22 A.ORI01 LTUBAL MANUAL 



Another type of glacial deposit is the " drumlin," or low, 



rounded mound, in height from 100 to 200 feet. Thousands of 

 these " drumlins " may be seen in western .New York between 

 Syracuse and Rochester, as one passes over the New York Cen- 

 tral railroad. 



FORMATION OF LAKES 



Prior to the ice age no lakes existed. Through many of the 

 pre-glacial valleys flowed streams. These valleys were deepened 

 by the glaciers. Some of the material carried by the ice was 

 deposited in the more narrow places, damming up the valleys, thus 

 forming lakes, some of them much larger than at present. As 

 in the case of Lake Ontario, the southern shore of which was the 

 " Ridge,*' extending through Niagara, Orleans, and Alonroe 

 counties, the numerous muck lands in this region are portions of 

 the bed of the old lake. The formation of these lakes, and in some 

 cases their decrease in size and volume of water, or their entire dis- 

 appearance, accounts' for the disappearance of ancient rivers, whose 

 old beds are now farm lands. At other times new rivers were 

 created or the volume and course of existing ones materially 

 changed. To illustrate: one such was called " Rome River," which 

 flowed westward from Little Falls, through Utica and Rome, and 

 into the basin now occupied by Lake Ontario. West Canada Creek 

 was then tributary to this river. The Sacandaga River flowed into 

 the Mohawk, instead of into the Hudson, as now. 



Estimates of the duration of the glacial period vary from 

 500,000 to l,5Q0,000t years. Southern New York was free from 

 ice w r hen the northern portion was still covered. Estimates vary 

 as to the length of time since the passing of the ice period, the 

 average being about 25,000 years. At the beginning of the glacial 

 period, the region of New York Stare was much higher than 

 today. Thia is clearly indicated by the Hudson River channel, 

 which has been definitely traced — a distinct trough in the ocean 

 bottom for 100 miles eastward from Sandy Hook. 



MORE RECENT CHANGES 



At the close of the ice age the land subsided to a level even 

 lower than today. Then, the lowlands of Long Island were under 



