Geology Along the Lewis & Clark Trail

Geologic Points of Interest

Dunes of the Ice Age

The bluffs at Council Bluffs are made up of loess, silt-dunes of the ice ages. Warm daytime temperatures caused high runoff of melt water from continental glaciers to the north, and filled the Missouri River Valley (A). This huge river nearly dried up during the night, when cold prevailing winds swept through the empty valley, winnowed away silt-size quartz grains, and deposited them on the eastern bank (B). This created the high bluffs on the eastern shore of the river (C). (Lewis and Clark, 7/28/1804)

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