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| Des Moines Till Member | |
|---|---|
| Name | Des Moines Till Member |
| Type | Till member |
| Age | Pleistocene |
| Period | Quaternary |
| Region | Iowa, Illinois, Minnesota, Wisconsin |
| Country | United States |
| Unitof | Wisconsinan glacial deposits |
| Thickness | variable |
| Lithology | diamicton, clay, silt, sand, gravel |
Des Moines Till Member The Des Moines Till Member is a Pleistocene glacial till unit recognized across parts of the Midwestern United States. It is an ice-contact diamicton that has been correlated with late Wisconsinan advances and is significant for reconstructing Laurentide Ice Sheet dynamics, Quaternary stratigraphy, and surficial geomorphology in states such as Iowa, Illinois, Minnesota, and Wisconsin.
The Des Moines Till Member has been described in stratigraphic reports and surveys by institutions including the United States Geological Survey, the Iowa Geological Survey, the Illinois State Geological Survey, and the Minnesota Geological Survey. Field studies by researchers affiliated with University of Iowa, Iowa State University, University of Illinois Urbana–Champaign, University of Minnesota, and University of Wisconsin–Madison have documented its physical characteristics, depositional relations, and mapping extents. The unit is often discussed in the context of regional syntheses by authors associated with the National Research Council (United States), the Quaternary Research Association, and during meetings of the Geological Society of America.
Stratigraphically, the Des Moines Till Member typically overlies older tills such as the Iowan Surface-related deposits and may be overlain by loess units correlated with the Peoria Loess or by younger alluvium associated with rivers like the Des Moines River and the Mississippi River. Lithologically, it is a matrix-supported diamicton composed of clay, silt, sand, and gravel with a diverse clast population derived from bedrock provinces including the Canadian Shield, the Superior Upland, the Driftless Area, and the Cedar Valley Formation. Petrographic and heavy-mineral studies reference provenance from formations such as the Galena Group, the St. Peter Sandstone, and the Jordan Sandstone. Sedimentologists have compared its fabric and clast imbrication to tills described in the Late Wisconsin glaciation literature and in regional reports by the Illinois State Museum.
Age control for the Des Moines Till Member draws on radiocarbon dates from associated organic horizons, optically stimulated luminescence (OSL) ages from overlying loess, and correlations with regional glacial chronologies such as the Wisconsin Glaciation and events tied to the Last Glacial Maximum. Correlative units in nearby areas include tills mapped as the Iowa Till and formations identified in Minnesota studies of the Des Moines Lobe and Iowan Surface. Chronostratigraphic work often references tephra correlations and paleomagnetic ties used in syntheses by the International Union for Quaternary Research and in compilations by the U.S. Quaternary Commission.
The Des Moines Till Member occurs across central and eastern Iowa, western Illinois, southern Minnesota, and southwestern Wisconsin, with surface exposures noted near towns and counties such as Des Moines, Polk County, Iowa, Story County, Iowa, Dubuque County, Iowa, Rock County, Wisconsin, and Winona County, Minnesota. Mapping products from the USGS National Geologic Map Database, state geological surveys, and county soil surveys (via the Natural Resources Conservation Service) delineate its extent in relation to glacial lobes like the Des Moines Lobe and drainage features including the Missouri River and Cedar River valleys.
Interpretations of genesis emphasize deposition directly from the base of the Laurentide Ice Sheet during a readvance or stillstand, with processes including basal shear, lodgement, and deformation of preexisting sediment. Analogs and process models are drawn from studies of ice-marginal environments such as those documented for the Laurentide Ice Sheet, Keweenaw Peninsula tills, and tills of the Appalachian Plateau in regional comparative research. Glacial geomorphologists reference features like drumlins, moraines, and ground moraines associated with the unit and invoke mechanisms described in texts by researchers at institutions including the Scott Polar Research Institute and the Institute of Arctic and Alpine Research.
The Des Moines Till Member is generally unfossiliferous in terms of in situ macrofauna because of glacial comminution, but it contains erratics and reworked fossil fragments derived from older Paleozoic and Precambrian units. Fossil fragments recovered from associated intertill beds or adjacent outwash units include brachiopods and trilobite fragments traceable to strata such as the Ordovician and Cambrian successions of the region, with taxonomic work referenced against collections at the Field Museum of Natural History, the Iowa Museum of Natural History, and the Illinois State Museum. Palynological investigations in related tills and adjacent loess sequences have provided pollen records used by researchers at Ohio State University and Michigan State University to infer late Pleistocene vegetational shifts.
From an economic perspective, the Des Moines Till Member influences surficial soil distribution, aggregate resources, and groundwater recharge characteristics important to municipalities such as Des Moines, Cedar Rapids, and Davenport. Engineering geology studies by departments at Iowa State University and consulting firms working with the Iowa Department of Transportation evaluate its bearing on foundation conditions, road cut stability, and till-derived aggregate used by contractors in projects involving the Federal Highway Administration and state DOTs. Quaternary deposits linked to the member also affect agricultural soil patterns mapped by the United States Department of Agriculture and influence land-use planning by county governments and regional planning commissions.