480 ELEMENTARY GEOLOGY 
Deposits. Moving forward, the ice dragged these rock 
fragments along, mostly beneath it as a ground mo- 
raine, but also m slight part within it. Excepting at 
its margin no morainal matter was carried on the back 
of the glacier, because no land projected above the 
surface. The ice was as clean and free from rock 
materials as the ice cap of Greenland. Even the 
highest mountains’ were covered (the Adirondacks of 
New York, the Green Mountains of Vermont, and the 
White Mountains of New Hampshire, for instance), 
and therefore the ice sheet was in places no less 
than a mile in depth. Where the glacial front stood 
in the sea, as it did east and southeast of New Eng- 
land, as it advanced it disappeared in icebergs, which 
floated away in the water; but where the termination 
was on the Jand, it extended to the poimt where 
melting exceeded supply, and there it ended in run- 
ning water. 
Along this land margin interesting results were brought about. 
As the water flowed from the ice, it sometimes formed marginal 
lakes where the glacier wall served to dam back the north-flowing 
streams; and in these lakes, beds of sediment were accumulated. 
In other places the streams emerging from the ice front crossed 
the pre-glacial divides, and cut them lower, so that when the ice 
withdrew from the land, some streams, whose pre-glacial course 
was northerly, were enabled to flow southward over their old 
divides, being then reversed. A very considerable part of the 
headwaters of the Alleghany River in New York and Pennsyl- 
