404 GLACIAL GRAVELS OP MAINE. 



look for agencies operating- along the whole coast. Horizontally these 

 changes take place within a zone generally not exceeding about 30 miles in 

 breadth, but sometimes, especially in the larger north-and-south valleys, 

 exceeding that limit. Some of the systems end some distance back from the 

 coast in marine deltas, and such are not here included among the coastal 

 o-ravels proper. The southern ends of such of the systems as reach nearest 

 the coast lie approximately at or near the northern ends of the bays or 

 fiords of the coast. Vertically the northern ends of the discontinuous sys- 

 tems are found at elevations from 50 up to 350 feet; their southern ends 

 have elevations rather less than 50 feet. 



The existence of numbers of glacial potholes near the shore proves the 

 presence of subglacial streams in the coastal region south of the ends of 

 the gravel systems. The scantiness or absence of gravels at the shore by no 

 means leads to an inquiry as to the local absence or feebleness of subglacial 

 streams near the sea. The problem is an entirely different one: How did 

 it happen that at nearly the same elevation all but five of the glacial rivers 

 along 200 miles of coast found themselves with so large a supply of water, 

 as compared with the sizes of their tunnels, that they were able south of 

 that line to sweep their tunnels clear of sediments, or nearlj^ so, while above 

 that level and to the northward they left, in channels within the ice, sedi- 

 ments that rapidly increase in quantity and continuity for 30 miles or more! 

 These changes are so great and so rapid that it is practicall)^ a revolution 

 we have to account for. 



Looking at the rapid transitions as we go north and south, we are 

 reminded that the zone of transition is approximately parallel with the 

 position of the ice front during the retreat, and we naturally seek for the 

 causes of these phenomena in some phase of the ice-sheet's structure or 

 behavior consequent on its final melting and disappearance. On the other 

 hand, when we look at the great differences between the osar rivers as to 

 size and length; when we see how parallel some of them were to the ice 

 flow while others were for long distances transverse; how some flowed in a 

 single drainage valley of the land while otliei's spanned several such valleys; 

 how some were in broad north-and-south vallej^s, whei-e the ice flow Avas 

 faster, while others were south of transverse hills, where the flow was slower; 

 and yet all but five end before reaching the shore, and there is no proof 

 that these five extend far beneath the sea ; when we think of the small ver- 



