66 GLACIAL GRAVELS OF MAINE. 



belt 20 to 40 miles wide next the coast there was a heavy fall of rain and 

 sleet. It is known as the "ice storm," because thick ice gathered on the 

 trees and broke down thousands of them, besides numberless branches. 

 The next July after the washout the matter eroded from the plain could be 

 seen overlying great numbers of limbs that had recently been broken off 

 and the tops of several small trees recently bent to the ground. The wash- 

 out, therefore, must have occurred during or soon after the ice storm. Evi- 

 dently the unusual rush of subterranean water was due to the snow melted 

 during the thaw, assisted by the rains of the subsequent ice storm. The 

 water seeped down through the porous gravel until it was stopped by the 

 till or solid rock, and it could then find exit onlj^ by flowing out from the 

 side of the gravel plain, which it did so rapidly as to effect the large erosion 

 above stated. 



We thus have not onlj^ the ordinary and unceasing erosion of porous 

 sediments by springs boiling up through them, but also from time to time 

 these extraordinary outbursts. The most destructive outbursts take place 

 in winter and spring. In Maine the ground ordinarily freezes in winter to 

 a depth of 2 or 3 feet, and it must often happen that the smaller outlets by 

 which the seeping waters escape will be frozen solid. The waters thus 

 temporarily dammed will accumulate until considerable pressure is attained 

 and will help to increase the velocity of the escaping water when at length 

 the ground thaws and a passage is forced. The dams or gorges which often 

 form in rivers when the ice breaks up in the spring must have the same 

 effect on porous valley alluvium. The pressure of the water above the ice 

 dam must sometimes cause a rapid seepage through coarse gravel and cob- 

 bles and the formation of erosive boiling springs at points below the dam. 

 As noted elsewhere, the erosive power of a stream when flowing out of a 

 mass of gravel is much greater than that of the same stream when sweep- 

 ing past the base of a body of the gravel. The remarkable amount of 

 erosion of osar-plains by even small streams is well illustrated near Knox, 

 between Canton and Livermore, and between Rumford and North Wood- 

 stock, as described elsewhere. It is noticeable that moderately coarse 

 gravel plains are eroded even more than fine sand. 



Universally, so far as my observation goes, the narrow ridges of glacial 

 gravel (kames and osars) have resisted erosion better than the large plains. 

 This fact seemed unaccountable until I began to investigate the action of 



