bluffline—perhaps only within the last few hundred years. In addition, ridges of loess once extended out into the valley, perpendicular to the present bluffline. These have been planed off, leaving steep, triangular-shaped faces known as truncated spurs. Older, higher floodplain levels (terraces) within the Missouri Valley also have been removed by erosion, though major tributaries such as the Soldier, Maple, and Boyer rivers still shelter these landform features and their geologic records. The principal landscape features formed along the Missouri valley margins since this erosional activity are alluvial fans. These apron-shaped deposits flare outward from smaller sediment-laden tributary streams that spill from the hills. On steep upper slopes within the hills, erosion also takes place by dislodgement of loess, encouraged by the pull of gravity. Catsteps, the striking stair-like features that mark many of the steeper slopes, result from periodic slipping and irregular downslope transfer of loess by gravity. These small slumps, good indicators of unstable slopes, also may form in response to trampling by livestock.
The physical properties of loess also contribute to a number of engineering problems and land-use hazards. For example, the style of roadcuts throughout the region reflects the capacity of exposed loess to maintain nearly vertical faces. Such excavations usually have steep single walls or are stepped back in a series of steep risers and horizontal treads. Cracks tend to develop along natural vertical partings or zones of weakness that often occur behind and parallel to these exposed faces of loess. Porosity and the natural steepness of loess slopes promote their relatively dry condition and apparent cohesiveness. During wet periods, however, when infiltrating water lubricates these vertical openings or when a rising water table saturates the base of an exposure, the seemingly stable loess can no longer bear its own weight. It collapses easily, sometimes resulting in serious landslides. Fresh scars on the landscape where these slope failures take place are prominent after heavy rains, and road-maintenance crews must periodically reopen roads blocked by slumped loess. The relocation of the War Eagle Monument on the bluffs at Sioux City was necessitated by recurring episodes of slope instability and collapse.
The erodibility of loess and its instability when wet pose other serious problems and land-use hazards in this region. Soil erosion rates are very high, and the amount of eroded sediment carried in streams draining the region is among the highest recorded in the United States. Gully erosion is especially pronounced, and these deep, narrow, steep-sided features are characteristic of the region’s smaller drainages (photo, left). Gullies lengthen headward, deepen, and widen quickly after rainstorms, cutting into cropland, clogging stream channels and drainage ditches, and forcing costly relocations of bridges and pipelines.
The Loess Hills are undoubtedly Iowa’s most fragile landform region in terms of susceptibility to erosion. The land is being shaped as rapidly as any terrain in the state. It is also clear from the geologic record that these dynamic episodes of erosion and deposition have been part of the scene for thousands of years. While gully erosion is intensified by stream channelization, loss of vegetation cover, overgrazing, cultivation, and other types of human disturbances, recent geological studies have shown that episodes of gully cutting and filling have occurred repeatedly during the last 25,000 years, significantly rearranging some of the thick accumulations of loess. Studies of buried gully fillings have also revealed previously unsuspected archaeological remains preserved because of these geological processes. The search for other archaeological sites can be guided by an improved understanding of the region’s landscape evolution.
The emphasis on erosion and the realization that much of its impact on the landscape is geologically quite young are important themes that will be repeated in other sections on Iowa’s landform regions.
Author: Jean C. Prior
Edited by: Drew Hutchinson