in this department of geology. 



The first important fact to be noticed in the formerly glaciated portion of 

 Northeastern North America, commonly known as the region of Northern 

 Drift, is the occurrence of large amounts of detrital material on the surface 

 which cannot be referred to the place of its origin in the same easy way in 

 which similar materials can be over the region of the Cordilleras, where it is 

 evident that the sand, gravel, and boulders have been carried down the slopes 

 of the adjacent mountain ranges by gravity, aided by currents of water, in a 

 manner which is natural, and in entire harmony with the existing topog- 

 raphy. In the Northern Drift region, on the other hand, such a connection 

 between the form of the surface and the movement of the detrital masses 

 appears, at first sight, to be altogether wanting, and certainly is, in many 

 places, very obscure. 



Here the marked difference in the topographical features of the two sides 

 of the continent is forced upon our notice. The western side is traversed by 

 numerous high chains of mountains, which give strong outlines to the profiles 

 of its surface. The eastern region, on the other hand, has few well-defined 

 ranges, and these only of moderate height. Much of the so-called Northern 

 Drift region is very nearly level, having an elevation of not more than from 

 500 to 1,000 feet. Another large area is a plateau of moderate height, rang- 

 ino; between 1,000 and 2,000 feet above the sea-level. Within the borders 

 of the United States there is nowhere, west of the Adirondacks, within the 

 limits of the Drift region, anything which can be called a mountain range, 

 until we reach the southwestern part of Lake Superior, where the trappean 

 rocks form a chain of hills, hardly anywhere, however, so much as a thousand 

 feet above the level of the lake. 



Still more perplexing is the fiict that in the centre or axis of the Drift 

 region, where the peculiar phenomena of the glacial epoch are most charac- 

 teristically displayed, there is an area of exceptional depression, a large 

 portion of which is below the sea-level, this and many thousand square 

 miles in addition being now covered Ijy water. This is the more remark- 

 able, because this is the region toward which the strise, to a certain ex- 

 tent, seem to converge, and where the continental ice sheet must have 

 originated, or over which it must at least have passed, if it ever had an ex- 

 istence. Instead of an elevated mountain range from which the glacial mass 

 descended, as has been seen to be the case in every other part of the world, 



