On the Map
Once known as the Gateway to the Northwest, Sioux City is part of a greater area that includes portions of northwest Iowa, northeast Nebraska, and southeast South Dakota, collectively known as “Siouxland.” For a large portion of geologic time, this region and most of Iowa were underwater as a transcontinental sea split North America nearly in half. These ancient seas deposited sedimentary rock over much of this region.
In the Precambrian Era (4.6 billion years-544 million years ago) volcanic activity was very common in this region, resulting in continental tears and the presence of volcanic rocks in the region. The Paleozoic Era (544-245 million years ago) was when the major layers of sediment and sedimentary rocks were formed in this region, including limestone, dolomite, sandstone, and shale.
Dinosaurs ruled the Earth for much of the Mesozoic Era (245-65 million years ago). In the Iowan seas marine giants like the plesiosaur dominated the region, along with several species of fish, sharks, and shelled marine creatures like ammonites.
Finally in the Cenozoic Era (65 million years ago-today) the seas receded. During the the Ice Age glaciers cut into the land and flattened it, and dispersed the sediment left by the seas. The finest grained silt sediment was carried by the wind and left in large mounds which we call the Loess Hills. Water runoff from the Rocky Mountains and the highlands of modern Wyoming and Montana followed the downhill path of least resistance and eventually formed what became the very livelihood of Sioux City herself: the Missouri River.