408 GLACIAL GRAVELS OF MAINE. 



development of the glacial gravels in belts marking certain stages of the- 

 retreat such as T have not attempted to mark on the map. Thus far I have 

 found only two such stages — one at the coast above described, and the 

 other the overwash aprons deposited in valleys above sea level. While it 

 is probable that the osars were deposited somewhat recessively, yet the 

 absence of well-marked stages traceable in the different systems, except 

 such as bear a relation to the old sea level, indicates that the retreat of the 

 ice alone was insufficient to account for the termination of so many gravel 

 systems at nearly the same elevation. Besides, where the zones of accu- 

 mulation and waste were so wide as they must have been in so great an 

 ice-sheet, it seems hardly probable that retreatal phenomena would take the 

 form of a great transition within so narrow a belt The ice must have- 

 extended 30 miles beyond Mount Desert Island at the time it flowed over- 

 that island if it had a surface gradient of 50 feet per mile, which is twice 

 the average gradient of the ice surface between there and Mount Katahdin.^ 

 Without allowing for berg discharge, the ice would reach 60 miles south of 

 the coast, and perhaps actually reached half or two-thirds that distance. 

 The coastal gravels may have been deposited 20 or more miles from the 

 ice front. Under these conditions it will require direct and positive evi- 

 dence to connect the peculiar development of the coastal gravels with any 

 marginal phase of retreatal action. Various modifications of the hypothesis- 

 suggest themselves, such as a coincidence of the subsidence of the St. 

 Lawrence Valley with the close of the period of deposition of the coastal 

 gravels, whereby the flow of ice from Canada over the St. John divide was 

 impeded and the development of the osars of the interior of the State 

 became more perfect than that of the coastal gravels, which was arrested 

 while in the earlier stages, etc. 



What conditions favorable to such a development as is exhibited by 

 the coastal gravels depended on the seal 



The subsidence of the land beneath sea level, especially a greater sub- 

 sidence toward the north, would destroy part of the effective "head" of the 

 subglacial streams. Most of the discontinuous osar systems lie in regions 

 that were beneath the sea throughout their whole length. The absence of 

 marine deltas favors the conclusion that numbers of the shorter osar rivers 



'Distance, 120 miles; elevation of surface at Mount Katahilin, 4,500 feet; at Green Mountain,. 

 Mount Desert, 1,500 feet. 



