Lessons in the Landscape: An Introduction to Iowa’s Geology

Written by Jean C. Prior

For many Iowans, there is a natural attraction to the land, and a curiosity about its shapes, composition and age. The field of geology answers these questions and explores how the Earth works. Geology invites us to look closely at our surrounding landscape, and also to examine its broader horizons and great depths, all with an expanded view of time. In encountering a towering bluff, a flowing spring, a fossil in a rock or a colorful agate on a sandbar, one finds beauty, discovery and a sense of the Earth’s past. Here in Iowa, that geologic history includes ancient seas, glacial ice, strong winds and flowing rivers. Looking into Iowa’s earth history leads to an increased appreciation of our surroundings, and an understanding of its link to the habitats of our native plants and animals. It also provides an interesting pathway to useful scientific knowledge.

Bedrock (540 million to 74 million years ago)

A captivating aspect of geology in Iowa is its scope. Starting with the oldest materials, the basic building blocks of the Iowa landscape include layered sedimentary rocks, especially limestone, shale, sandstone and dolomite. These are the hardened remains of ancient deposits that accumulated in shallow tropical seas, coastlines and deltas between about 540 million years ago and 74 million years ago. Sometimes these rocks contain fossil remains of animal and plant life that lived in both marine and terrestrial environments. Such paleontological treasures can include crinoids, brachiopods, corals, trilobites, and impressions of leaves. 

This bedrock foundation is most easily observed in northeast Iowa. Here sedimentary rocks of Paleozoic age dominate a surprisingly scenic region, the Paleozoic Plateau. Intriguing caves, springs, and sinkholes, known as “karst” features, result from the flow of underground water through natural vertical and horizontal fractures in shallow limestone and dolomite. 

Bedrock spanning various geologic ages is also seen as layered outcroppings along roadcuts, in rock quarries and along deeper river valleys throughout the state. A notable exception are reddish outcrops of Sioux Quartzite, a rock of durable quartz grains found in the far northwest corner of Iowa. This is the oldest bedrock seen anywhere in Iowa at about 1.6 billion years. Also notable on the Bedrock Geology Map of Iowa is a prominent circular feature, the Manson Meteor Impact Crater. It is invisible at the land surface because it is buried beneath glacial-age deposits.

Bedrock Geologic Map of Iowa, 2010, Iowa Geological Survey, University of Iowa Hydroscience and Engineering, Iowa City.

Glaciers (2.5 million to 10,000 years ago)

This brings us to the second basic building material of Iowa’s terrain, the much younger sediments left by glaciers pushing into the Midwest, and by the strong winds and swift meltwaters that accompanied the glacial activity. These deposits consist of loose pebbly clay (known as “glacial drift”), silt, sand and gravel. 

Glaciers originated in the Canadian arctic and advanced over all or parts of Iowa numerous times between 2.6 million and 10,000 years ago. Today, Iowa’s oldest glacial deposits are more than 500,000 years old and are seen across the Southern Iowa Drift Plain. These rolling hills and well drained landscapes show the long-term effects of stream erosion. 

During later periods of glacial cold between about 30,000 and 14,000 years ago, Iowa was free of ice, but the glacier’s mass lay close by in states to the north. Summer melting carried glacial grindings down meltwater rivers, especially the Missouri. During winter periods of low flow, strong westerly winds carried silt from the broad floodplain, and draped a gritty blanket of “loess” across most of Iowa. These deposits are unusually thick along the eastern margins of their Missouri Valley source. Later erosion of this silt gave rise to the unique, sharply ridged topography of Iowa’s Loess Hills region. 

During a peak period of glacial cold, between 21,000 and 16,000 years ago, the ice-free landscapes across northern Iowa were exposed to tundra-like conditions, with seasonal freezing and thawing of permafrost in the ground. Over time, these erosion cycles leveled out the once hilly terrain. The result is quite apparent today in the more gently rolling landscapes of both the Iowan Surface and Northwest Iowa Plains regions. The Iowan Surface in particular displays an unusual number of glacial boulders - called “erratics” - left concentrated at the land surface. 

Then, into a moderating climate came a final surge of glacial ice into north-central Iowa. This ice advance is known as the Des Moines Lobe, named for the city that now sits at the southernmost reach of the glacier. These are Iowa’s freshest glacial deposits (only 15,000 to 12,000 years old), and they contrast sharply with the rest of the state. This distinctive landscape still shows the imprints of a slowly stagnating glacier – poorly drained, with numerous wetlands, lakes, and ridges called “moraines.”

Fascinating fossils from Ice Age deposits in Iowa range from mammoths, mastodons, giant sloths, musk-ox and caribou; to smaller Arctic fox, shrews and lemmings; to tiny snails, insects and fossil pollen grains. 

River Alluvium

The last basic building material of Iowa’s landscape is alluvium. These are porous deposits of sand and gravel left by flowing rivers and streams. Today’s rivers have been sculpting Iowa’s landscape since the glaciers melted away. In fact, many of today’s rivers are quite narrow compared to their wide valleys. The broad valleys were carved by glacial meltwater floods, and the abundant gravel and sand deposits along Iowa’s larger valleys reflect the swift, powerful currents that left them in place. Modern floods and sediment moving in waterways continue today as the most dominant geologic process at work on Iowa’s landscape.

Geology Today

Geology is tied to other aspects of our state’s natural and cultural history. It helps explain the distribution of soils, native plant and animal communities, and even routes of exploration and settlement. Early geologists found lead ore, around the Dubuque area, coal and shale in southern Iowa, gypsum deposits around Fort Dodge, shale and peat in northern Iowa, abundant limestone in eastern Iowa, and sand and gravel throughout the state.

Geology is important in today’s society. The distribution and arrangement of Iowa’s bedrock, glacial and alluvial deposits affect the location of drinking water supplies, mineral and rock resources, waste disposal sites, and vulnerability to natural hazards (flooding, landslides, sinkholes, and mined-land collapse). It is useful for Iowans to understand our state’s geologic framework and the geologic and hydrologic systems that operate on and beneath the land. By connecting Iowans with geologic concepts, easily within their grasp and nurtured by their curiosity, we can bring well-informed decisions to Iowa’s natural resource and environmental issues. 

Sources:

  • Stratigraphic Column of Iowa 2017, Iowa Geological Survey, University of Iowa Hydroscience and Engineering, Iowa City, 1 p.
  • Bedrock Geologic Map of Iowa, 2010, Iowa Geological Survey, University of Iowa Hydroscience and Engineering, Iowa City, 1 p. double-sided.
  • Landform Regions of Iowa (map), 2017, Iowa Geological Survey, University of Iowa Hydroscience and Engineering, Iowa City 1 p, double-sided.
  • Landforms of Iowa, 1991, J.C. Prior, University of Iowa Press, Iowa City, 153 p.
  • Iowa’s Geological Past: Three Billion Years of Change, 1998, Wayne I. Anderson, University of Iowa Press, Iowa City, 484 p.
  • Iowa Geology: Annual issues 1980 to 2001 contains non-technical articles about interesting and important aspects of Iowa’s geology. Iowa Geological Survey website/publications.
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