40 IOWA STUDIES IN NATURAL HISTORY 



lava flows, or by the draining of lakes, have no genetic rela- 

 tion with diastrophism. On such surfaces the number of 

 diastrophic movements is one less than the total number of 

 cycles. 



The Nature of Movements 



Not only the number but the nature of diastrophic move- 

 ments should be determinable in an interpretation of the 

 erosional history of a region. There are several possible 

 cases: (1) uniform uplift of the whole surface ; (2) uplift 

 with tilting; (3) uplift with warping ; (4) uplift with fault- 

 ing; (5) subsidence of the surface, with the four possible 

 phases as outlined for uplift. 



Uniform Uplift: If a peneplain were formed and then 

 uplifted uniformly, there would be a change in altitude but 

 not in attitude. If the general slope of an old erosion sur- 

 face, be its remnants on the summits or at intermediate 

 levels in a topography, is approximately the same in direc- 

 tion and amount as the slopes of other graded erosional sur- 

 faces in the region, the inference would be that the uplift 

 had been uniform. The difficulty with this point lies in 

 the fact that no old erosion surface is perfectly flat, and it 

 is difficult to determine whether consecutive graded surfaces 

 are parallel. Also a graded plain might be uplifted uniform- 

 ly and yet not be parallel with a younger peneplain if the 

 streams had higher or lower gradients at the close of the 

 second cycle than at the close of the first. 



However, if a peneplain represented by even-crested sum- 

 mit areas is practically parallel with an intermediate plain, 

 and with present valley flats on which streams are at grade, 

 the conclusion would be warranted that both the uplift 

 which inaugurated the second cycle and the uplift starting 

 the third cycle were practically uniform. 



Uplift with Tilting: If a raised and partly dissected 

 surface which was once a peneplain has a generally uni- 

 form slope throughout a given region, but is not parallel 

 with an intermediate peneplain or with graded streams be- 

 low it, either because its angle or direction of slope is dif- 

 ferent, the conditions suggest that the uplift which in- 



