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THE GEOLOGY OF THE DES MOINES RIVER VALLEY
Tropical Climate of the Paleozoic Era along the river today were deposited in shallow Glacial erratic at Duckworth Creek, in Lacey-Keosauqua State Park.
The Des Moines River helps to tell the story tropical seas, near-shore environments, and in Courtesy of Iowa Geological Survey.
of Iowa’s creation as it cuts down through rivers and estuaries that dominated the ancient
sediment and stone exposing the bedrock that landscape. During the Mississippian Period, 325 can be seen in Lacey-Keosauqua State Park.
provides clues to the deep history of the land. to 335 million years ago, an arid tropical climate They are metamorphic or igneous in origin,
Bedrock exposures can be seen in the wooded prevailed. The seas became shallow and began testifying to the great distances that they were
bluffs along the river and in the steep ravines to retreat. Eventually they withdrew and the dry transported.
of smaller creeks that flow into the river from land was exposed to a long period of erosion
the south and west (good exposures are vis- that lasted more than 10 million years. As the The last episode of glaciation in southeastern
ible in Lacey-Keosauqua State Park). Because of continent drifted into a humid tropical, equato- Iowa took place about 500,000 years ago. The
continental drift, what is now North America rial climate zone during the Pennsylvanian Period more recent Illinoian and Wisconsinan glacial
occupied a more southerly global position during around 310 million years ago, shallow seas and advances did not reach as far south as Van Buren
the Paleozoic Era than it does today (Witzke swamps once again covered southeastern Iowa. County (Hallberg 1980; Tassier-Surine 2004:51).
2004:55). During the Mississippian and Pennsyl- The Pennsylvanian “coal measures” found across After the glaciers retreated from the northern
vanian Periods Iowa lay near the equator. The southern Iowa were deposited in swampy low- portions of the region, a mantle of windblown
sandstone and limestone beds that are exposed lands of the near-shore environments of the time loess was deposited across the land. Loess is
(Witzke 2004:55). exceptionally uniform, fine-grained particles
Iowa’s global position relative to the equator during the Late made up mostly of quartz. The particles are
Mississippian period 325 million years ago: http://cpgeosystems.com/ Changing Environment – The Pleistocene Ice so fine that they could be blown around by the
images/NAM5.jpg. Age to the Present wind and, under the right conditions, piled up
like deep snow drifts. Because the grains are
More recent geological deposits visible along the angular, they stick together and can form steep-
Des Moines River valley are from the Quater- sided bluffs like those seen in the Loess Hills in
nary Period which began about 2.6 million years western Iowa.
ago. By this time the continents had reached
the positions they now hold. The earliest part of
the Quaternary Period is the Pleistocene epoch,
commonly known as the Ice Age. The Pleisto-
cene was characterized by episodic advances of
continental glaciers. Southern Iowa was covered
by glaciers many times during this period. The
glaciers deposited thick layers of gravelly till and
sand across the landscape and occasionally left
behind large boulders, called erratics, that were
transported within or beneath the ice. A num-
ber of erratics, some several feet in diameter,
6 A River of Unrivaled Advantages—Life Along the Lower Des Moines River
