Geology of Wildcat Den State Park
by Brian J. Witzke
Wildcat Den State Park encompasses 417 acres along the Pine Creek drainage in Muscatine
County, between Muscatine and Davenport. It is an area of exceptional beauty marked
by imposing sandstone exposures. The park was formally dedicated in 1935 to preserve
its scenic geology, diverse plant communities, archaeological resources, and historic
sites. The restored Pine Creek Grist Mill, constructed in 1848, is one of the finest
examples of a mid-nineteenth century grist mill to be seen anywhere in the country.
A series of well maintained hiking trails affords the visitor easy access to many points
An array of geological resources in the park provides fascinating glimpses into
Iowas ancient past. The oldest bedrock, exposed along the margins of Pine
Creek, includes about 25 feet of fossiliferous limestone and dolomite ("magnesian
limestone") of the Cedar Valley Group. These rock layers formed from lime
sediments that accumulated in shallow tropical seas that covered the region during the
Devonian Period about 375 million years ago. The Devonian strata are best displayed
below the dam next to the grist mill, where the beds are disrupted by small-scale
faulting. Fossil corals, stromatoporoids (extinct sponge-like animals), fossil
brachiopod shells, and skeletal pieces of crinoids ("sea lilies") are preserved
in the bedrock along Pine Creek.
A slot through the dramatic sandstone bluffs along Pine Creek valley
takes visitors close to rocks that originated in a river channel 310 million
years ago (Pennsylvanian). Photo by Robert McKay.
A considerable hiatus in the geologic record separates the Devonian strata from the
overlying Pennsylvanian shale and sandstone units in the park. These Pennsylvanian
strata reach thicknesses of about 100 feet and accumulated during the so-called "Age
of Coal." During this geologic period, eastern Iowa straddled the equator, and
humid tropical forests stretched along ancient river courses and coastal lowlands that
were present. Swamps accumulated masses of vegetation, later to become coal.
Salt-water estuaries encroached up the river valleys, as evidenced by brackish-water
fossils and by sediments that reflected tidal rhythms. Deposits of gray shales and
mudstones accumulated about 315 million years ago, yielding the soft rocks of the
Caseyville Formation, which typically form overgrown soggy slopes along the lower valley
walls of Pine Creek. The shales contain abundant organic material, and fossil ferns
and scale tree foliage have been identified within the park. Thin coal
beds are also associated with this formation at Wildcat Den and in nearby areas of
Stone used to construct the mill pond dam and a foundation for the 1848 Pine
Creek Grist Mill came from the limestone outcropping seen to the right. These
fossil-bearing strata were deposited in warm offshore seawater 375 million years ago
(Devonian). Photo by Robert McKay.
Picturesque sandstone cliffs and glens form the most noteworthy landscape features of
Wildcat Den State Park. These sandstones belong to the Cherokee Group and were deposited
about 310 million years ago (during Middle Pennsylvanian time), subsequent to the
underlying shales and mudstones. These strata contain the bulk of Iowas coal
resources, known especially from south-central Iowa. Springs and seeps occur at the
contact between the porous Cherokee sandstone and the underlying, less permeable
Caseyville shale (e.g., iron-rich springs near the base of Steamboat Rock).
The sandstones were deposited in ancient river channels and record multiple episodes of
erosion and deposition. Crossbedding of the quartz sand is prominent, and records
the general southwesterly direction of flow within the rivers. In some places,
iron-oxide cements form dramatic swirls and bands in the sandstones, and these are related
to later chemical precipitation from mineralized groundwater. The delightful
sandstone precipices and ravines spur our imaginations with a variety of images, reflected
in part by the fanciful names that have been given to some of the prominent features,
including Steamboat Rock, Devils Lane, and Devils Punchbowl.
Rock overhangs and small sheltered caves result from weathering and erosion
along zones within the sandstone that had different sedimentary histories. Photo by
Later chapters of the geologic history of Wildcat Den State Park are interpreted from more
recent sediments that mantle the bedrock. The area was overridden by continental
glaciers several times during the Pleistocene Ice Age. The most recent
of these was the Illinoian glaciation which spread from Illinois into easternmost Iowa
about 300,000 years ago, displacing the ancestral Mississippi River channel westward into
eastern Iowa. Remnants of these glacial sediments are scattered in the parks upland
The most recent glacial advance (Wisconsinan) pushed southward across northern Iowa and
areas of Wisconsin and Illinois, but did not reach into eastern Iowa. However,
deposits of wind-blown silt (loess) did blanket much of the area and were derived from
river valleys that carried huge volumes of sediment-laden glacial meltwater. Two
silt units are recognized at Wildcat Den. The older Roxana Silt dates from about 55,000 to
28,000 years ago. The overlying Peoria Loess, which accumulated between 21,000 and
12,000 years ago, is typically about 20 feet thick in the uplands of the park.
Dissolved iron is abundant in groundwater that moves through these
cross-bedded sandstone outcrops. Rust-colored, iron-oxide stains and cemented zones are
often seen on rock faces. Photo by Ray Anderson.
Other important landscape changes occurred between 25,000 and 11,000 years ago, as eastern
Iowa began to assume a more familiar form. Of special note, the channel of the
Mississippi River shifted to its present location between Rock Island and Muscatine, and
the modern stream drainage configuration began to develop.
The most recent aspects of the geologic evolution of Wildcat Den are found in the
sediments, landforms, and cultural remains of the Holocene, the last 10,000 years.
The advent of intensive agriculture and settlement in the area over the past 150 years
have also modified the landscape and stream courses.
As we visit Wildcat Den State Park today, our experience is enriched by an appreciation of
the history contained in the rocks, sediments, and landforms. Wildcat Den will serve
as a clear reminder of our states natural heritage for generations to come, a proud
connection to this land we call Iowa.
Adapted from Iowa Geology 1999, Iowa Department of Natural