ANDKOSCOGtilN LAKES POETLAND SYSTEM. 219 



Androscoggin River, which here is flowing southeastward. The gravel 

 soon turns southwest and ascends the valley of the west branch of the 

 Concord River through Milton and Bethel to the top of the divide near 

 North Woodstock, which is fully 125 feet above Rumford Point, and per- 

 haps as much as 140 feet. From the Androscoggin River to North Wood- 

 stock this valley affords an instructive study. The average slope is not 

 far from 25 feet per mile. The bottom of the valley was once occupied 

 by an alluvial plain from one-eighth to near one-half of a mile in breadth. 

 The osar ridge near the mouth of the Concord is lost in the plain of finer 

 sediments soon after it leaves the Androscoggin River. South of this point 

 a ridge is found along the axis of the valley. It is from 10 to 60 feet in 

 height, and is locally known as the " Whalesback." Both sides of the valley 

 are bordered by terraces having nearly the same height as the centralridge, 

 but composed of somewhat finer drift. Near the Androscoggin River the 

 material is sand. Going southward, it becomes coarser until, at North 

 Woodstock, we find only coarse gravel, cobbles, and bowlderets. Both the 

 central ridge and the lateral terraces are usually bordered by rather steep 

 banks. They are simply uneroded portions of the original plain which 

 extended across the valley. Two vallej^s of erosion have been formed, one 

 on each side of the central ridge. These erosion valleys, where observed, 

 do not cut down to the till, hence the osar-plain must have been originally 

 of great depth. The valley is only about 8 miles long, and the small brook 

 that flows in it does not receive any large tributaries. It is quite too 

 small to have deposited, even in the highest floods, such a gravel plain as 

 once filled the valley. Indeed, at first it seemed to me surprising that it 

 could have eroded the two large valleys on each side of the "Whalesback." 

 It was not until I had studied the remarkable erosive power of boiling 

 springs that I could assign any physical cause for so great an erosion in so 

 short a valley. 



The alluvial terraces of the Androscoggin Valley rise from 30 to 50 

 feet above the river at the mouth of the Concord. The Androscoggin at the 

 time it stood at its highest level must have backed up the valley of the 

 Concord for 2 miles or more, and would fill that valley with more or less 

 river alluvium. At North Woodstock the gravel rises 70 or more feet above 

 the highest terrace of the Androscoggin at Rumford. It is thus proved 

 conclusively that the gravel along the North Woodstock Pass was not 



