APPALACHIAN CYCLES 34 I 



destructive agencies new powers. The peneplain is attacked and 

 carved into valleys and hills, the valleys being rapidly cut down 

 to base-level, while the divides and hills are much more slowly 

 removed. If time enough be granted, the rugged country formed 

 from a dissected peneplain is in its turn worn down to a second 

 peneplain at a lower base-level. This alternate upheaval and 

 wearing down together constitute a cycle of denudation, from base- 

 level back to base-level. A complete cycle is one in which the 

 whole region is reduced to a peneplain before the reelevation 

 occurs, and a partial or incomplete cycle is one which is inter- 

 rupted by upheaval before the region is cut down, and only small 

 and local peneplains have been formed. From a study of an old 

 region several cycles of denudation may frequently be made out, 

 represented by the remnants of dissected peneplains at different 

 levels preserved in the harder rocks. The successive adjustments 

 of the drainage system are a valuable auxiliary in working out the 

 history of the cycles. 



As an excellent example of these cycles of denudation whose 

 marks are preserved in the structure of the rocks, we may take 

 the Appalachian Mountains, which have been studied with great 

 care. The cycles have been worked out elaborately, but only an 

 outline of the more striking events can be given here. 



These mountains began as a great geosyncline in which 

 throughout the vast lengths of the Palaeozoic era (see p. 365) 

 were accumulated enormously thick masses of shoal water sedi- 

 ments. At the close of that era a number of crustal movements 

 set in, crushing the sides of the geosynclinal trough, and crum- 

 pling the mass of strata contained in it into a series of roughly 

 parallel, closed, inclined, or overturned folds, forming doubtless 

 a very lofty range of mountains. During the long ages of the 

 Mesozoic era (see p. 441) the mountains were attacked and worn 

 down by the destructive agencies ; and by the time the Creta- 

 ceous period was reached (see p. 474) the range had been re- 

 duced to a peneplain, with only a few hills rising above its 

 almost featureless level, — hills which are now the peaks of west- 

 ern North Carolina, the highest points of the range at present. 



