ORIGIN OF THE MIDDLE SEGMENT OF THE MISSOURI RIVER VALLEY

Much of the upper and lower Missouri River valley (MRV) aligns closely with Wisconsinan and pre-Illinoian ice-margin positions, respectively. The position of the middle segment, between Sioux City and Kansas City, has been ascribed to recessional margins of unspecified pre-Illinoian glaciations, but specifics remain elusive. Nevertheless, several lines of evidence indicate that the middle segment of the MRV originated during the last pre-Illinoian glaciation or between that event and deposition of Illinoian-age loess. The spatial distribution of the youngest pre-Illinoian till further suggests that this course was an ice-marginal position along the western terminus of the last pre-Illinoian glaciation.

First, six pre-Illinoian tills are present in western Iowa and eastern Nebraska. Laminated clayey silts are widely preserved between these tills on both sides of the MRV. These silts likely were deposited within a network of ice-dammed lakes that formed repeatedly as ice advances blocked eastward flowing streams from the Great Plains. The fact that these lake sediments can be traced far eastward into Iowa means that the MRV was not present then to intercept the eastward drainage as it does now. Second, the thickness and grain size of pre-Illinoian loess decrease away from the Great Plains, but show no obvious relationship to the present MRV, a marked contrast with younger Illinoian and Wisconsinan loess. Thus, the MRV was a major drainage for glacial meltwater and source of loess during the Illinoian and Wisconsinan, but not during most of the pre-Illinoian. Finally, all but the youngest pre-Illinoian till (the “A1”) is widely present on both sides of the MRV. The A1 till, which is quite distinct lithologically from older tills, has not been found west of the MRV, even though it is commonly preserved just east of the valley. This pattern suggests that the middle segment is also an ice-marginal position, and that nearly the entire course of the modern Missouri River was established along ice margins of various ages.