Fillmore County lies within the eastern portion of Nebraska's loess plain, a region of soil deposited by the wind between 25,000 and 13,000 years ago, forming a plain that slopes to the southeast. The Big Blue and its tributaries have incised channels into the loess surface in places, but in much of the county the original plain remains. These loess-plain regions are characterized by extensive upland flats with shallow depressions, lined with fine-grained and relatively impermeable silt, and tend to form shallow ephemeral wetlands when filled with rain or snowmelt; such wetlands range in area from less than to more than .