ELEMENTS IN SAND-PLAIN FORMATION 453 
shows conclusively that the forward growth of the delta must 
have been extremely rapid, for an ice margin bathed constantly 
by the waters of the sea or a glacial lake could not long remain 
stationary. That the time of growth was a short one was early 
urged by Davis and is generally admitted by those familiar with 
such deposits. Even in the case of many of the larger plains 
it seems clear that the time of growth should be measured by 
months rather than by years, and the assumption that they are 
the result of a single season’s stream work is not unwarranted. 
If this view is accepted, the question at once arises as to 
whether the plain represents the deposits of the whole, or only a 
part of the season of ablation. The first thought would natu- 
rally be that they represent the whole, but on more careful con- 
sideration this seems less probable. The observations upon sand- 
plains show that there was practically no backward melting of 
the ice during their formation. The retreat, then, must have 
taken place under conditions more favorable to the melting or 
breaking up of the margin of the ice sheet than those existing 
during the formation of the plains, and to maintain that the 
sand-plains represent the whole of the summer periods of melting 
would mean the reference of the periods of retreat to the winter 
season, a conclusion not in harmony with the laws of nature. 
As an alternative, it might be considered that the sand-plains 
represent summers of only moderate warmth, while the periods 
of’ melting were characteristic of seasons of a considerably 
higher average temperature. The melting of the ice, the dis- 
charge of the glacial streams, and the amount of detritus, would 
all have been increased under such conditions. In reality we 
find that often only a slight deposition of sediments took place. 
between the stages of sand-plain growth, especially when plains 
are but short distances apart, as in the case of the Bar- 
rington and Nyatt Point sand-plains, hereafter to be described. 
Here the intermediate area is practically free from deposits of an 
inter-sand-plain period, indicating that the retreat of the margin 
took place during a period when little detritus was being set 
free from the ice by ablation. 
