The Loess Hills of western Iowa were formed from 20,000 to 10,000 years ago of finely ground windblown silt from the glacial deposits. As the Pleistocene glaciers melted, the Missouri Valley became a major channel for tremendous amounts of water. Each winter season, as the quantity of melt water was reduced, large areas of flood-deposited sediments were left exposed to the wind. Silt, clay and fine sand were lifted by the wind, carried to the east and deposited. The bedrock of Harrison and Monona Counties is limestone upon which lies a mantle of drift, sand, gravel, loess, and alluvium. It determines most of the surface features in the counties today.
Three layers of loess have been deposited in the Loess Hills--Loveland, Post-Kansan and Post-Iowan. The Loveland Loess is a water deposit formed during the melting of the Kansan Glacier. It is a compact, heavy, reddish clay which is very sticky when wet. It is highly valuable in the dry hills because of its ability to retain moisture.
The Post-Kansan Loess is fine and compact and light blue in color. It takes up moisture slowly when wet but retains it relatively well.
The Post-Iowan Loess is the immediate subsoil of the upland regions of the hills. It can occur up to 90 feet in depth, deeper closer to the Missouri River. It breaks vertically into irregular columns causing frequent slippage and faulting at the surface.
The thickness of the loess and differences between the soils formed in the loess are related to the distance from the source of the loess. The loess is the thickest in the bluffs, reaching more than 200 feet deep in some areas. In places, mainly on steep hillsides adjacent to stream valleys, the Wisconsin loess has been removed by geologic erosion. Here, glacial till, or in a few places the Loveland Loess deposit is exposed on the surface.
The Loess Hills are an outstanding example of two basic geologic processes the strong influence of past eolian or wind deposition as well as erosional sculpture of the land. These origins contribute to potential environmental hazards of slope failure and collapse. In addition, the association of the loess, the topography, and the vegetation combine for a classic display of the interdependence between geology and ecology. In this high relief area, the terrain supports a mosaic of unique ecological niches.
The soils of the Loess Hills State Forest were formed in loess, alluvium, and glacial till; most soil series formed in a loess parent material . Loess is yellowish-brown, wind-deposited material consisting largely of silt particles. This specific soil forming factor is what gives Iowa's Western Loess Hills their name as well as their peculiar form. The steep bluffs, comprised solely of loess soil, rise to between 150 and 250 feet above the Missouri River bottom land. The native vegetation of these soils was almost exclusively prairie grasses, with some timber along streams and drainage ways.
The steep, ridged topography, combined with the special physical properties of loess, create some problems in the Loess Hills. The angles of the slopes often range from 50 to 75 degrees; this is due to the geo-technical or engineering properties of the soil. The loess, composed primarily of coarse silt particles, has a very low shear strength when water-saturated, so that often it cannot bear its own weight. However, when relatively dry, the loess develops a greater apparent cohesion; this allows the loess to maintain the spectacularly bold bluffs and ridges-forms along the Missouri River valley. These special soil properties impose some serious limitations on any development involving roads and buildings.
Loess is easily eroded by running water. This factor, combined with its collapsibility, contributes to another major problem -- soil erosion and resultant gullying. Some of the highest soil erosion rates in the nation, averaging about 40 tons per acre per year, have been documented in this region. The resulting high sediment loads in local streams necessitates continual maintenance of drainage ditches and stream channels and results in detrimental conditions for many aquatic species.
The Loess Hills State Forest is made up of six general soil associations that are located in Harrison County and five general soil associations that are in Monona County. The Nine different soil associations of the two counties are the Sarpy-Albaton-Carr, Albaton-Haynie-Onawa, Luton-Keg, Kennebec-McPaul-Nodaway, Hamburg-Ida-Monona, Monona-Ida-Napier, Luton-Salix-Blencoe-McPual, Kennebec-Zook-Mcpaul, and the Hamburg-Ida-Castana-Napier.