ICE-RETREAT IN GLACIAL LAKE N ETON SET 193 



plains, which were deposited in standing water at an altitude of 

 250 feet. These plains have an aggregate area of at least five 

 square miles, and a probable average thickness of 75 feet. Certain 

 reasons have already been given (pp. 183, 184) why these deposits 

 cannot be regarded as derived from the surrounding uplands, but the 

 most conclusive argument is their immense bulk, considered in connec- 

 tion with the absence of erosion features on the surrounding uplands. 

 That the material came from the ice, therefore, there can be no 

 doubt. The question then is: Were they deposited by subglacial 

 or by superglacial streams ? 



Half a mile southeast of East Sharon the plains, which here con- 

 sist of sands and of gravels with pebbles up to several inches in 

 diameter, stand at an altitude of 245 feet, or over 100 feet above 

 the valleys on the east, north, and west, across which the materials 

 must have been borne. If the gravels were transported by sub- 

 glacial streams, they must have been suddenly lifted a distance of 

 100 feet. To move a pebble 2 inches in diameter along a level 

 bottom requires a current with a velocity of 2.8 feet per second; 

 to lift it upward requires a velocity of about 3 feet per second. This 

 would correspond to an upward pressure along the assumed ice- 

 tunnel across the valley to the north of 43 pounds to the square 

 inch, and to indicate a head of 2 feet per mile in a straight conduit 

 like that of a circular pipe 6 feet in diameter. In a conduit with 

 the many irregularities of a glacial tunnel the head would probably 

 have to be 10 per cent, greater, or approximately 3^ feet per mile, 

 in order to give the observed velocity. Slightly lower figures would 

 be given by larger conduits. Such a head could be found at a dis- 

 tance of many miles to the northwest. In living glaciers subglacial 

 streams may possibly exist for considerable periods of time, but it 

 seems beyond the ground of probability that a subglacial stream, 

 with the pressure indicated, could continue over hill and valley for 

 long distances beneath the stagnant ice-sheet without finding a pas- 

 sage for the more ready escape of its waters. 



The temperature of a subglacial stream is always, according to 

 observations, slightly above the freezing-point. The corrosive action 

 of a stream, with the velocity and pressure indicated, would rapidly 

 eat the roof of its tunnel, until during a somewhat extended period, 



