126 IOWA STUDIES IN NATURAL HISTORY 



monoclinorium which extends far beyond the boundaries of 

 the Driftless Area to the south and southwest. This move- 

 ment left the surface high above the level of grade, with a 

 stream divide on the axis of the fold. This event took place 

 after the Niagaran epoch of the Silurian period and prob- 

 ably after the Pennsylvanian period, but before the Creta- 

 ceous. The date may be set roughly at the close of the 

 Paleozoic era. 



Following its initiation, the surface was eroded in one or 

 more cycles and was brought to the condition of a plain 

 with a relief of less than 200 feet, the Dodgeville plain. The 

 cycle was not complete, but a stage at least as late as early 

 old age was reached. The divide at La Crosse was probably 

 obliterated before the close of this cycle. The stage in the 

 history of the region was probably brought to an end at 

 some time during or at the close of the Tertiary period. 



Probably in late Tertiary time the gravel-strewn Dodge- 

 ville plain was uplifted almost uniformly to the amount of 

 approximately 180 feet. 



The uplift mentioned in the last paragraph inaugurated 

 a new cycle of erosion known as the Lancaster cycle, which 

 continued probably until the advance of the pre-Kansan 

 glacier in the earliest part of Pleistocene period, by which 

 time a second peneplain, the Lancaster plain, had been 

 formed. Neither was this cycle of erosion complete. How- 

 ever, the surface at this time was much more nearly flat 

 than the present surface. The surface doubtless lacked 

 something of having gone so far in its stage of reduction 

 as was the case during the Dodgeville cycle. 



The Lancaster erosion cycle was interrupted most likely 

 at some time soon after the retreat of the pre-Kansan 

 glacier in the early Pleistocene, by a diastrophic uplift with- 

 out tilting or warping, amounting to 600 feet or more. This 

 movement raised the Lancaster plain to levels high above 

 grade and inaugurated a third cycle of erosion. 



The details of the post-Nebraskan, pre-Wisconsin history 

 of the Driftless Area are not known; but the history seems 

 to have been one of erosion interrupted locally and tem- 



