94. NEW YORK STATE MUSEUM 
When the ice front paused for a considerable time upon a rather. 
flat surface, the debris-laden streams emerging from the ice formed | 
what is called an overwash plain by depositing layers of sediment | 
over the flat surface. The finest illustration of such an overwash 
plain in the State is all of that part of Long island lying just south 
of the great terminal moraine, and known as the Jamaica plain | 
toward the east (see plate 12). 
When the ice front extended across a more rugged country, 
with valleys sloping away from the ice, the large glacial streams, 
heavy laden with debris, caused more or less deposition of materials 
on the valley bottoms often for many miles beyond the ice front. 
Such deposits, known as valley trains, are especially well developed 
along most of the large south-flowing tributaries of the upper | 
Susquehanna river in the Southwestern plateau province. | 
Glacial boulders, or erratics, have already been referred to; they — 
are simply blocks of rock or boulders from the top of the ice or 
within it which have been left strewn over the country as a result 
of the melting of the ice. They vary in size from small pebbles 
to those of many tons weight (see plate 40), and are naturally most — 
commonly derived from the harder and more resistant rock forma- 
tions. Thus erratics from the Adirondacks are very numerous in — 
east-central New York, some having even been transported to the 
southern border of the State. Erratics are often found high up 
on the mountains, and sometimes they have been left stranded in 
remarkably balanced positions. 
A very extensive glacial deposit, called the ground moraine, is 
simply the heterogeneous, typically unstratified, debris from the 
bottom of the ice which was deposited, sometimes during the ice 
advance, but most often during its melting and retreat. When it is 
mostly very fine material with pebbles or boulders scattered through 
its mass, it is known as fill or boulder clay. The pebbles or boulders 
of the till are commonly faceted and striated as a result of having 
been rubbed against underlying rock formations. 
Another type of glacial deposit of unusual interest is the drumlin 
which is, in reality, only a special form of ground moraine material 
or till. The typical drumlins of New York State are low, rounded 
mounds of till with elliptical bases and steeper slopes on the north 
sides and with long axes parallel to the direction of ice movement 
(see plate 42). In height they rarely exceed 200 feet, being most 
often less than too feet. The origin of the drumlins has not yet 
been satisfactorily determined, though it is known that they formed 
near the margin of the ice either by the erosion of an earlier drift 
