378 CLIMATIC CONDITIONS OF THE GLACIAL EPOCH. 



delicately laminated, form a part of the detrital mass, marking the passage 

 of the " diluvial " into the " alluvial " deposits. The sides of the rivers fre- 

 quently exhibit a greater or less number of terraces, showing that the water 

 once flowed in these valleys in much larger volume than it now does. 



As we proceed westward, the character of the geological formations 

 changes, and at the same time that of the overlying detrital materials. The 

 rock in place being mostly sedimentary and non-crystalline, the presence in 

 the drift of hard metaraorphic and granitic materials becomes a more strik- 

 ing phenomenon, since the fact that they were brought frona a distance is 

 evident to the most superficial observer. With much rolled material of a 

 coarser kind of both local and foreign origin, we find a large and sometimes 

 a predominating quantity of finer detritus, including both clay and the pecu- 

 liar deposits called by the names of" loess" and " bluff formation." Indeed, 

 the clay which usually forms the lower member of the detrital series becomes 

 an extremely persistent one when we reach the vicinity of Lakes Michigan 

 and Superior. 



As a rule, the detrital formations described and included under the term 

 "drift" are most extensively developed in New England and on the southern 

 borders of the Great Lakes. Li the former region they extend to the sea 

 without marked change of character ; but westward are spread over the land 

 surface, gradually thinning out as we recede from the lakes, and becoming 

 more and more confined to the vicinity of the pi-esent rivers or to channels 

 of now obliterated streams. In the great river valleys, on the other hand, 

 the traces of materials of northern origin, becoming finer and finer, may be 

 observed far beyond the region where anything of the kind can be discovered 

 in the adjacent higher lands. 



The facts thus stated — in the most general way, as will be admitted — 

 appear at first sight to indicate that the detrital deposits in question are 

 simply the result of the action of water. The predominance of sand and 

 gravel, their occurrence to a large extent in the stratified form, the alter- 

 nations of coarse and fine materials — these seem to be unquestionable 

 evidences of an aqueous origin. 



That the drift should have been borne, on the whole, in a general south- 

 erly direction — over a large part of the region, at least — need not e.xcite 

 surprise, when we call to mind the fact that the present drainage of much of 

 the area covei'ed by the drift is to the south. The larger rivers — as the 

 Hudson, Connecticut, and Mississippi — have an almost due south course, and 



