120 IOWA STUDIES IN NATURAL HISTORY 



therefore, that the last diastrophic movement in the history 

 of the Driftless Area was a subsidence of about 150 to 200 

 feet and that it took place sometime before, during, or just 

 after the partial filling of the valleys by fluvio-glacial 

 debris. 



If it be assumed that this subsidence was accompanied 

 by tilting, so that the south portion of the Driftless Area 

 subsided more than the north portion, the apparent parallel- 

 ism of the Dodgeville plain, the Lancaster plain, and the 

 rock-bottomed valleys, and the more gentle slope of the 

 present Mississippi flood plain would be explained. This 

 assumption is rendered unnecessary, however, if it be con- 

 sidered that the present Mississippi has a sufhciently great- 

 er volume and lighter load than all previously existing 

 streams, to allow it to develop and maintain a gradient one- 

 tenth as steep as any preceding gradient. 



The conclusion is reached, therefore, that at least four 

 different diastrophic movements affected the Driftless Area, 

 namely, (1) uplift with warping and tilting which initiated 

 the land surface; (2) a nearly uniform uplift of about 180 

 feet interrupting the Dodgeville cycle; (3) a nearly uni- 

 form uplift of 600 feet or more which started the excavation 

 of the deep valleys ; and (4) a subsidence, perhaps accom- 

 panied by tilting, which raised the level of grade to that of 

 the present Mississippi river. 



THE DATES OF EVENTS 

 The whole history presented in this paper is limited in 

 time between the Niagaran epoch on the one side and the 

 Wisconsin epoch on the other. The sequence of events has 

 already been worked out. The accuracy with which the 

 dates of these events can be stated depends upon the ac- 

 curacy with which the ages of the upland plains can be de- 

 termined. There has been disagreement concerning the 

 ages of these plains and perhaps the final conclusion will 

 have to await further work, but strong evidence now at 

 hand leads to the conclusions here presented. 



