From the Iowa Wildlife Action Plan:
The glacial history and topography of each landform affect the type and distribution of current wildlife habitats and agricultural land use.
The Loess Hills
(Tallgrass Prairie) is a unique landform that formed at the end of the last Ice Age about 18,000 years ago. The formation is only one to fifteen miles wide but is about 200 miles long extending from near Sioux City, Iowa to St. Joseph, Missouri. Although deposits of windblown soils (loess) are found in many parts of the world, nowhere else but in China do they reach as high as in Iowa where some of the hills are more than 200 feet above the adjacent Missouri River valley. The Loess Hills landform has other features that are easily noticed. Bedrock is exposed naturally in only a few places and the soil has unique physical properties. If the topsoil on the slope of a hill is removed, the exposed loess will erode quickly and deep gullies will form. Even when covered with topsoil, loess can slump, often in a unified way across a slope creating “cat-step” ledges along the sides of hills. However, when a loess hill is cut vertically the exposed wall will stand for decades.
The Des Moines Lobe
(Prairie Potholes) has a landscape that is gently rolling with abundant moraines, shallow wetland basins or potholes, and a few relatively deep natural lakes. This landform still retains the imprints of recent glacial occupation. Loess is entirely absent. The most prominent landform patterns left by the Wisconsin glacier on the Des Moines Lobe are the end moraines. The Des Moines Lobe is part of the Prairie Pothole Region that extends north and west into western Minnesota, eastern North and South Dakota, and the Canadian Prairie Provinces. Most of the potholes have been drained with ditching and underground tile lines to make way for agriculture. Agriculture was also responsible for greatly increasing the rate at which streams and drainage patterns developed in this geologically young landform.
The Southern Iowa Drift Plain
(Tallgrass Prairie) is the largest of Iowa’s landforms. Like the Des Moines Lobe, it is composed almost entirely of glacial drift, but the Pre-Illinoisan glaciers that deposited material in this part of Iowa were much older. As a result, deep glacial drift, ranging from a few to several hundred meters, is the only evidence of their occupation. Instead of poorly drained and relatively level landscapes, streams have had time to erode the land surface and form well-defined drainage systems. Hilltops have similar elevations that reveal the approximate level of the land surface constructed by the last ice sheet. As erosion slowly dissected this landscape, a layer of loess ranging from 2 to 10 meters was deposited over the glacial till. Throughout the Southern Iowa Drift Plain the terrain varies considerably, but the pattern of relief resulting from its history of erosion is the dominant feature of the region. Many of the larger rivers had glaciers standing in their headwaters at the time the Des Moines Lobe was ice-covered. These valleys obtained much of their present width, depth, and alluvial fill from flooding as the Wisconsin ice sheet melted away from northcentral Iowa. In many places the rivers have cut through the glacial drift into the underlying sedimentary bedrock. The rough wooded terrain adjoining these valleys supports many scenic and recreational areas and important wildlife habitat.
The Iowan Surface
(Eastern Tallgrass Prairie) landform extends over a large region of northeastern Iowa and is characterized by long, gently rolling slopes, low relief, and open views of the horizon. Pre-settlement vegetation in this region was primarily prairie with heavily wooded floodplains along the larger watercourses. Drainage networks are well developed, but stream gradients are low with some scattered areas of poor drainage and natural wetlands. The area was once part of the Pre-Illinoisan Southern Iowa Drift Plain but experienced large-scale and more destructive erosion events, the latest occurring during the coldest part of the Wisconsin glaciation 16,500 to 21,000 years ago. Frost action, down slope movement of water-soaked soil materials, and strong winds were the dominant geologic processes in this region. Layers of loess are thin and scattered. Glacial boulders are numerous and many are very large. Elongated ridges and isolated oblong hills called pahas occur in the southern part of the Iowa Surface region. These features are covered with a mantle of silt and sand believed to have accumulated in response to strong northwesterly winds that occurred during the period of glacial cold. Soils mapped on the larger pahas indicate they developed under forest vegetation rather than prairie. Karst topography occurs in the northern part of the landform where cavities in the underlying limestone bedrock collapsed and formed sinkholes. Fens are also present but more scattered than in the Des Moines Lobe.
The Northwest Iowa Plain
(Eastern Tallgrass Prairie) contains many of the terrain features and geologic materials present in other landforms and is similar in appearance to the Iowa Surface with a uniform low relief. This landform was and still is a relatively treeless, gently rolling landscape. Despite these similarities, the landscape differs from other regions because of a combination of factors. The western uplands of this region are underlain with highly eroded, Pre-Illinoisan glacial tills. The eastern part of these tills is covered with later glacial deposits from an early Wisconsin glacial advance. The entire region was then subjected to vigorous erosion activity that accompanied the later advance of the Wisconsin ice sheet. As a result, features of a freshly glaciated landscape were lost as a well-established branching network of streams formed over the entire region. The deeper thickness of the loess mantle, the overall elevation of the land surface, and the present precipitation and vegetation distinguish the Northwest Iowa Plains from the state’s other landforms. Windblown loess is abundant and nearly continuous across the region ranging in thickness from 4 to 16 feet. Altitudes throughout the Northwest Iowa Plains are uniformly higher than any other portion of the state and topographically are continuous with the High Plains of the Dakotas. Average annual precipitation is lower than other parts of the state. Thus, the region is higher, drier and less timbered than any other in the state. Although bedrock exposures are rare in the Northwest Iowa Plains, the oldest bedrock in Iowa (Precambrian-age Sioux Quartzite) occurs here along the Big Sioux River.
The Paleozoic Plateau
(Prairie to Hardwood Transition) is the most distinctive of Iowa’s landforms because of its abundant rock outcroppings, karst topography, a near absence of glacial deposits, many deep narrow valleys, cool-water streams, and heavily wooded uplands. Numerous gorges and ravines cause abrupt local changes in the direction of slopes and exposures. These sites provide abundant cool, moist and wooded habitats rich in diverse communities of plants and animals. Seeps and springs are common features along valley sides where strata of varying permeability are exposed and signify subterranean drainage systems. Ice caves and cold-air (algific) slopes are unique to this area. Unusual microclimates associated with these features support a particularly rare and sensitive biological habitat in Iowa. The steep rocky slopes are unsuited for agriculture and remain heavily forested. Remnant prairies occur on southand west facing slopes. Ecologists believe these prairies were more extensive before the suppression of naturally occurring fires following European settlement.
, often called floodplains, are constructed by water flowing off of the landscape and carrying with it boulders, cobbles, gravel, sand, silt, and clay. This process of erosion creates a dendritic-shaped landform of nearly level corridors with varying widths depending on the size and reach of the river. These corridors are largest along the Missouri and Mississippi Rivers but can be found along streams throughout the other landforms. The floodplain is a dynamic landform that is frequently disturbed, sometimes drastically, by flood and drought events. Stream channels may be cut off leaving backwater sloughs or oxbow lakes. Large-scale vertical changes may also occur within the floodplain due to the deposition of alluvium that forms terraces and benches. These structures are level but are elevated above existing floodplains by a distinct slope. Smaller tributaries that enter the floodplain of a larger river often form alluvial fans that may cover older floodplain materials. During low flow periods, wind becomes an important factor in the transport of materials. Exposed sand or soil having little or no vegetation to hold it place can be blown onto floodplain and terraces as well as onto higher elevations along valley margins. Sand dune topography occurs downwind of valley floors.