EROSION AL HISTORY OF DRIFTLESS AREA 41 



augurated the dissection of the upper plain was accom- 

 panied by tilting. If an upland peneplain and an inter- 

 mediate peneplain are essentially parallel, but are not 

 parallel with an existing and undissected peneplain, the up- 

 lift which started the second cycle was probably uniform 

 and the movement starting the third cycle was a tilting 

 movement. The best evidence of tilting in the renewal of 

 lands by diastrophism is a lack of parallelism between uni- 

 formly-sloping, consecutive, graded, erosion surfaces. 



As in most rules, there are limitations in the application 

 of this one. Lack of parallelism between consecutive ero- 

 sion surfaces, provided it is a matter of amount rather than 

 direction of slope, may be due to difference in the gradients 

 of final grades of the drainage system under diflferent con- 

 ditions at different times. At the close of a first cycle the 

 streams may have been small and carrying heavy loads, 

 with resulting high gradients and a relatively steeply slop- 

 ing peneplain. The uplift may have been uniform, but in 

 the second cycle larger streams carrying lighter loads may 

 have developed gradients lower than those of the first cycle, 

 and the two erosion surfaces would diverge upstream. By 

 the reversal of the sequence, two peneplains might be 

 caused to converge upstream, without tilting. 



Even differences in the direction of slope of two pene- 

 plains in a region might be obtained without tilting, if con- 

 ditions of structure, proximity to the sea or climate were 

 so changed, during uplift, as to cause reversal or diversion 

 of drainage in the second cycle. 



It is probable that these exceptions might lead to con- 

 clusions that tilting has taken place where it has not in 

 some cases, and that tilting has not taken place where it 

 has in others. Perhaps only the more pronounced cases of 

 tilting can be distinguished by this method. 



Uplift with Warping: Warping during uplift of a plane 

 erosion surface would result in an irregular obliquity be- 

 tween this surface and lower peneplains. The two erosion 

 surfaces would converge and diverge in many directions 

 and at many angles. In a region where such obliquity ex- 

 ists between a summit plain and an intermediate plain, but 



