Post-settlement alluvium lies on top of rich organic top soil.

Deep, rich soils predominate at Herbert Hoover National Historic Site, despite land use changes that resulted in the loss of 40 inches of topsoil in eastern Iowa in the last century. This land falls within the Heartland Bread Basket, which still produces more corn and soybeans than anywhere else in the world.

The landscape consists of gently rolling terrain caused by water erosion. Uplands range in elevation from 710 feet to 780 feet above sea level. Soils originated from loess (fine wind-blown silt) deposition on top of ancient glacial material. The soils consist of five distinct silty-clay-loam types with moderate to slow permeability and are susceptible to sheet erosion.

Native prairie encouraged development of rich topsoil. About 48 inches of rich native topsoil overlays the loess. Another topsoil layer of approximately 24 inches lies above the native layer near the creek. Floodwater from the creek carries this soil from the upper watershed and deposits it on the floodplain. Geologists in Iowa refer to this soil layer as “post-settlement alluvium,” since the origin is soil erosion from modern land use upstream.