468 GLACIAL GRAVELS OF MAINE. 



08AK BOEDER CLAY. 



This interesting deposit is so fully discussed in connection with the 

 Anson-Madison system^ that little need here be added. The general 

 conception which I have foi-med of it is as follows: 



First an osar was deposited iii a narrow channel, just as the other 

 ridges were. This channel was subsequently broadened by lateral melting 

 and erosion of the ice so as to become one-eighth to one-fourth of a mile 

 wide, and in some cases wider. If a large glacial river flowed in this broad 

 channel, an osar terrace was formed within it. If the supply of water was 

 small, its motion in so broad a channel was necessarily slow, and even the 

 fine clay could be precipitated This clay is as truly a glacial sediment as 

 the sand and gravel, yet the titles "eskers" and "osars" have come to be 

 applied to ridges of coarser matter, and hence I give a special name to the 

 plain of clay that borders the central ridge. Structurally I can not dis- 

 tinguish it from the plain of sand and gravel that borders the central i-idg-e 

 of the broad osar. The character of the sediments depended simply on the 

 velocity of the stream that flowed in the broad channel. The evidence is 

 conclusive that this border clay was contained in a channel inclosed wholly 

 or in part by ice. This evidence is stated elsewhere and need not here be 

 repeated. The border clay is found only in level regions below the eleva- 

 tion of about 400 feet, and the slow velocity of the water may in several or 

 most cases have been due to the sea backing into the channels. 



In several places below the highest sea level what appears to be border 

 clay contains marine fossils. This is the case unless reaches of clay depos- 

 ited in the open sea alternate with border clay in the course of the same 

 system. But the border clay has in these cases been covered by more or 

 less clay deposited in the open ocean after the melting of the ice at the sides 

 of the broad ice channel. It will require a more detailed field examination 

 than I have been able to give these deposits in order to determine what 

 propoi'tion of the clay was deposited in the open sea after the melting of 

 the ice of the whole region, and what was left in the bottom of broad chan- 

 nels and formed long fiords by which the sea penetrated for considerable 

 distances, perhaps several or many miles, into the thin ice-sheet of late gla- 

 cial time. It was in such broad channels that the narrow marine deltas were 



■ See p. 180; also Cliutou system, East Vassalboro branch, p. 170. 



