440 GLACIAL GRAVELS OF MAINE. 



field between osars deposited respectively by subglacial and superficial 

 streams are not so definite as is desirable. Probably all the field phenoinena 

 can be accounted for on either hypothesis, though sometimes only by 

 cumbrous complications that in the end must break down any hypothesis 

 that has to resort to them. Often in the last fifteen years I have discov- 

 ered what was hoped to be an unmistakable and crucial test, only to find 

 my quest unsuccessful. 



Some of the elements of the problem of the osars have been set forth 

 above. It remains to correlate them with others in order to get a more 

 general view of the osars, their history and causation. This is reserved 

 for a subsequent chapter. 



BROAD OSAKS OR OSAR TERRACES. 



Several of the osars, after preserving the form of a two-sided ridge 

 with arched cross section for a distance of 5 to 30 miles from their north 

 ends, expand into a level-topped plain varying in breadth from one-sixteenth 

 to three-fourths of a mile. These plains or terraces contain no kettleholes 

 proper, though the surface is sometimes gently undulating. More often it 

 is very level. The material of the plain is usually rather fine gravel and 

 sand. In some cases they are found as terraces on hillsides far above any 

 ordinary stream, and can without difficulty be at once pronounced as of 

 glacial origin. But they often extend across the whole of the valleys in 

 which they are situated, and so closely resemble valley drift that they can 

 with difficulty be distinguished from that form of alluvium. The principal 

 tests for distinguishing the two kinds of sediments are the following: 



1. Topographicall)^, the broad osars occupy the same position with 

 respect to the osars lying north of them as they would if they were depos- 

 ited by the same glacial rivers. The existence of the osar north of them 

 indicates that a glacial river flowed from the north to the point where the 

 osar expands into the broader plain. This river must be accounted for. It 

 could not disappear except in a lake or the sea, or by flowing out of the ice 

 into a valley where the ice had already melted. But the osar terrace is not 

 a delta proper, showing a complete transition from the gravel to sand and 

 finally clay. It was not deposited in a lake proper or in the sea; at least the 

 velocity of the water was only partially checked. The glacial river must 



