GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 273 
Importance of Sand Plains in Southern New England. 
It may truly be said that sand plains are the most plentiful and the 
best-developed form of stratified glacial deposits in New England. Their 
abundance and relative perfection, along with their significance, make 
them a fitting subject fora study of late glacial history ; at the same time, 
the relative — one might almost say complete — absence of other records 
of ice-front lakes in this area, such as wave-cut benches and scoured and 
ageraded outlet channels, makes sand plains doubly important. It is 
natural, then, that good work has been devoted to sand plains, and 
that the progress in the understanding of them has come from eastern 
Massachusetts. 
The first clear presentation of what a sand plain is came from 
Davis in 1890 (a). Much detailed work is also recorded in papers by 
Woodworth, Gulliver, and Fuller; and more recently the correlation of 
sand plains, in order to make out the history of ice-front lakes, has 
yielded definite results to Crosby, Grabau, and Clapp. Perhaps the 
most concise example is Grabau’s report on glacial Lake Bouvé (b), in 
which the history of a small ice-front lake in the southern part of the 
Boston Basin is rather thoroughly treated. 
Discordance of Level of Sand Plains. 
It is common in eastern Massachusetts to find that the sand plains in 
a single lake basin vary in altitude. Possibly the lakes were continually 
suffering changes of level ‘by reason of temporary blocking of their out- 
lets by icebergs or ice-dams, or perhaps as a result of great variation of 
supply of water from the melting ice. In that case we should expect 
to find deltas at all sorts of levels. Controlling conditions as irregular 
as the formation of ice-blockades would involve great irregularity in 
delta levels. 
If, however, there is no such disturbing element in an ice-front lake, 
one can still see how the deltas in a single lake might vary in level; for 
if we consider again the normal lake history already outlined, we find 
that from the natural succession of lower and lower water-planes, as the 
ice retreats across the basin uncovering lower and lower outlets, there 
must be lower and lower sets of deltas. Sand plains, then, might mark | 
different levels in the same lake; but in this case the variation would be 
perfectly systematic. Instead of a promiscuous arrangement of high and 
low deltas due to irregular blocking of outlets, there would be a definite 
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