EROSION AL HISTORY OF DRIFTLESS AREA 99 



eral northerly and southerly slopes from the axis of the 

 arch were not steep, but are believed to have been steep 

 enough to control the general courses of the streams. The 

 details of the courses might be influenced by the minor 

 structures such as anticlines, synclines, accentuations of 

 the monocline, faults, joints, etc., by local irregularities in 

 the surface, or by slight differences in resistance. The con- 

 ditions during this first stage of adjustment are illustrated 

 in Fig. 28. 



Fig. 28. Block diagri'am illustrating: the drainage conditions in the Driftless Area, 

 as they should have been in the initial stage of stream adjustment. 



As the main streams on the two limbs of the arch cut 

 downward they would, somewhere in their courses, pene- 

 trate the resistant Niagara dolomite and reach the relative- 

 ly non-resistant Maquoketa shale. On this soft formation 

 the main streams would develop broad valleys and would 

 send out tributaries (Fig. 29). 



When maturity of the erosion cycle was reached and the 

 inter-valley divides had been made narrow, the south-flow- 

 ing main stream, having greater volume, or a higher gradi- 

 ent, or flowing on less resistant material than the stream 

 on the opposite side of the arch, might work headward 

 through the main divide and steal water by reversion of 

 the main stream flowing in the opposite direction. (Fig. 

 30). In this case it seems that the pirate stream would 

 work headward down the course already established by the 



