. Studies in natural history. Natural history; Natural history. EROSIONAL HISTORY OF DRIFTLESS AREA 13 first cycle and the lower portion in the second cycle of erosion. There are, however, distinct limitations to the use of in- terrupted profiles as criteria of rejuvenated streams. Any stream which flows across resistant to non-resistant rock, and which has not yet brought the resistant rock to grade, may develop an interrupted profile in a single cycle of erosion. (Fig. 2). If a relatively flat surface emerged. Fig. 2. Diagram showing how interrupted profile abc may be developed in a single c
. Studies in natural history. Natural history; Natural history. EROSIONAL HISTORY OF DRIFTLESS AREA 13 first cycle and the lower portion in the second cycle of erosion. There are, however, distinct limitations to the use of in- terrupted profiles as criteria of rejuvenated streams. Any stream which flows across resistant to non-resistant rock, and which has not yet brought the resistant rock to grade, may develop an interrupted profile in a single cycle of erosion. (Fig. 2). If a relatively flat surface emerged. Fig. 2. Diagram showing how interrupted profile abc may be developed in a single cycle of erosion with reference to sea level cd, due to inequalities in the hardness of the rock formations. from the sea and came to stand distinctly above its sur- roundings, as in the case of a plateau, water would run down the gently sloping summit surface and down the steep bordering slope and would acquire an interrupted profile which would last until the lower portion of the stream had worked headward to the very source of drainage. Any region therefore having unequally resistant rock or any flattish surface bordered by a distinct descent, may have streams with interrupted profiles. In the abstract, interrupted profiles in the streams of a region merely suggest that the region may be in the second cycle; they do not furnish strong evidence, much less proof, of a second cycle. They amount to strong evidence, only after all other possible interpretations have been eliminated by careful study in the field. Stream Terraces The uplift of a surface in which a stream has previously reduced its bed to grade and developed a valley flat causes the stream to intrench itself in the flood plain and form. 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
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