GLACIAL EROSIOK AND TRANSPORTATION, 229 



England and the extensive lime formations of the Appala- 

 chian region in America shall all be cu^solved and carried in 

 invisible solutions to the sea. The Mammoth Cave is but 

 the remnant of larger, longer, and more numerous caverns 

 which have honey-combed vast regions in Kentucky and in 

 Tennessee. Many of the extensive valleys of that region are 

 but the depressions formed by the falling in of the to 

 innumerable caverns. 



The rate of chemical erosion on limestone rocks is not 

 easy to estimate. The most elaborate attempt of which I am 

 aware is that of Professor A. L. Ewin£ in the Nittany Val- 

 ley, of Huntingdon county. Pa.* 



This valley, which is known by different names, extends 

 through a considerable portion of the Appalachian region. It 

 consists of the remains of a great anticlinal fold, which, had it 

 not been eroded away, would form an immense mountain-like 

 plateau over 20,000 feet above its present height. As it is. the 

 floor of the valley is composed of the upturned edges of the 

 lower Silurian limestone, eroded through a thickness of 

 feet. The valley is flanked on either side by the overlying 

 Medina sandstone, which forms monoclinical ridges from 600 

 to 1,000 feet above its floor. 



From data carefully collected. Professor Ewin-jf ascer- 

 tains that the amount of solid matter annually carried out of 

 the valley in chemical solution is equal to a layer of 2TTT3- °^ 

 a metre in thickness. Hence, to .lower the surface t-:» the ex- 

 tent of one metre by this process would require 29,173 

 year- — that is, it would take about 9,000 years to remove one 

 foot from the surface. 



It is safe to assume that had the rocks of this region been 

 similar to those of the bordering mountain- in their nature and 

 power to resist dynamical agencies, we should have in place of 

 Nittany Valley a mild anticlinal plateau somewhat above the 



* See " Proceedings of the American Association for the Advancement of 

 Science," vol. xxxiii. 1S^4. p. 4 4. The paper was published in the "Second 

 Geological Survey of Pennsylvania, TV pp. 451-i54. 



