32 



IOWA STUDIES IN NATURAL HISTORY 



nectioii an intermediate plain may be defined as one having 

 a position intermediate between tiie summits of the high- 

 est elevations and the bottoms of the deepest valleys (Fig. 

 10). Or intermediate plains might be defined as plains 



Fig. 10. Diap:rarr.matic section illustrating an ideal intermediate plain. 



above which stand erosion remnants and below which are 

 valleys. The erosional history of such a region as is shown 

 in Fig. 10, would seem to involve two cycles of erosion and 

 to be somewhat as follows : ( 1 ) the formation of a land 

 surface at levels at or above the present summits, (2) the 

 reduction of the region and the formation of a peneplain, 



(3) the uplift of the region, rejuvenating the streams, and 



(4) the development of the valleys. This sort of topography 

 seems to be and is strong evidence of more than one cycle 

 of erosion, although it hardly amounts to proof. 



Plains having similar relations to erosion remnants and 

 valleys might be structural. It is conceivable that streams 

 might cut through soft surficial material, to a thick, hard 

 formation of rock, then find further degradation retarded 

 to such an extent that by processes of widening, the soft 

 material might be removed over wide areas before the hard 

 formation is cut through, leaving only a few remnants 

 above the level of the top of the resistant formation. The 

 surface in this stage might resemble a peneplain. Finally, 

 the streams might sink themselves below the hard forma- 

 tion and develop valleys at levels below the structural plain. 

 However, it seems difficult to conceive that, even under the 

 most favorable circumstances, intermediate plains of wide 

 extent could be formed in this way. Rock terraces might 

 be so formed, but hardly plains which spread across divides 

 from valley to valley. Such structural plains, also, should 

 be parallel with rock structure and everywhere located on 

 rocks more resistant than their surroundings. 



