EROSIONAL HISTORY OF DRIFTLESS AREA 77 



(6) In a region where so much erosion has taken place 

 it would seem that the relative hardness of the resistant 

 formations would express itself in topography if there had 

 been but one cycle. The most resistant of the three should 

 stand the highest, and the least resistant the lowest. But 

 the general southward slope of the Dodgeville plain is un- 

 interrupted by differences in rock hardness, although it is 

 doubtless true that local resistant rocks have influenced 

 local irregularities in its surface. (Compare Fig. 16 with 

 Figs. 13 and 14) 



(7) If the upland surfaces in the Driftless Area are en- 

 tirely structural it would seem likely that the local anti- 

 clines and synclines which interrupt the general monoclinal 

 dip would cause undulations in the surface of the Dodge- 

 ville plain. In the Galena and Elizabeth quadrangles, where 

 a portion of this plain is well known and where the anti- 

 clines and synclines have been carefully mapped, there is 

 no apparent relation between the altitude of the upland 

 surfaces and the folds. The plain bevels the local folds 

 without any expression of the structure in the topography. 

 It is true that there are a few high parts of the upland 

 plain which correspond roughly with anticlines, such as at 

 Waukon, but there are also many high places where there 

 are no anticlines, many high places over synclines, and 

 many low places on anticlines. No general effect of local 

 structure can be observed in the topography of the plain. 

 The higher portions of the upland plain are to be inter- 

 preted as resistant portions of the rocks or as original 

 inter-stream areas, not as arched structures. 



(8) It is abnormal for the divides of a surface to be 

 lowered greatly before evidences of old age in adjacent 

 valleys appear. During youth of the normal cycle of ero- 

 sion the main work of the streams is in the development of 

 valleys and the dissection of the original surface. Maturity 

 is ushered in when the upper flat approaches thorough dis- 

 section and lasts until lower flats are formed and come to 

 constitute an appreciable portion of the surface. Most of 

 the work of lowering the original divides is accomplished 



