406 GLACIAL GRAVELS OP MAINE. 



beds of the streams, such as rocks or low hills. At first these places are 

 few and at long intervals, but as the channels still more enlarge, sedimen- 

 tation occurs at shorter intervals, until at last it is practically continuous. 

 A deposit once formed that the ice can not pusli forward becomes a nucleus 

 around which more gravel gathers. The resulting narrowing of the 

 channel aids in its further enlargement, and thus in process of time very 

 great masses are collected. The causes of enlargements of the channels 

 have already been noted. 



One class of the coastal gravel deposits demands special attention. 

 These are numerous massive mounds, also plains up to 5 or 10 miles in 

 length and half as broad. They often contain kettleholes, but their glacial 

 character is that of massiveness, and they are by no means so conspicu- 

 ously ridged as the plexus of reticulated kames. Often the kettlehole is 

 simply a depression in what would otherwise be a mesa or plain with a 

 rather level or gently undulating surface.^ 



These deposits are sometimes bordered in part by hills, against which 

 they he hke terraces, but they usually end in bluffs, rising above the 

 adjacent land. They, were evidently formed within ice walls either wholly 

 or in part. One of the most remarkable of these bluffs is that along tlie 

 top of which the base line of the Coast and Geodetic Survey was measured 

 in Deblois and Columbia. Now, if even the largest of the glacial rivers 

 flowed into lakes as large as the largest of these plains, the}^ would have 

 d^eposited deltas showing a horizontal assortment of sediments from coarse at 

 the proximal to fine clay and rock flour at the distal end. Instead, these 

 plains are composed of rather coarse matter — sand, gravel, cobbles, etc. — 

 with but little horizontal classification. Some of them are 150 feet in 

 height and must contain 10 to 15 square miles of surface. 



The absence of such reticulated ridges as are found at the proximal 

 ends of the moraine deltas proves that the subglacial rivers did not flow 

 into very large open bodies of water. It is a better interpretation that a 

 small channel- or lakelet open to the air was first formed. These gradually 

 enlarged by the lateral melting of their walls, partly by the heat of the 

 inflowing waters, but most rapidly from heat directly absorbed from the 

 sun. The subglacial and perhaps to some extent the superficial streams 



' See the descriptions of the Portlaud, Eeadfield-Brunswiok, and Staudisli-Buxtou systems, 

 also of the gravels of Gorliam, Charlotte, Freeport, Aubnrn, .lonesboro, Columbia, and Deblois. 



