them confining units, or aquitards, which inhibit groundwater flow. Meltwater, as well as groundwater, tends to preferentially flow through subglacial sediment deposits, given that they are sufficiently porous, soft, and deformable. The flow regime is assumed to be either darcian or advective flows, although the precise method is often speculated and debated. Soil formation and fertility During the last glaciation, much of the Midwestern and Central United States were covered by glaciers. As the glaciers retreated and melted, much of the land was covered in till plains. These till plains were the basis from which the present day soil formed from. The parent material which these soils formed from varies greatly from one area to another, and is dependent on the path of the glacier which deposited the initial glacial till. This has caused different soils to form over time, but most generally share similar properties such as a loamy texture and poor drainage if derived from glacial till. The soils developed from till plains have become some of the most productive agricultural regions in the United States, due to the combination of thick till deposits forming rich fertile soils, and the gently rolling to flat topography being very suitable for farming practices. See also Glacial till plains (Ohio) Southeastern Wisconsin Till Plains (ecoregion) See plain article for other types of plains. References Glaciology Geomorphology