Northwest Iowa Plains Vigorous erosion that accompanied the glacial cold also produced open rolling hills across this region. Loess is abundant, a reflection of its nearness to the Missouri and Big Sioux river valley sources. Land elevation is uniformly higher and precipitation lower than elsewhere in Iowa. Paleozoic Plateau Narrow valleys deeply carved into sedimentary rock of Paleozoic age and a near-absence of glacial deposits define this scenic region. Fossil-bearing strata originated as sediment on tropical sea floors between 300 and 550 million years ago.
Rock layers vary in resistance to erosion, producing bluffs, waterfalls, and rapids.
Shallow limestone coupled with the dissolving action of groundwater yields numerous caves, springs, and sinkholes. Backwater sloughs and oxbow lakes on larger floodplains mark former channel meanders. Most rivers originated during melting of various ice sheets that covered all or parts of Iowa.