Ancient River Channels
by Robert M. McKay
The scenic and recreational attractions along Iowa's rivers are unique to the valleys in which they flow. Geologically, these valleys are the result of flowing water eroding soft glacial-age materials or older, more resistant bedrock materials. Along steeper sides of the valleys, one frequently sees thin soils mantling prominent rock bluffs. These bluffs often are composed of limestones and dolostones, which are the biological and chemical products of tropical marine seaways that covered vast portions of North America between 65 and 550 million years ago. At other localities, however, river downcutting has exposed massive vertical bluffs of sandstone, sometimes forming steep-walled box canyons.
Scenic examples of such bluffs and canyon-forming sandstones are found along several stretches of the Des Moines valley, the Iowa and Raccoon river valleys, and a few smaller stream valleys in eastern and southern Iowa. Many of these picturesque sandstones have been included in Iowa's state parks, preserves, or county conservation lands because of their exceptional scenic and botanical qualities. Good examples include Wildcat Den State Park in Muscatine County; Dolliver, Ledges, and Elk Rock state parks in Webster, Boone, and Marion counties; Woodman Hollow State Preserve in Webster County; and Cedar Bluffs Natural Area in Mahaska County.
The majority of these bluff-forming sandstones are 300-million-year old (Pennsylvanian-age) deposits that are part of the Cherokee Group, a major rock sequence throughout much of Iowa, Missouri, Nebraska, Kansas and Oklahoma. These sandstones typically form elongate, ribbon-shaped sand bodies that were deposited in sinuous channels of ancient rivers and deltas. The sandstone deposits are usually oriented northeast to southwest and are usually less than one mile wide. Their thickness may exceed 100 feet. These ancient river channels meandered across Iowa as part of a vast equatorial ecosystem of lowland rivers and coastal deltas. They drained the interior of North America from the Appalachian Mountains in the east to the edge of a large seaway to the southwest. During this time the coastal margin migrated back and forth between Oklahoma and Illinois, and the river and delta systems deposited clay, mud, peat and channel sands on the land mass between the mountains and the sea. In the course of geologic time, these sediments were compacted and cemented into the familiar sandstone, shale, and coal beds seen today.
Iowa's modern rivers, which had their origins during the relatively recent glacial history of the continent, have removed the covering of glacial deposits and exposed the ancient channel sandstones to view. These rock formations are valuable to Iowans as local sources of groundwater for wells, and also as cool, moist, forested retreats in our state and county parks. During historic settlement of Iowa, many of these channel sandstone deposits were quarried for building and foundation stone. Perhaps the best known examples of this use are the sandstone buildings of the Amana Colonies.
Adapted from Iowa Geology 1994, No. 19, Iowa Department of Natural Resources