152 



GLACIAL GRAVELS OF MAINE. 



of coarser gravel, which were afterwards covered by at least 15 feet of fine 

 gravel, and finally sand and clay, more nearly horizontally stratified and 

 extending across the valley before mentioned. This remarkable depression 

 is bordered much of the way by a steep bank of till, especially on the 

 north. It extends for about 3 miles along the base of the high hills, when 

 it comes out to a very low north-and-south pass through the range. To the 

 west of this pass rises Hogback Mountain, and I will term it the Hogback 

 Mountain Pass. For a short distance east of this pass the bottom of the 

 U-shaped valley containing the osar-plain is rather stony, then for one- 

 fourth mile or more there is a curious narrow bog, occupying the northern 

 part of the valley, while on the south side is a level terrace, apparently 

 composed of till. This teiTace is several feet higher than the bog. A 

 cross section of the valley at this point, is shown in fig. 18. 



This part of the. valley is bordered by a blufi'of till 20 to 25 feet high. 

 It is as steep as the banks of most streams, and shows every mark of an 



erosion cliff. In this part 

 of the valley only very 

 local drainage takes place, 

 since even the little south 

 branch of the Half Moon 

 Stream enters the valley 

 to the east of this point. These facts establish the following conclusions: 

 The osar river came to the northern base of the high hills and turned 

 southwest along a small previously existing valley. This valley consisted 

 of a valley in the rock which had become deeply filled by till. The stream 

 flowing in the valley eroded the till to a considerable depth, leaving its 

 channel bordered by clifi"s of erosion. The narrow bog above described 

 was once an erosion channel deeper than the rest of the glacial channel. 

 Originally it formed a small lake, but by degrees has become peated 

 over. As the velocity of the osar river diminished during the final melting, 

 the osar-plain was deposited in the lower part of the channels, though 

 near the highest point of the region crossed but little if any gravel was 

 deposited. 



At the north end of Hogback Mountain Pass the system we have been 

 tracing from Hartland and St. Albans is joined by a tributary branch, which 

 begins near Freedom Village. It follows a low pass southward, over a hill 

 about 100 feet high, where its course is bordered by a bluff" of till so steep 



Fig. 18 Sectioij across channel eroded 



channel of 



1 the till; lIontTille. 



