. Studies in natural history. Natural history; Natural history. 32 IOWA STUDIES IN NATURAL HISTORY nection an intermediate plain may be defined as one having a position intermediate between the 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. Diagrammatic 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 foll
. Studies in natural history. Natural history; Natural history. 32 IOWA STUDIES IN NATURAL HISTORY nection an intermediate plain may be defined as one having a position intermediate between the 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. Diagrammatic 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 the
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