RETEEATAL PHENOMENA. 391 



glacial rivers would not only be attacked from the front lengthwise of their 

 courses, but often might be cut off b j the sea penetrating transverse valleys 

 and thus arresting their flow at some point many miles to the north of their 

 previous mouths. Thus, from the north end of the Georges River discon- 

 tinuous osar system in Searsmont it is a less distance eastward to Belfast 

 Bay than southward to the coast at Thomaston or St. George. 



In marking the lines of synchronous retreat of the ice front the deposits 

 we have to depend on are, first, terminal moraines; second, marine glacial 

 deltas; third, frontal or overwash aprons of glacial sediments. There are 

 many such deposits in Maine, but unfortunately they are several or many 

 miles apart and no contemporaneous deposits connecting them have as yet 

 been recognized. In the table on page 393 these deposits are divided into 

 classes. The order of deposition was first determined for each glacial ri^'-er 

 sejDarately. Of two neighboring deltas the one that was north of a line 

 passing through the other parallel to the general coast line was assumed to 

 be the later deposit. This assumption is unsafe, but is the best practicable 

 test at present. Obviously the rate of retreat of the ice front is determined 

 by the ratio between the rate of terminal melting and ice flow. Naturally 

 the symmetry of the retreat is much marred in a hilly country, and may be 

 still more in a country where the deeper valleys for 100 miles or more from 

 the coast were beneath sea level. If the ice melted more rapidly before 

 the advancing sea than on the land above sea level, then long bays on the 

 fiords of the sea would deeply penetrate the border of the ice; if slower, 

 there would be formed lobes of the ice reaching as capes into the sea. Thus 

 the long. Penobscot and Kennebec valleys Avere at that time below sea level 

 for more than 100 miles. Their general trend is nearly parallel with that 

 of the ice flow, and both open northward into numerous tributary valleys. 

 They were thus favorably situated for a rapid rate of ice flow. 



Two glacial rivers flowed from the Penobscot Valley eastward through 

 low passes into the valley of Union River, where they deposited large 

 marine deltas that demand considerable time for their deposition. One of 

 these overflows was from Clifton to Otis, the other from Greenfield to 

 Aurora. Manifestly in both cases the ice lingered in the Penobscot Valley 

 all the time the}^ were forming, while the open sea prevailed over the valley 

 of Union River. Both glacial rivers departed from places in the Penobscot 

 Valley that were considerably below the highest sea level. The distance 



