452 GLACIAL GKAVELS OF MAINE. 



bottom, and then the stream might, under favorable circumstances, build 

 up a single ridge having a shallow channel on its top, an exaggerated sub ■ 

 aqueous form of the ridge built above the sea by rivers at their deltas. 

 The buoyancy of the water would enable such a ridge to have much 

 steeper slopes than if formed above the water. As a glacial stream under 

 these conditions entered the sea or a lake it would naturally, as the flow 

 became less rapid in autumn, fill up the channel or channels in which it 

 had previously flowed, so as to leave the ridge with a broad flattish or 

 uneven top, or sometimes even rounded. If such a ridge wei'O beneath the 

 sea, the subsequent action of the waves would round off the summit to the 

 lenticular or rounded form. 



5. A ridge forms in the lee of an island, rock, mass of ice, or other 

 obstruction situated in the midst of a sediment-bearing stream. Such a 

 ridge has previously been described as forming in the Presumpscot River 

 below a bridge pier. 



All the above cases are instances of the general property of running 

 water to drop its sediments as its velocity decreases. The distance to which 

 the glacial rivers could extend ridges after issuing from the mouths of their 

 chamiels depended on the size and velocity of the rivers. At Litchfield 

 Plain the different ridges become confluent and are lost in the sand plain 

 within one-fourth of a mile, and at the small marine delta in Amherst the 

 sand jDasses into clay within the same short distance. On the other hand, in 

 the larger deltas it is 3 to 5 miles northward from where the ridges become 

 confluent to where they plainly were deposited in ice channels. It is diffi- 

 cult to determine the line between the ridges deposited in the sea in front 

 of the ice and those within channels near the margin, if for no other reason 

 than that the ice must have been retreating while the delta was forming 

 and the one formation would follow and overlie the other in the retreat. 

 If this retreat was for any great distance, the later sands ought to overlie 

 the earlier gravel ridges deposited near the ice front of the eai'lier times. 

 Thus far I have found no field evidence of such an order of deposition, 

 and it is doubtful if we can admit more than 1 to 3 miles of retreat while 

 the larger deltas were in process of formation. In addition to the retreat 

 of the ice front, we have to consider also the possibility that the sea was at 

 the same time rising or falling. 



