466 GLACIAL GEAVELS OF MAINE. 



The point did not occur to me while in the field and was not specially 

 stiidied, but I have no note of transverse ridges having a different con- 

 tour from the longitudinal ridge adjacent. 



Moreover, we must assume that the ice front was retreating. When in 

 the retreat the ice had receded to a given point, the streams at that point 

 were ready to cease to be, as hitherto, glacial, and were about to become 

 frontal. The matter poured out from the front of the ice would overlie 

 the previously deposited sediments. In the case of a narrow subglacial 

 channel changing to a broad osar channel, the retreat of the ice would in 

 part be equivalent to the gradual and recessive melting of the roof of the 

 vault, and then the subsequent lateral enlargement of the canyon thus 

 formed. In this case there are two retreats to be considered — one of the 

 ice over the channel and the other of the ice at the sides of the channel. 

 When, as is true in many cases, there is a ridge of coarse matter in the 

 midst of the osar-plain, we may consider it deposited in a subglacial chan- 

 nel. There are many ways in which such a channel could change to a 

 broad channel open to the air. Perhaps as plausible a theory as any is that 

 often the roof melted recessively northward at the same rates. If so, the 

 matter of the broad plain would be, with respect to the receding subglacial 

 stream, frontal matter, and this would account well for its rather horizontal 

 stratification and level surface. But though frontal with respect to the roof 

 of the subglacial stream, it was contained between walls of ice in whole or 

 in part, and hence was glacial with respect to the regions over which the ice 

 had all melted. 



Now, as in the retreat of the ice as a whole the glacial streams had 

 continued to pour out their sediments from the ice front over the previously 

 deposited reticulated gravels, they would at once begin to fill up the kettle- 

 holes and change the ridged to a level plain. The fact that the ridges over 

 large areas still preserve their individuality proves that but little frontal mat- 

 ter was deposited upon them. This in many cases can readily be accounted 

 for, and when a good relief map is obtained perhaps all the cases can be 

 explained. In the region of southwestern Maine under discussion the north- 

 and-south series of gravels are connected by a number of east-and-west 

 series. The latter probably date from the last part of the kame period, 

 when the glacial Avater could escape eastward by subglacial or englacial 

 channels or over the ice of the valleys more easily than over the hills to the 



