240 



SCIENCE 



[N. S. Vol. XXXVII. No. 946 



that seems adequate is the precession of the 

 equinoxes, having a variable but average 

 period of about 21,000 years. Taylor has 

 studied the Cincinnati-Mackinae moraine 

 series from this viewpoint, and concludes 

 that the fifteen rather equally spaced mo- 

 raines represent 75,000 to 150,000 years, 

 using the minimum length of the preces- 

 sion period.^ The eight or ten morainic 

 belts which we now recognize in western 

 New York may correlate with that many 

 on the Cincinnati-Mackinae meridian, but 

 the recession of the ice front on the Hud- 

 son-Champlain meridian probably repre- 

 sents a much longer time than the Ohio- 

 Michigan series. 



If the changes in geologic climates be due 

 to variation in solar radiation it is conceiv- 

 able that some minor secularity might be 

 responsible for the oscillations of the ice 

 front. Variation in the amount of carbon 

 dioxide content of the atmosphere can more 

 reasonably be invoked to explain the larger 

 and mare irregular changes in ancient cli- 

 mates than for the shorter and more regu- 

 lar changes that caused the ice front oscilla- 

 tions. The same is true of continental ele- 

 vation as a cause of colder climate. 



GEOLOGIC EFFECTS OF THE ICE SHEET 



Erosional work. — The subject relating to 

 glaciers that has been the cause of the 

 greatest difference of opinion is the erosive 

 power or destructional work. The writer 

 will here not discuss seriously glacial ero- 

 sion in general, but only so far as it applies 

 to New York." 



That mountain glaciers abrade their val- 

 leys and by moderate erosional work 

 change the V-shape to the U-shape has long 

 been apparent. The destructive work at 

 the head of the glacier in production of 

 cirques is fully recognized, although this is 



'Journal of Geology, Vol. 5, pp. 421-465, 1897. 



* For the argument in general see Bull. Geol. 

 Soc. Am., Vol. 16, pp. 13-74, 1905. 



largely atmospheric effect. All argument 

 for deep erosion by glaciers based on the 

 abrasional or plucking action of mountain 

 or stream glaciers fails when applied to 

 New York, as there were no effective moun- 

 tain glaciers in New England and New 

 York, at least not during the waning of the 

 Laurentian ice body. The ice disappeared 

 from the more elevated tracts, while linger- 

 ing in the lowlands. Whatever the erosive 

 power attributed to mountain glaciers of 

 Norway or New Zealand, it can not be in- 

 voked here, as New York had no such gla- 

 ciers. We have to consider only the work 

 of a continental glacier. Whatever de- 

 structive effects an ice cap may have under 

 its central or subcentral mass, it has long 

 been admitted that it is not a vigorous 

 erosive agent in its border zone or dissipa- 

 ting belt. The district in New York, the 

 Finger Lake area, which has been used in 

 illustration of glacial valley erosion, was al- 

 ways in merely the outer zone, or that of 

 predominant deposition by the Laurentian 

 glacier. All students of New York geology 

 practically agree on the lack of vigorous 

 ice erosion over all the rest of the state. 

 Those who have worked in the Adirondacks 

 and in the Champlain and St. Lawrence 

 valleys have noted the proofs of a weak 

 erosion.^ It has been shown by Gilbert and 

 the writer that erosion was weak on the 

 Ontario lowland of western New York. 

 The claim for deep erosion has been only 

 for the valleys of the Finger lakes, specially 

 Cayuga and Seneca, the claim based chiefly 

 on anomalous topographic features. 



The advocates of glacial deepening of 

 the valleys appeal either to vigorous cur- 

 rents at the bottom of the ice sheet or to 

 the tongue-like lobations of the ice front, to 

 deeply gouge the bottoms of the valleys so 

 as to produce hanging side valleys and 



"See N. T. State Museum Bull. 145, pp. 147, 

 171-172, 1910. 



