THE GEOLOGICAL HISTORY OF NEW YORK STATE 87 
Pew ERNARY PERIOD, INCLUDING NEW YORK IN THE 
GREAT ICE AGE 
The fact of the Ice age. The Quaternary is the last great period 
of earth history, and it still continues for it has led up to the present- 
day conditions. This period was ushered in by the spreading of 
vast ice sheets over much of northern North America and Europe, 
which must take rank as one of the most interesting and remark- 
able occurrences of geological time. On first thought the existence 
of such vast ice sheets seems unbelievable, but the Ice age occurred 
so short a time ago that the records of the event are perfectly clear 
and conclusive. The fact of this great Ice age was discovered by 
Louis. Agassiz in 1837, and fully announced before the British 
Scientific Association in 1840. For some years the idea was 
opposed, especially by advocates of the so-called iceberg theory. 
Now, however, no important event of earth history is more firmly 
established and no student of the subject ever questions the fact 
of the Quaternary Ice age. 
Some of the proofs of the former presence of the great ice sheet 
are as follows: (1) polished and striated rock surfaces (see plate 
39) which are precisely like those produced by existing glaciers, 
and which could not possibly have been produced by any other 
agency; (2) glacial boulders or “ erratics”’ which are often some- 
what rounded and scratched, and which have often been trans- 
ported many miles from their parent rock ledges; (3) true glacial 
moraines, especially terminal moraines, like that which extends the 
full length of Long Island and marks the southernmost limit of 
the great ice sheet; and (4) the generally widespread distribution 
over most of the glaciated area of heterogeneous glacial debris, both 
unstratified and stratified, which is clearly transported material and 
typically rests upon the bedrock by sharp contact. 
Ice extent and centers of accumulation. The best known exist- 
ing great ice sheets are those of Greenland and Anarctica, especially 
the former which covers about 500,000 square miles. This glacier 
is so large and deep that only an occasional high rocky mountain 
projects above its surface, and the ice is known to be slowly moving 
outward in all directions from the interior to the margins of Green- 
land. Along the margins, where melting is more rapid, some land 
is exposed, but often the ice flows out into the ocean where it 
breaks off to form large icebergs. 
The accompanying map (figure 29) shows the area of nearly 
4,000,000 square miles of North ‘America covered by ice at the 
