138 GEORGE C. MATSON 



also occur over the TuUy limestone, which caps the soft Hamil- 

 ton shales. In general, the amount of fall in the postglacial 

 gorges diminishes toward the north, although there are some 

 exceptions to this rule. 



The principal cause for this gorge condition, in the lower part 

 of the tributary valleys, is the fact that the streams have been 

 turned from their old channels and forced to cut new ones. 

 Another cause which has contributed to the same end is the 

 widening and deepening of the main valley by glacial erosion. 

 In some cases the streams have possibly been turned aside by a 

 moraine dam, but the most common obstructions are the deltas 

 deposited in the ice-dammed lakes'' which formed in front of the 

 continental glacier. These lakes fell to successively lower levels 

 when lower outlets were uncovered, and the streams continued 

 to flow along the course occupied when the lakes fell. The new 

 channels are mostly south of the old course, and Professor Tarr^ 

 has attributed this to the effect of the prevailing north winds on 

 the waters of the extinct lake. Coy Glen, being exposed to the 

 south winds and protected from the north winds, lies north of its 

 old channel. 



Inter glacial gorges. — In every case where the streams have 

 been turned aside in the manner just described there is a lower 

 course in the same broad valley, which is itself a gorge. Pro- 

 fessor Tarr has already called attention to these gorges in his 

 Physical Geography of New York? 



Interglacial (.^) gorges. — In central New York there are numer- 

 ous gorges which are broader than the postglacial valleys and 

 partially obscured by glacial till, showing that they were formed 

 either during preglacial or interglacial times. This class of val- 

 ley is especially well illustrated in Six Mile Creek, where its rela- 

 tion to the broad, mature preglacial valley is well shown. In one 

 case, near Taghanic Valley, lake beds containing fresh-water 

 fossils have been found beneath the till. 



One naturally thinks of these gorges as being interglacial in 



' H. L. Fairchild, Bulletin VI, Geological Society of America (1895), pp. 353- 

 74 ; T. L. Watson, Report, New York State Museum (1897), pp. 55-117- 



"^ Physical Geography, N. Y., p. 177. ^Pp. 178, 179. 



