116 IOWA STUDIES IN NATURAL HISTORY 



and straight streams with the Lancaster plain. Even- 

 crested summit areas, antecedent streams and fluvial de- 

 posits on divides constitute evidence of one ancient cycle, 

 and an intermediate plain which is a partial peneplain, in- 

 trenched meanders and associated sets of crooked and 

 straight streams afford evidence of another one. Below 

 the intermediate plain the streams in their deeply excavated 

 valleys show that a third cycle is involved. 



It is believed that the surface of the Driftless Area has 

 been eroded in at least three cycles, the first known one 

 being represented to-day by the Dodgeville plain, the second 

 one by the Lancaster plain, and the third one by the pres- 

 ent valleys below the Lancaster plain. These cycles are 

 called the Dodgeville cycle, the Lancaster cycle and the 

 present cycle respectively. 



THE HISTORY OF DIASTROPHISM 



The first recorded diastrophic event involved in the ero- 

 sional history of the Driftless Area caused the warping of 

 the strata to form the anticline with its axis crossing the 

 Mississippi river at or near La Crosse, and with its south 

 and north dipping limbs, together with the gentle and local 

 anticlines and synclines on the limbs of the larger fold. 

 This movement may or may not have accompanied or 

 caused the final withdrawal of the Paleozoic seas. It was 

 an uplift of the surface with warping. 



After the initial movement and the establishment of the 

 land surface, after the streams had reached grade and de- 

 veloped the Dodgeville plain, an uplift occurred which in- 

 terrupted the Dodgeville cycle and inaugurated the Lan- 

 caster cycle. Although the Dodgeville plain is not perfect- 

 ly parallel with the Lancaster plain nor with the present 

 flood plain of the Mississippi river, this uplift was not ac- 

 companied by marked warping or tilting. The local irregu- 

 larities of the Dodgeville and Lancaster plains are clearly 

 due to erosion rather than to diastrophism and are neglect- 

 ed in the following estimates. The relative directions and 

 amounts of slope of the Dodgeville plain, the Lancaster 



