. Studies in natural history. Natural history; Natural history. 98 IOWA STUDIES IN NATURAL HISTORY to the south from the divide. In the knowledge that the formation of this arch antedated the establishment of the present course of the Mississippi river and that the various rock formations do not thin out appreciably in approach- ing the crest of the arch, it is possible to reconstruct the original surface for a line following the present course of the Mississippi river. This surface is found to lie at 960 feet at Bellevue, 1030 feet at Dubuque, 1520 feet at Prairie du Chien, 1850 feet at Lansi
. Studies in natural history. Natural history; Natural history. 98 IOWA STUDIES IN NATURAL HISTORY to the south from the divide. In the knowledge that the formation of this arch antedated the establishment of the present course of the Mississippi river and that the various rock formations do not thin out appreciably in approach- ing the crest of the arch, it is possible to reconstruct the original surface for a line following the present course of the Mississippi river. This surface is found to lie at 960 feet at Bellevue, 1030 feet at Dubuque, 1520 feet at Prairie du Chien, 1850 feet at Lansing, 2000 feet at La Crosse, 1870 feet at Winona, and 1410 feet at Minneapolis. A section showing this original surface and the attitude of the strata beneath it for the whole length of the Mississippi river from Minneapolis to Bellevue is shown in Fig. Fig. 27. A section showing the original surface of the Driftless Area and the struc- ture of the strata along a line now followed by the Mississippi river. The total horizontal distance is approximately 250 miles. Vertical scale: 1 inch equals 360 feet. Such a surface would be eroded by streams which would exhibit different stages of adjustment in different stages of the erosion cycle. In the first stage of adjustment streams would form, flowing south and north from the crest of the arch. From the main streams, tributaries would develop which would curve headward up the slope of the plain toward the divide from either side. As all the streams in this first stage were flowing on the Niagara dolomite there were only slight differences in resistance, and the courses of the streams would be determined primarily by the topographic slopes which were in turn determined by the structure. The gen-. 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
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectnaturalhistory, booky
