TESTS OF SLTBGLACIAL OE SUPEEFICIAL DEPOSITIOjST. 439 



lower parts of the Mattawamkeag Valley near Kingrnan. This enlarges 

 our claims for superglacial osar rivers from small streams near the ice front 

 to the long osar rivers themselves. 



Thus we here discover no crucial test between the two rival theories, 

 though the difficulties of the superglacial hypothesis are increased with 

 every complication, such as that involved in the claim of their ability to 

 form glacial lakes in which stratified gravels were deposited, and hence must 

 have reached to the bottom of the ice, or nearly, and that, too, at a distance 

 of 10 miles back from the ice front. It is a matter of observation that pools 

 which presumably would expand into lakes in a time of stagnation of the 

 ice movement are formed in Greenland where large surface streams flow 

 beneath the surface and escape subglacially, but no instances are recorded 

 where they form very deep lakes and escape supei-ficiall}^. 



In all cases known to me where the osars went up and over rather high 

 hills with long valleys to the north, such as the Portland system north of 

 North Woodstock, the Smyrna series north of Danforth, the Bridgton series 

 north of Baldwin, the north end of the Peru-Buckfield system, and others, 

 the field phenomena prove that the gravels of earliest deposition north of 

 the higher hills were deposited in rather narrow tunnels and that the streams 

 had considerable velocity. There are several cases of reticulated ridges on 

 the northern (up) slopes, which may, perhaps, be accounted for on either 

 theory. Where broad osars or lake deltas are found in such situations they 

 are plainly a rather late if not a retreatal phenomenon. 



A sufficient cause, as it appears to me, has been pointed out for the 

 restriction of the subglacial tunnels north of these hills, but I know of none 

 on the superglacial hypothesis. At Kingman we may perhaps account for 

 the absence of broad-channel phenomena by the convenience of the broad 

 channel or lake at Macwahoc, but in other places there is no such way of 

 accounting for the lack of broad-channel deposits in the valleys north 

 of hills. 



On the whole, I conclude that the subglacial hypothesis is strengthened 

 and the superglacial weakened by the behavior of the glacial rivers where 

 they crossed transverse valleys and hills. 



It is not here meant to assert that all the broad osar channels date from 

 so late a period of the ice-sheet as that assumed in this discussion. 



It must be admitted that the various tests for distinguishing in the 



