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Summary of Natural
Resources Alternating layers of Pennsylvanian age sedimentary rock form the bedrock of the County. Varieties of shale, siltstone, sandstone, limestone and coal underlay the uplands and outcrop along the river corridors. The surface topography and drainage of the County is dominated by three geological influences, the Des Moines Lobe of the Wisconsinian Glacier Landform of north-central Iowa, the Southern Iowa Drift Plain and the Raccoon River System which has cut into these landform regions. The northern 4/5, or about 80%, of the County is covered by Wisconsinian-age glacial till on the uplands and recent alluvium in the drainage basins. The southern 1/5, or about 20% of the County is covered by a loess cap on pre-Illinoian glacial till with recent alluvium in the drainage basins. Drainage patterns are well developed along tributaries extending south of the Raccoon River System. The Middle and South Raccoon Rivers flow along the boundary of two landform regions, creating a meeting place for plant and animal communities. Three distinct, regional, vegetative patterns meet in Dallas County. The tall grass prairie associated with the southern extent of the Wisconsinian Glacier Landform Region, the eastern deciduous forest associated with the Ohio River watershed, and the oak-hickory forest region of the Ozark Plateau. Aquatic resources found within the County include about 100 miles of river corridor including the Des Moines River, Beaver Creek and the Raccoon River System. All three branches of the Raccoon River have created valleys with a topographic relief of 150’ to 200’. Until the late 1800’s, the swale and swell topography of the Wisconsinian Landform Region created a mosaic of upland marsh, wet prairie and mesic prairie.
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