454 MYRON L. FULLER 
A consideration of the conditions obtaining during the clos- 
ing stages of the ice sheet leads to the conclusion that the 
periods or steps of the retreat in the vicinity of the Barrington 
Plain were characteristic of spring, corresponding more or less 
roughly to the present months of March, April and May. 
Following is a summary of the reasons: (1) The insignifi- 
cant development, or complete absence of back-sets in the most 
typical sand-plains, show that the retreat was not characteristic 
of the summer period. (2) The fall represents a waning, and 
the winter a cessation of all the conditions that can in any way 
be regarded as favorable to the ice retreat. The retreat cannot, 
therefore, be regarded as characteristic of these periods. (3) 
Though the actual precipitation may have been no greater than 
in the winter months, precipitation in the form of rain probably 
reached its maximum in the latitude of northern United States 
during the months of early spring. It is well known that water 
attacks ice much more rapidly than air at the same, or even 
higher temperatures. The period of spring rains must, then, 
have been one of rapid ablation. (4) The precipitation of the 
winter months must have been mainly in the form of snow, which 
according to Upham, would reach a maximum within a com- 
paratively short distance of the margin. Under the influence of 
the spring rains the deep snow must have rapidly melted, helping 
swell the glacial, and especially the superglacial streams to 
sizable turrents. The rapidity with which such superglacial 
streams cut into the ice is well shown by some of the superglacial 
streams of Greenland, which, though usually short and of small 
size, have often sunk tosome considerable depth into the ice. 
Such streams near the margin of the waning ice sheet would 
have rapidly cut through the ice to the very bottom, leaving 
detached pieces of various sizes and shapes which, however, 
would melt with comparative rapidity. (5) The streams during 
the spring, being fed mainly from the melting snow or direct 
precipitation, would carry a proportionally small amount of 
sediment, and the detritus instead of being deposited at a single 
point, as in the case of the sand-plains, would be distributed 
