EXTENT AND TOPOGRAPHICAL EFFECTS OF EROSION. 



5. EXTENT AND TOPOGRAPHICAL EFFECTS OF 

 EROSION OYER THE CONTINENTS. 



1. Extent of erosion. — The outlining of mountain-ridges and 

 valleys has been in part produced by subterranean forces frac- 

 turing the strata ; but the final shaping of the heights is due to 

 erosion. This cause has been in action from the earliest time, and 

 nearly all rocks not calcareous have resulted from the erosion of 

 pre-existing formations. The Appalachians have probably lost by 

 denudation more material than they now contain. Mention has 

 been made of faults of even twenty thousand feet along the 

 course of the chain from Canada to Alabama. In such a fault one 

 side is left standing twenty thousand feet above the other, equi- 

 valent in height to some of the loftier mountains of the globe; 

 and yet now the whole is so levelled off that there is no evidence 

 of the fault in the surface-features of the country. The whole 

 Appalachian region consists of ridges of strata isolated by long 

 distances from others with which they were once continuous. 

 Fig. 103 represents a common case of this kind. It is supposed 

 by some geologists that the Appalachian and western coal-fields 

 were once united, and that, in western Ohio and other parts of the 

 intermediate region, strata thousands of feet deep, from the Lower 

 Silurian upward, have been removed, and this over a surface 

 many scores of thousands of square miles in area. This view has 

 been questioned on a former page. Whether true or not, there is 

 no doubt that the anthracite coal-fields of central Pennsylvania 

 were once a part of the great bituminous coal-field of western 

 Pennsylvania and Virginia (fig. 559, p. 323). They now form iso- 

 lated patches, and formations of great extent have been removed 

 over the intervening country. The Illinois coal-region is broken 

 into many patches in consequence of similar denudation and 

 uplifts. 



In New England there is evidence of erosion on a scale of vast 

 magnitude since the crystallization of its rocks. On the summit- 

 level between the head-waters of the Merrimac and Connecticut, 

 there are several pot-holes in hard granite ; one, as described by 

 Professor Hubbard, is ten feet deep and eight feet in diameter, 

 and another twelve feet deep. They indicate the flow of a 

 torrent for a long age where now it is impossible ; and the period 

 may not be earlier than the Post-tertiary. Many other similar 

 cases are described by Hitchcock. 



These examples of denudation are sufficient for illustration. 



