176 GLACIAL GEAVELS OF MAINE. 



the principal terrace. The slopes of these " horsebacks" are about as steep 

 on the side away from the river as the erosion cliff which forms the bank 

 of the river. The depression between the central ridges or horsebacks and 

 the alluvial terraces at the sides is of varying breadth up to one-eighth 

 of a mile or a little more. This depression, being of varying depth and 

 confined between the central ridge and the lateral terraces, presents a suc- 

 cession of basins or kettleholes. Some of these are so deep as to contain 

 lakelets without visible outlets. Some of these ponds are said to rise and 

 fall with the water of the river. One pond in Moscow is said to be 40 feet 

 deep in time of high water. By aneroid its surface was several feet higher 

 than the river; hence it must be fed by springs from the side of the valley 

 faster than the water seeps through the alluvium into the river. All this 

 favors the hypothesis that there is a body of coarse alluvium along the 

 central part of the valley rather easily permeable by water. In some 

 other respects the interpretation of the facts is beset with difficulties. The 

 ridges plainly have the appearance of being uneroded portions of an 



Fig. 20. — Section across Kennebuc Valley, a, present situation of river. 



alluvial plain which once extended across the valley. It is not so easy to 

 account for the basins and lakelets in a vallej^ of erosion. Here is the 

 bottom of a so-called channel of erosion 50 to 75 feet higher at one place 

 than at another only a short distance up or down stream. Such ups and 

 downs are of frequent occurrence in the depression situated between the 

 ridges and the terraces at the sides of the valley away from the river. If 

 this depression has been cut down into the alluvial plain by water, then we 

 must account for the very unequal depth of the channel. This question 

 will be referred to hereafter. If we assume that the two-sided ridges result 

 from the unequal erosion of a once continuous alluvial plain, why did not 

 the river continue enlarging its channel laterally instead of forming a new 

 one on the other side of the valley, leaving the central portion of the plain 

 imeroded! The true answer to this question probably is that the alluvium 

 of the central part of the valley is composed of so much larger stones that 

 it is less easily eroded than the drift at the sides. In many places the 

 •central ridge is largely composed of cobbles and bowlderets, and for some 



