760 ABSTRACTS OF PAPERS 



Discussion 



Prof. D. W. Johnson : Field observation and the inspection of the excellent 

 maps of the Yosemite Valley prepared by Mr. Matthes impress me with the 

 important control exerted by joint planes on the details of cliff sculpture in the 

 Yosemite, and also with the remarkable fidelity with which contours represent 

 such details when the right man is behind the pencil. An examination of Mr. 

 Matthes' map when it first appeared convinced me that Branner's explanation 

 of the notches beside the Yosemite Falls would not apply in a number of the 

 cases cited, and an examination of those notches last summer confirmed my 

 belief that many of the notches result from weathering along joint planes and 

 have never been occupied by streams. Only a few weeks ago did I learn that 

 Mr. Matthes had come to the same conclusion a year or two earlier. 



Mr. Matthes apparently supports the theory that stream erosion on the 

 Jointed rocks is competent to explain the peculiar features of the Yosemite, 

 sometimes ascribed to glacial erosion; in other words, the joint structure ex- 

 plains the broad bottomed, overdeepened valley. I should like to suggest 

 another interpretation, namely, that the glacial overdeepening of the main 

 valley and the oversteepening of its walls has made possible the remarkable 

 cliff sculpture guided by joint planes. Not until the glacier formed its deep 

 trough, with almost vertical sides, could weathering and gravity work so 

 effectively as to produce the present topography. Overdeepening by normal 

 stream erosion even in jointed or fissured rocks does not seem competent to 

 produce a deep, open, flat-floored valley, whose tributaries hang 2,000 and more 

 feet above the valley floor. 



FURTHER LIGHT ON THE GORGE OF THE HUDSON 

 BY JAMES F. KEMt 



(Abstract) 



The paper gave the latest evidence furnished by the deep borings in the 

 Hudson Valley at the Storm King crossing of the New York City aqueduct, 

 and cited the results of the construction of the Pennsylvania Railroad tunnels 

 opposite Thirty-third street, New York. The facts were interpreted, involving 

 a discussion of the general problem of glacial overdeepening. 



Discussion 



Dr. J. W. Spencer : I should like to ask if Professor Kemp has any evidence 

 of this being the former northward development of the Hudson Channel north 

 of Storm King, as topographic features suggest such diversion. Lake Cham- 

 plain is 400 feet deep, besides which there has been considerable northward 

 warping in recent post-Glacial time, and the depth of the Saguenay is about 

 900 feet. The Gulf of Saint Lawrence shows a recent subsidence of 2,000 feet 

 or more. The topography suggests that the Highlands once formed a divide 

 between a branch of the northern Saint Lawrence drainage and the more 

 rapid grades to the drowned Hudson River canyon extending to more than 

 100 miles seaward from New York. 



Professor Ke]\tp replied that there were ledges of Hudson River slate all 

 across the river a few miles above Troy, and that unless there were a buried 



II 



