AN EXAMINATION OF THE ROLE OF BIOTURBATION ON STONE LINE FORMATION ON THE IOWAN EROSION SURFACE IN SOUTHEASTERN MINNESOTA

The Iowan Erosion Surface (IES) of northeastern Iowa and southeastern Minnesota has long been a topic of controversy. The earliest arguments revolved around the question of whether this region was formed by an “Iowan” glacial advance of late Pleistocene age. A part of this question revolved around the origin of the pebble band, or stone line, in the soils of the IES, which overlies the glacial drift and is covered with thin loamy material. R.V. Ruhe and associates seemed to put these questions to rest with a comprehensive study conducted in the 1960's, which concluded that the “Iowan” drift did not exist and this region was a landscape highly eroded by water with a thin layer of loess or loam over an older “Kansan” or “Nebraskan” (now simply lumped together as Pre-Illinoian). The stone line was explained as an erosional lag covered by this thin layer of loess or loam. Even though the controversy of the Iowan Erosion Surface is widely believed to have been resolved, the region is still not fully understood. Other processes, such as bioturbation, have been overlooked as means by which this surface has evolved. Many studies have been conducted that document how organisms translocate and restratify sediments. The primary area of activity is the upper soil horizons and the activity may result in finer materials, such as sand or silt, being transported to the surface, while larger materials, such as pebbles or stones, being left behind. This project explored the soils of the IES to determine whether bioturbation had a role in the formation of the stone line in order to shed more light on the geomorphic processes that combined to form this complex landscape. The preliminary results of various laboratory tests, including particle size analysis, organic material content and optically stimulated luminescence dates will be presented as argument for the role of bioturbation on the soils of the Iowan Erosion Surface.