476 GLACIAL GRAVELS OF MAINE. 



HISTORICAL RELATIONS. 



In a preceding- cliaiiter the manner of the retreat of the ice has been dis- 

 cussed and the hnes of the front have been marked on the map (PI. XXXI) 

 as they are supposed to have been at various ])eriods. The lines of retreat 

 seem to indicate not only that the melting took place from above down- 

 Avard but that it was most rapid at the margin. They furnish no proof that 

 an}'- large bodies of stagnant ice were isolated from the main body by the 

 melting of the ice to the north of it, unless the ice situated south of east- 

 and-west glacial rivers be so considered. Thus, near Oxford there is proof 

 that, at a time when a broad plain of sand was being deposited, it was kept 

 from spreading into Tliompson Pond by the presence of ice in the basin of 

 that lake. The valley of the Little Androscoggin Eiver may at this time have 

 formed an arm of the sea from Oxford or Norway to Auburn. (See p. 225.) 

 In the Androscoggin Valley in Gilead and Shelburne, New Hampshire, also 

 in the Kennebec Valley from Embden northward, and elsewhere, the valley 

 drift often does not spread into lateral valleys. This suggests that these 

 laterals were filled by ice at the time the central plains were being deposited. 

 While thus there are indications that glacial channels often broadened till 

 they covered all the valleys in which thej were situated, and thus the purely 

 glacial sediments deposited in channels back from the ice front passed by 

 deo-rees into frontal bodies of overwash, the probability is that the retreat 

 of the ice as a whole took place from the margin and the glacial stream 

 channels were bordered by ice until the retreat of the general frontal line 

 back to that place. 



1. In valleys containing osars the larger glacial rivers were already 

 estabhshed, draining areas 5 to 10 or more miles in width. The same pro- 

 cesses that collected the glacial gravels with so few visible ravines of erosion 

 in the ground moraine or till sufficed to accumulate the material of the val- 

 ley drift in the channels of the glacial streams. In all cases the smaller 

 tributary subglacial streams seldom left gravels, except for short distances 

 near the main osars. This indicates that their channels were small as com- 

 pared to the flow of water. Most of their work, including tracts of erosion 

 of the ground moraine, glacial potholes, etc., has been covered out of sight 

 by the euglacial till. In a word, we have in the glacial streams a machinery 

 for diffused erosion without the ravines required by the hypothesis of till 

 erosion by rains and streams after the melting of the ice. 



