Upham.] 
242 
L April 17, 
received, at least in some instances, from one to six feet of till 
yearly deposited over their whole surface, so that the accumulation 
of these hills to their heights of 50 to 150 or 200 feet required only 
from two or three decades to a century of j^ears. Indeed, where 
they exhibit no such banding, I have thought that sometimes their 
entire formation may have been more rapid, so that the most mas- 
sive drumlins, like the largest esker ridges, were probably depos- 
ited in so short a time that their beginning, growth, and completion 
would occupy considerably less than a man’s lifetime. The drum- 
lins appear to have been heaped up beneath the ice-sheet within a 
few miles back from its margin, or perhaps occasionall}’- within even 
less than one mile, as seems to be suggested by Mr. Barton’s obser- 
vations. Where they are scattered over any extensive area, as 
raround Boston, probably they were not all in progress of deposition 
contemporaneously, but were successively accumulated as the ice- 
margin retreated. The plentiful occurrence of drumlins upon cer- 
tain belts of country, as in northeastern Massachusetts, in southern 
New Hampshire, from Spencer, Mass., to Pomfret, Conn., and in 
central New York, may therefore contribute important informa- 
tion, when they shall be fully studied, concerning the outlines of 
'the receding glacial boundary during the dissolution of the ice- 
sheet, supplementing the evidence supplied by marginal moraines 
:and kames, eskers or osars, and deflected striae. 
While some steps of progress seem to be gained toward a knowl- 
edge of the manner and time of deposition of the drumlins, the 
questions as to how the ice-sheet could amass these hills, and why 
they are distributed in abundance upon some districts, but are ab- 
sent or represented only by a few examples upon other large areas, 
remain still unanswered. Their distribution, however, is to so large 
a degree independent of topographic features, and of the character 
of underlying rock-formations, that the explanation of their origin 
and grouping appears more likely to be found in variable conditions 
of the glacial movements resulting from secular climatic changes 
during the final melting of the ice. 
Mr. Geo. H. Barton then presented a preliminary paper on the 
“Drift of Certain Portions of Middlesex County.” 
