. Studies in natural history. Natural history; Natural history. EROSIONAL HISTORY OF DRIFTLESS AREA 99 eral northerly and southerly slopes from the axis of the arch were not steep, but are believed to have been steep enough to control the general courses of the streams. The details of the courses might be influenced by the minor structures such as anticlines, synclines, accentuations of the monocline, faults, joints, etc., by local irregularities in the surface, or by slight differences in resistance. The con- ditions during this first stage of adjustment are illustrated in Fig. Fig. 28.
. Studies in natural history. Natural history; Natural history. EROSIONAL HISTORY OF DRIFTLESS AREA 99 eral northerly and southerly slopes from the axis of the arch were not steep, but are believed to have been steep enough to control the general courses of the streams. The details of the courses might be influenced by the minor structures such as anticlines, synclines, accentuations of the monocline, faults, joints, etc., by local irregularities in the surface, or by slight differences in resistance. The con- ditions during this first stage of adjustment are illustrated in Fig. Fig. 28. Block diagram illustrating the drainage conditions in the Driftless Area, as they should have been in the initial stage of stream adjustment. As the main streams on the two limbs of the arch cut downward they would, somewhere in their courses, pene- trate the resistant Niagara dolomite and reach the relative- ly non-resistant Maquoketa shale. On this soft formation the main streams would develop broad valleys and would send out tributaries (Fig. 29). When maturity of the erosion cycle was reached and the inter-valley divides had been made narrow, the south-flow- ing main stream, having greater volume, or a higher gradi- ent, or flowing on less resistant material than the stream on the opposite side of the arch, might work headward through the main divide and steal water by reversion of the main stream flowing in the opposite direction. (Fig. 30). In this case it seems that the pirate stream would work headward down the course already established by the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original University of Iowa; State University of Iowa; Making of America Project. Iowa City, Ia
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectnaturalhistory, booky
