488 GLACIAL GEAVBLS OF MAINE. 



while the sea stood at high level, the two rivers combined would have filled 

 up the bay, as I conceive. Yet the land slopes at this time must have been 

 almost as steep as at present, and were much steeper than when the sea 

 stood at its highest level. The conditions would be favorable to transporta- 

 tion from up the valleys, yet the late deltas are comparatively small. The 

 most reasonable interpretation is that the supply of sediment fell off greatly 

 as soon as the ice had melted. 



SIZES OF THE VALLEY-DRIFT RIVERS. 



Professor Dana postulates in the Connecticut Valley a river large enough 

 to fill all the space between the terraces — a condition inadmissible in Maine. 

 The broad osars and the uneroded valley di-ift all point to sedimentation by 

 the rivers open to the air, as taking the form of rather level plains, not as 

 high terraces bordering a deep central channel. 



The hypothesis that there was a greater elevation of the interior than 

 of the coast region of Maine helps clarify some heretofore very doubtful 

 points of interpretation. At elevations extending from 350 to 450 or 500 

 feet are plains of valley sediments up to 5 miles in breadth, and in a few 

 cases they are somewhat wider. If these great sheets are valley drift, they 

 demand very large rivers. But if they are in large part marine beds, i. e., 

 fluviatile deltas formed offshore in bays or fiords, we do not need so large 

 streams to account for them. From the sea margin back to the ice these 

 rivers were dependent, like ordinary rivers, on the annual precipitation. 

 Within the ice-covered area their waters were glacial. But the drainage 

 systems of the ice-sheet did not conform to those of the land. Any attempted 

 comparison of the sizes of the valley-drift rivers with the present rivers must 

 take into account the amount of glacial waters that was diverted from one of 

 the present valleys by glacial streams, or that was brought into it. Such 

 calculations are necessarily difficult. The valley drift is more abundant in 

 valleys that once contained the larger glacial rivers — that is about all we 

 know. 



Valley-drift time was relative. In each valley it lasted from the melt- 

 ing of the ice until the supply of glacial water was all cut off. Whatever 

 inay have been the annual precipitation, the flow of the valley-drift rivers 

 was not only that due to this precipitation, but also that due to the net 

 melting' or wastage of the ice-sheet. 



