880 



GLACIAL GRAVELS OF MAINE. 



Fig. 28.— Marine clay oTerlying base of osar, and itself covered -with a capping of 

 gravel; Corinth. 



but, so far as I have noted, only in places where the ridges would be 

 exposed to the surf. The fact could be accounted for in two ways : 



1. The ridg-es were first deposited within ice walls. Subsequently the 

 ice melted and sea water covered the ridges. Marine clay, or in some cases 

 kame or osar border clay, was now laid down, covering the bases of all the 

 ridges and the whole of the smaller ridges or osars, and a thin sheet of 

 clay may have been spread over the tops of even the highest ridges that 

 were under the sea. During the retreat of the sea to its present level the 



surf must have suc- 

 cessively beat upon 

 every part of the 

 land as it emerged 

 from the water. In 

 exposed situations 

 the waves would be 

 able to erode the upper portions of such gravel masses as rose above the clays 

 and to leave the matter in quaquaversal or anticlinal stratification along the 

 lower slopes of the ridges and extending out a few feet or yards over the clay 

 previously deposited upon the base of the gravel. In Carmel, Clinton, 

 Detroit, and many other towns, wells dug in the flanks of the osars almost 

 invariably are dug through a thin stratum of gravel, then through clay 

 containing shells, and finally into a deep stratum of gravel in which water 

 is found. The upper gravel 

 extends only a short dis- 

 tance from the central ridge. 



2. According to an- 

 other theory, the sand and 

 gravel which overlie the clay on the flanks of the osars may have been 

 brought there by glacial streams. On this theory some of the coarser 

 matter was swept out to sea for short distances beyond the retreating ice 

 front and deposited over the marine clays that had already been laid down 

 in the open sea. The theory would make the sand and gravel overlying 

 the marine c\aj a sort of marine delta. The subject will be discussed more 

 fully later. If the glacial gravel at Portland overlies the fossiliferoiis 

 marine clays, it may do so in the manner here indicated, or if at the base 

 it overlies the clays, this would form a fourth arrangement of the gravels 



Fig. 29. — Marine clay and sand in tbe midst of osar gravel ; Hermon Pond. 



