352 R. S. TAKR — LAKE CAYUGA A ROCK BASIN. 



direction from which the ice moved, must have aided in increasing the 

 depth. 



Chamberhn,"^ in discussing the possible southerly continuation of the 

 preglacial Cayuga river, points out a fact which, in considering the erosive 

 power of glaciers, is of much importance. He shows that to obtain a 

 great accumulation of drift we must have great erosion, just as in the 

 case of railroad construction cutting must be made before filling. He 

 therefore considers that it is more conservative to hold that there is con- 

 siderable erosion and considerable filling than that there is practically 

 no erosion and vast accumulation. The mass of morainic drift at the 

 head of Cayuga valley indicates much erosion, and Chamberlin was the 

 first to point this out, although few seem to have understood the impor- 

 ance of the conclusion. He says : f 



" It seems to the writer, therefore, quite clear that tlie assumption that involves 

 the least glacial corrasion is not that which denies to ice any notable action in the 

 modification of the topography of the rock surface, but rather that which assumes 

 that the rock prominences were ground down and some of the great channels of 

 flow excavated, while the remaining depressions were filled with the debris." 



Rhythm of Deposition and Erosion. — It is a striking fact in this region, 

 and one which the author has noticed elsewhere, in parts of New England, 

 that near the ice margin there is a rhythm of ice-work ; that is to sa}'^, 

 there is an alternation of erosion and deposition. On a small scale there 

 is the same phenomenon illustrated in drumlins, whether we explain 

 them by one or the other of the two most probable theories. Like a 

 series of waves with trough and crest, the ice appears to move, or per- 

 haps better, like the meandering river, depositing at one point and erod- 

 ing at another with a rhythmic curve of repetition. In the valley of 

 lake Cayuga there is a moraine at the south, then a zone of rock excava- 

 tion, followed to the northward by extensive deposition (or comparative 

 freedom from- erosion) J in the form of parallel drift hills of druralinoidal 

 outline, and again, I believe, by extensive erosion in lake Ontario basin. 

 What is the explanation of this ? Is it universal ? This rhythm of 

 glacial work impressed itself upon the mind of the author in New 

 England, where much less developed, and here in central New York it 

 appears to be illustrated in almost ideal perfection. 



Northivard Flow of Cayuga River. — Was the direction of flow of this pre- 

 glacial Cayuga river northward or southward ? Chamberlin § says upon 

 this point — 



* Third Ann. Rep. U. S. Geol. Survey, 1883, p. 356. 



t Third Ann. Rep. U. S. Geol. Survey, 1883. p. 356. 



I Johnson: Trans. N. Y. Acad. Nat. Sci., vol. i, 1882, pp. 77-80. 



g Third Ann. Rep. U. S. Geol. Survey, 1883, p. .3.55. 



