44 IOWA STUDIES IN NATURAL HISTORY 



in altitude of the surface are too abrupt to be accounted for 

 on the basis of warping. 



Subsidence: It is a quibble whether or not the sub- 

 sidence of a surface starts a new cycle of erosion. How- 

 ever, the study of the erosion cycles in a region may yield 

 evidence of subsidence. If it can be proven that all the 

 graded streams of a region have their beds at levels far 

 above beds which they previously occupied, it seems most 

 likely that the surface of the region has subsided. If the 

 establishment of grades below previous grades indicates up- 

 lift, the establishment of new flood plains above previous 

 erosional surfaces is an equally strong indication of sub- 

 sidence. If uplift raises a previously graded surface above 

 grade, subsidence lowers valley bottoms below the level of 

 grade. 



This principle seems to be illustrated in the upper Mis- 

 sissippi valley region, where the Mississippi river and its 

 main tributaries are at grade 100 feet or more above the 

 bedrock beneath. The fills in this region consist of glacial 

 and fluvio-glacial drift. It seems likely that the surface 

 subsided after the deep valleys were cut. 



Another possible interpretation is that the streams were 

 not so heavily loaded before the filling as now, or were 

 larger then than now, and consequently were able to reduce 

 their valleys to a lower depth limit in relation to the Gulf 

 of Mexico than is possible now. 



By the application of the principles outlined above for 

 uplift, it might be determined whether subsidence was uni- 

 form or was accompanied by tilting, warping or faulting. 



The Amount of Movement 

 The interpretation of erosional histories furnishes some 

 basis for determination of the amount of each diastrophic 

 movement. Streams which have reached their depth limits 

 in a first cycle of erosion may degrade their beds below 

 these old graded levels in the second cycle by approximate- 

 ly the amount of the uplift which rejuvenated them. The 

 difference in altitude between two consecutive graded sur- 

 faces, therefore, is roughly the measure of the amount of 

 the uplift which interrupted the one cycle and started the 



