viii MAN AND THE GLACIAL PERIOD. 



tant new evidence, among others by authorities of no less 

 eminence and special experience in glacial investigations 

 than Professor Dana,* Mr. Warren Upham,f and Profess- 

 or Edward H. Williams, Jr. J Professor Williams's inves- 

 tigations on the attenuated border of the glacial deposits 

 in the Lehigh, the most important upper tributary to the 

 Delaware Valley, Pa., are of important significance, since 

 the area which he so carefully studied lies wholly south of 

 the terminal moraine of Lewis and Wright, and belongs 

 to the portion of the older drift which Professors Cham- 

 berlin and Salisbury have been most positive in assign- 

 ing to the first Glacial epoch, which they have main- 

 tained was separated from the second epoch by a length 

 of time sufficient for the streams to erode rock gorges in 

 the Delaware and Lehigh Eivers from two hundred to 

 three hundred feet in depth.* But Professor Williams has 

 found that the rock gorges of the Lehigh, and even of its 

 southern tributaries, had been worn down approximately 

 to the present depth of that of the Delaware before this 

 earliest period of glaciation, and that the gorges were 

 filled with the earliest glacial debris. 



A similar relation of the glacial deposits of the atten- 

 uated border to the preglacial erosion of the rock gorges 

 of the Alleghany and upper Ohio Eivers has been brought 

 to light by the joint investigations of Mr. Frank Leverett 

 and myself in western Pennsylvania, in the vicinity of 



* American Journal of Science, vol. xlvi, pp. 327, 330. 



f American Journal of Science, vols, xlvi, pp. 114-121 ; xlvii, pp. 

 358-365 ; American Geologist, vols, x, pp. 339-362, especially pp. 

 361, 362; xiii, pp. 114, 278; Bulletin of the Geological Society of 

 America, vol. v, pp. 71-86, 87-100. 



\ Bulletin of the Geological Society of America, vol. v, pp. 

 13-16, 281-296 ; American Journal of Science, vol. xlvii, pp. 33-36. 



# See especially Chamberlin, in the American Journal of Sci- 

 ence, vol. xlv, p. 192 ; Salisbury, in the American Geologist, vol. xi, 

 p. 18. 



