ORIGIN OF THE VALPARAISO MORAINE, VAN BUREN COUNTY, MICHIGAN

The Valparaiso moraine in southwestern Michigan was originally mapped by Leverett and Taylor (1915) as a morainal system formed along the southeastern margin of the Lake Michigan lobe of the Laurentide Ice Sheet. More recent work places the final glaciation of this area into the Crown Point Phase of the Michigan Subepisode (13,800-15,500 yr B.P.). Ongoing surface mapping and stratigraphic investigations indicate that the “moraine” is a series of upland tracts formed by a variety of ice-marginal and non ice-marginal glacial processes. Much of the upland is probably better described as a till plain. In these areas a discontinuous diamicton caps a rolling upland surface. One small part of this surface contains about 10 drumlins trending east-southeast. A rotasonic borehole on one of the drumlins revealed a lacustrine sequence approximately 30 m thick, which fines downward to a gray, clay-rich offshore unit and then coarsens downward to a hard, gray diamicton. This lacustrine unit appears to be widespread in the Valparaiso upland, suggesting that a sequence of proximal to distal lacustrine depositional environments persisted at the site as the ice retreated following deposition of the lower diamicton, becoming proximal again throughout the readvance that deposited the upper diamicton. Highly deformed lacustrine sediments immediately underlie the upper diamicton. Impoundment of this proglacial lake to the east was provided by the higher terrain of the Kalamazoo moraine. Ice-marginal landform assemblages make up both the eastern and western margins of the upland. The term, “Valparaiso moraine” should probably not be retained for this set of diverse landform sediment assemblages.