64 GLACIAL GEAVELS OF MAINE. 



to erosion (the common tlieory) until it is proved that erosion is now going 

 on at such a rate as to justify the induction that the terraces could have 

 been eroded within the time that has elapsed since the Valley Drift period. 

 Thus, for instance, the Kennebec and Sandy rivers are bordered for many 

 miles by bluffs or terraces 50 to 80 feet high, and between these bluffs lies 

 a valley one-fourth to three-fourths of a mile wide. On the erosion theory, 

 there is a very large amount of denudation and transportation to be accounted 

 for. We have already noted that areas of the marine sands and clays from 

 1 to even 5 miles in diameter have been eroded by rains and streams to a 

 depth of 10 to 70 feet or more. 



According to a common theory of stream erosion, the terraces were 

 eroded directly by the rivers as they wandered back and forth over their 

 flood plains, or by their lateral branches. On this theory the base of every 

 bluff or terrace was once washed by the river or its tributaries, at least in 

 time of flood. This process of erosion by meandering can be seen in oper- 

 ation in many valleys, and is no doubt a common, and in greater or less 

 (jgo-ree a universal, process. But there is in operation in Maine a process 

 which is often far more efficient in eroding wide valleys than meandering. 



We have seen that the upper stratum of the valley drift is usually 

 coarser than the lower. Hence the surface waters soak readily through the 

 porous upper stratum until they reach the rather impervious underclay. 

 They then seep laterally through the basal layers of the sand and gravel 

 and along the top of the clay until they find exit in the form of boiling 

 springs. The same thing happens at the plains of glacial sand and gravel, 

 only in this case the water is generally arrested by the till. Thus, boiling 

 springs often reveal the presence of glacial gravel hidden beneath marine 

 clay. The reasoning is as follows: Large boiling springs are rare in the 

 till, unless for a short time while the snow is thawing in the springtime. If 

 such a spring issues from a suspected ridge, the ridge is more likely to be 

 glacial gravel than till. The decisive test is fm-nished by the stones found 

 in the boiling spring and its outlet, which will be well rounded if the spring 

 issues from a mass of glacial gravel, and will not be the ordinary tillstones. 



A fine instance of recent erosion by springs can be seen a short dis- 

 tance south of Solon Village. The plain of the valley drift which occupies 

 the valley of the Kennebec River here extends for one-half mile or more 

 east of the river. Back from the river at varying distances up to one- 



