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It is well known that several systems of drainage lines have been impressed upon the surface of the present state of Iowa, only to be successively wiped out by the hand of time. Not to mention possible earlier ones, a well-marked drainage system was cut into the Saint Louis and older strata prior to Des Moines time. Upon the (relative) subsidence of the land during Des Moines time the valleys were filled and drainage lines obliterated to the farthest limits of deposition of Coal Measures rocks. Differences of nearly 400 feet in the altitude of the Saint Louis surface near Des Moines give evidence of the vigor of the erosive forces and the lapse of time during which degradation was active. At least some of the Coal Measures outliers of eastern Iowa may occupy depressions cut during this period, and, as Doctor Calvin pointed out, the land surface of that time, probably stood higher than at present, since the base of the Coal Measures sandstones of the Iowa City outlier is sixty feet below present river level. Beyond the eastern limits of the Des Moines strata the drainage systems doubtless continued for a long time, though at times sluggish and ineffective as erosive agents.

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Proceedings of the Iowa Academy of Science





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©1914 Iowa Academy of Science, Inc.



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