Pre-Illinoian glacial stratigraphy in north-central Missouri

Five pre-Illinoian tills are recognized and named informally in northern Missouri, near the southernmost margin of the pre-Illinoian Laurentide ice sheet. The three youngest tills (McCredie formation) have high (50-60%) expandable clay mineral contents and normal remanent magnetic polarity. The next oldest till (Moberly formation) has low (30-40%) expandable clay content and reversed polarity, The oldest till (Atlanta formation) has very low or no expandable clay minerals. Its remanent magnetic polarity is unknown, The sequence of four pre-Illinoian tills above the Atlanta formation probably correlates with the similar sequence of four pre-Illinoian ''A type'' tills in western Iowa and eastern Nebraska. The upper four tills in Missouri are also correlated, in order of increasing age, with the three members of the Wolf Creek Formation and the older Alburnett Formation in eastern Iowa. The oldest till in Missouri possibly correlates with the ''C type'' till in western Iowa, Pre-Illinoian Laurentide ice reached the same approximate southern margin at least five times. Those advances deposited tills which maintain characteristic compositions over distances of at least 350 km along flow lines. (C) 1996 University of Washington.