274 



DENUDATION, OR GENERAL EROSION. 



Again, although it is probable that the erosive power of glaciers is 

 greater than that of rivers, yet their action is so much more local, both 

 in time and space, that we believe we may take the average rate of 

 river-erosion as a fair representative of the average rate of denudation. 

 Amount of Denudation. — A mere glance at the figures below will 

 show in a general way the manner in which geologists estimate the 

 amount of denudation in certain regions. In almost all countries, 

 especially in mountain-regions, we find slips varying from a few feet 

 (Fig. 247) to many thousands of feet perpendicular (Fig. 227). There 



are slips in the Appalachian chain which are 

 estimated to be 8,000 and even one 20,000 

 feet, in the Uintah 20,000, in the Wahsatch 

 40,000 feet, perpendicular. And yet in 

 most cases the escarpment, which would 

 otherwise exist, is completely cut away, so 

 that no surface-indication of the slip exists. 

 Evidently in such cases there must have 

 been erosion on the elevated side, at least equal to the amount of slip, 

 and probably much greater. The dotted line represents the probable 

 original surface. 



Sometimes the horizontal strata of isolated mountain-peaks corre- 

 sponding to each other (mountains of erosion) show that these are but 



Fig. 247 



Fig. 218.— Denudation of Red Sandstone, Northwest Coast of Ross-shire, Scotland. 



scattered fragments of a once high plateau, which has been removed by 

 erosion, as shown in the annexed figure (Fig. 248) and in many of the 



Fig. 249.— Section across Middle Tennessee. The dotted lines show the amount of matter removed. 



figures on pages 269 and 270. In such cases the erosion must have 

 been at least equal to the height of the peaks, and may have been to 



Fig. 250.— Section through Portions of England. 



