80 IOWA STUDIES IN NATURAL HISTORY 



conncctins' the cccGtr.s, cuch an the divide from Cashton to 

 Prairie du Chien, described above (Fig. 17), seem to sug- 

 gest that there was once a plain on non-resistant, as well 

 as resistant material. Thess connecting ridges are ap- 

 parently remnants of an upland surface once continuous 

 across inter-cuesta areas. (7) If the Dodgeville plain is 

 an old peneplain there is no necessity for conceiving that 

 the divides have been reduced by hundreds of feet in a 

 cycle of erosion in which the streams have scarcely reached 

 grade. (8) There are distinct erosion remnants standing 

 on the plain, far from present drainage lines. In some cases 

 these remnants consist of material which is more resistant 

 than that outcropping on the adjacent plain. In other cases 

 the rock of the remnant and the rock of the plain are the 

 same. There seems to be no reason why these divides 

 should have been reduced in such a way as to leave rem- 

 nants above their general flat surfaces, unless the streams 

 reached grade at or near the levels of these surfaces. (9) 

 The presence of stream deposits at many places on the plain 

 not only appears as a fatal objection to the single cycle 

 theory, but it seems practically to demonstrate that the 

 Dodgeville plain is a raised peneplain. (10) Both Creta- 

 ceous sediments west of the Driftless Area and Tertiary 

 deposits to the south must have been derived at least partly 

 from erosion in the Driftless Area and both bear evidence 

 that the land of their sources was low and approaching the 

 peneplain stage. (11) The fact that Tertiary deposits of 

 the great Mississippi embayment, extending north toward 

 the Driftless Area, lie in a gently sloping plain which, if 

 projected, would coincide with the Dodgeville plain, is dis- 

 tinctly in favor of the peneplain theory. (12) As will be 

 explained more fully within the next few pages, there are 

 lense-shaped bodies of the softer formations underlying the 

 Dodgeville plain in two or three places, which show that the 

 surface over them was brought to grade. (13) In addition 

 there are within the Driftless Area other strong evidences 

 of more than one cycle of erosion, yet 'to be described, which 

 in combination with the even-crested summit areas increase 

 the value of these upland flats as evidences and the com- 



