FORMATION OF ROCK BASINS. 351 



that the extreme believers in this form of glacial erosion are not nearer 

 the truth than those who, like the author, have been inclined to doubt 

 the power of the ice in this direction ? 



Since beginning this study it has occurred to me to question whether, 

 as in my own case, the term rock basin has not implied mentally more 

 than it does in reality. In cross-sections and longitudinal sections of lakes 

 having a depth of several hundred feet the depth is greatly exaggerated. 

 Lake Cayuga, for instance, in the scale adopted by the engineering de- 

 partment of Cornell University, has a depth of more than two inches, 

 while in reality it should be, on the horizontal scale adopted, only a slight 

 bulging of the lower part of a line. On the scale used in that map, if the 

 vertical scale were the same as the horizontal, as it appears to be at a glance, 

 the depth of the lake would be nearly four miles.* This same point is 

 brought out by Ramsay for lake Geneva.f Lakes, even deep ones, are 

 therefore mere scratches when compared with the mass of ice. Their 

 depth is extremely slight compared with their width and length, and the 

 ice does not have to go down into a deep hole and ascend in order to 

 erode the basin. . On the contrary, it needs only to wear irregularly, and, 

 where conditions are favorable, this may be easily done. 



In the Finger lake region the ice, moving from the northward, after 

 entering the valley occu^^jied by lake Ontario, found its progress inter- 

 fered with b}'- the rising New Yorlc-Pennsylvania plateau. Naturally the 

 north-and-south valleys furnished lines of easiest escape, and naturally, 

 also, the ice motion was here more powerful and the ice deeper. That 

 the latter was true is proved by the fact that, even without the added 

 depth due to ice erosion, these valleys were, at the beginning of the glacial 

 invasion, at least 700 or 800 feet below the general upland level. This 

 increase in thickness means, other things l^eing favorable, an increase of 

 erosive power. While this of itself is a pure deduction, the facts in the 

 field prove its truth, for at the head of the lake valleys the moraine is 

 thick and extensive, while upon the uplands it is comparatively incon- 

 spicuous. Greater dei)Osition means a greater supply, and this indicates 

 more erosion and more motion to bring new supplies. 



It is a striking fact that almost all of these lakes have their northern 

 end in the hard Helderberg limestone and their southern ends and 

 deepest parts in the softer shales of Marcellus and Hamilton age. More- 

 over, the dip is gently southward. Perhaps even without these pecul- 

 iarities the ice might have eroded a rock basin; but cetainly this soft, 

 crumbly, thin bedded and well jointed shale, dipping away from the 



* Brigham : Bull. Am. Geograph. Soc, vol. xxv, no. 2, 1893, p. 15. 



t Physical Geology and Geography of Great Britain, fourth edition, 1874, p. 170 ; Quart. Jour. Geol. 

 Soc, vol. xviii. 18G2, p. ISo. 



