t)avis.] 
480 
[May 18 , 
2. GLACTAL DEPOSITS IN GENERAL. 
Let us consider especially the forms taken by the land waste 
when dropped near the margin of the ice. The type of these 
forms is seen in the terminal moraine. It consists chiefly of 
dragged, carried or washed material, which accumulates near the 
margin of the ice, where the activity of transportation weakens. 
In its best development, a terminal moraine is continuous over 
long distances and the formation of its entire extension marks a 
definitely limited chapter in geological time. It is confluent 
backwards with the sheet of till or ground moraine that has been 
dragged and washed a less distance forward from its source. It 
is continuous forwards with an overwasli of gravel, sand, and 
clay, distributed as evenly as the foreground will allow, by the 
waters escaping from the ice. The distinctness and size of the 
moraine are functions of many variable factors, the chief of which 
are the supply of detritus, the activity of its carriage by move- 
ment of the ice or by glacial streams, the duration of glacial 
action, and the precision of the balance between supply and melt- 
ing, by which the margin of the ice-sheet is held steadily on a 
single line. A steady balance, however, is seldom maintained 
during any long lapse of time. The ice margin generally oscillates 
back and forwards. The deposits formed during the forward ad- 
vance are not preserved intact ; they are more or less completely 
obliterated by the subsequent dragging and grinding of the ice 
over them. Those formed during the final retreat of the ice are 
preserved with no more change than is determined by the later 
sub-aerial processes of destruction. As a rule, the surface of 
the uppermost glacial deposits in New England has suffered so 
little from the action of general denuding agencies that we must 
conclude that the disappearance of the ice-sheet from over New 
England occurred at a comparatively recent date. 
During the early . stages of the retreat of the ice front, it is pos- 
sible that a considerable forward movement of the ice ma}^ have 
been maintained ; but when the melting had advanced so far as to 
reduce the thickness of the sheet below the measure required for 
motion over our rugged hills, the ice must then have become 
stagnant — as suggested by Chamberlin — lying quiet on the land 
till it melted away. Under such conditions the deposit at the 
margin of the decaying sheet would consist only of materials con- 
