Saga of the Des Moines River Greenbelt

Chapter 2

Though its inhabitants have dramatically altered the appearance of the Des Moines River Valley during their brief occupancy, much more awesome changes occurred during the millennia before their appearance. Two major agents in the formation of this land were: (1) the continental seas, which determined the types of underlying rock, and (2) the glaciers of the Ice Age, which shaped the topography, formed the drainage patterns, and laid down soil materials.

The Pennsylvanian Seas

Each of the various ancient seas that covered the land left layers of sediments and sedimentary rocks~ but only the last one in any given area is significant. An extremely ancient sea, the Pennsylvanian, was the last to cover the Greenbelt (except for a small portion of northern Webster County, where the last sea was the Jurassic, a more recent sea).

A unique characteristic of the Pennsylvanian Period (310-265 million years ago) was that the equator at that time was a little north of Iowa, and the climate of central Iowa was tropical! Giant scaly-barked fern trees grew in the coastal swamps of the very shallow Pennsylvanian Sea. As they stood in the stagnant warm waters, the trees turned to coal. Most of Iowa’s coal, in fact, is found in the Greenbelt area; coal was once second only to good soil in resource value. The history of many an Iowa town revolved around coal. ‘Ibwns lived and died in response to coal supply and demand.

Limestone, in part derived from the hard shells of ancient marine life, is economically important in the area (see Figure 3). Sandstone outcrops, deposited as the deltas of great rivers which once emptied into the Pennsylvanian Sea, occur in such places as the Elk Rock and Red Rock bluffs of Marion County, in the Ledges of Boone County, and in Dolliver State Park in Webster County. Shale deposits are also present.

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Except for the Fort Dodge gypsum of the Jurassic Period, the bedrock of the Greenbelt is remarkably uniform as to type.

Fossilized imprints of tropical trees of Pennsylvanian Coal Bearing Period. Found in an abandoned coal mine in 1961 on the Des Moines River near Pella (Dr. Donald Huftman)

The Ice Age or Pleistocene

The Ice Age, the time of the passage of a succession of glaciers, occurred relatively recently, geologically speaking, from about 2 million to 10,000 years ago. The glaciers carried rock materials from northern lands and deposited them in Iowa (see Figure 4). These deposits, called till or drift filled in the uneven places, including the river channels, thus leveling the land but disarranging the drainage patterns. Fine materials were laid down and provided excellent soil minerals. Bigger rocks were laid down, too, often where the glacier paused for a time. These piles of coarse materials were called moraines, an example of which may be seen in the line of low hills across Boone County.

In some places, huge chunks of ice remained after the main body of the glacier had passed on. These, when melted, formed the sloughs and lakes so common in the northern counties of the Greenbelt.

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After a glacier melted, forces of water and wind attacked the land, and the process of erosion began. The Greenbelt is divided into two topographic-drainage regions as determined by the amount of time each region was out from under the glacier and thus subject to erosion:

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Figure 4—Glaciation in Iowa.

(Iowa Geological Survey)

(1) South of the City of Des Moines, the last glacier to pass over this southern area was the Kansan, a very ancient glacier. Hundreds of thousands of years have elapsed since the passage of the Kansan glacier, and streams have therefore had a long time to tear down the land and sculpture it into rolling hills interspersed with areas of level upland divides.

The river presently has little work to do so it flows along sluggishly, and instead of carrying erosive sediments, it drops them into its bed. As the river lays down the silt and sand, it takes the course of least resistance and flows around these deposits rather than through them thus cutting great meanders.

The river cuts laterally rather than downward, creating a wide area of flat bottomlands, very rich in alluvial soils, but until the completion of the Red Rock Dam in 1969, subject to devastating floods.

The uplands back from the river bottoms are eroded and leached, and their thin soils, as well as uneven topography, are often poor for cropping and are instead used for grazing. Land values are among the lowest in the State (see Figure 8). This area south of Des Moines is part of the region known today as the Southern Iowa Nature Area (see Figure 5).

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(2) North of the City of Des Moines, the last glacier was the Wisconsin (Des Moines Lobe), the latest of the glaciers to invade Iowa. The topography and drainage of the area covered by the Wisconsin glacier is therefore very young The streams were filled with glacial deposits, and drainage patterns were completely disarranged; they have not yet had the time to completely reestablish themselves. The land is flat and dotted with sloughs and lakes, although man has in this present century drained many of these recently glaciated lands.

The river is working hard to drain the land, cutting downward rather than laterally resulting in steep banks, many of them heavily forested and only narrow belts of bottomland. Its course is relatively straight with but few meanders.

The land back from the river valley is flat and very fertile both as a result of the rich minerals deposited on it by the glacier (glacial till), as well as the rich humus it has derived from the tall grass of the prairies (see Figure 6). The area, which includes North Polk, Boone, Hamilton, and Webster Counties, has the most fertile, most valuable soils in Iowa (see Figures 4 and S for correlation of Wisconsin glacier deposits and high land values). The combination of fertile soils and flat topography makes it possible to cultivate almost every inch of land. It is today referred to as the Cash Grain Area of Iowa (see Figure 5),

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In summary, the Greenbelt owes (1) its mineral resources— coal, clay, sandstone, limestone, and gypsum—to its various ancient seas, principally the Pennsylvanian and (2) its soils, topography, and drainage to the glaciers and the time they disappeared from the land.