The bedrock beauty of Whitewater Canyon

by Jean C. Prior

The word “canyon” is rare among Iowa place names. But there it is, on a U.S. Geological Survey topographic map, identifying a deep, twisting, forested segment of Whitewater Creek.

As the creek flows from southern Dubuque County into northeastern Jones County, its valley abruptly narrows to a constricted gorge bound by steep bedrock bluffs, a series of sharp turns and tight meander loops deeply entrenched into the surrounding landscape: Whitewater Canyon. The zig-zag course takes nearly three miles to cover a two-mile, straight-line distance. Below the canyon, the creek valley broadens again before emptying into the North Fork of the Maquoketa River.

Looking out across the rolling countryside from Highway 151 or county roads, you could miss this canyon all together. Only on near-approach does the sudden, 150-foot drop into this geological gem come into full, spectacular view.

In addition to great scenery, this canyon offers lessons in Iowa geology, both here and throughout the state.

Rock solid foundations
Such eye-catching features in Iowa’s scenery are usually the result of underlying geological conditions. The landscape-changing factor in this case is bedrock: durable dolomite—a magnesium-rich limestone.

This rock formation, which lies much deeper under central and southwest Iowa, was deposited as limey mud and shell fragments in a warm, clear, shallow sea that covered the interior of North America about 430 million years ago (Silurian age). In fact, the exposed bedrock contains many marine fossils.

Though this bedrock is most exposed in the canyon itself, you can catch glimpses of it nearby: in weathered rock knobs poking through to the land surface, in layered strata exposed along road cuts and in local stone quarries. The usual mantle of glacial-age clays and silts that masks Iowa’s bedrock elsewhere across the state is quite thin here. Upstream, Whitewater Creek and its tributaries have occasional brief contacts with bedrock, but nowhere more dramatically than in beautifully carved Whitewater Canyon.

Forces & fractures
Looking into the canyon from blufftop overlooks, one’s eye is drawn to its straight, sheer walls of rock and to the winding course of both creek and valley below.

The bold bluffs actually follow fractures, or planes of weakness, through the bedrock. These developed earlier in the region’s geologic history when rigid bedrock masses fractured under long-term stress associated with regional warping of the Earth’s crust. The resulting vertical fractures, called “joints,” tend to occur in parallel sets and at nearly right angles to each other.

Whitewater Creek, taking the path of least resistance through the dolomite, follows first one fracture trace and then another, often with right-angle turns. This erosion pattern is repeated in deep vertical crevices that open behind and parallel to major cliff faces, in tributary ravines entering the canyon at nearly right angles, in rectangular bedrock “chimneys” standing apart along the bluffs, and in angular blocks of bedrock lodged on the canyon’s lower slopes. State preserves and parks—such as White Pine Hollow, Backbone, Maquoketa Caves and Palisades-Kepler—are other places where Silurian bedrock produces dramatic effects on both topography and drainage patterns.

In addition to these pronounced vertical features, visitors will notice prominent horizontal ledges, overhangs and recesses. These occur along “bedding planes” that reflect the sedimentary rock’s original accumulation on an ancient sea floor. These layers, accentuated by differences in their resistance to erosion, provide picturesque relief to the rock faces as well as convenient footholds for vegetation.

Water power
Though highly resistant to erosion, dolomite is a carbonate rock, which makes it subject to the dissolving action of groundwater. Its porous, weathered surface is marked by small irregular cavities and pits called “vugs.”

Larger openings, even caves, can develop from chemical reactions to percolating groundwater—as well as by mechanical slippage of bedrock slabs to form angular openings along fracture traces. Such “karst” features—which also can include sinkholes, springs and algific slopes—are strongly associated with these Silurian rocks in eastern Iowa. Occasional groundwater seeps within the canyon are a reminder that these rocks also compose the Silurian aquifer, a major source of drinking water for eastern Iowans.

Common ground
Geology and ecology form a strong partnership in today’s canyon environment. The rugged landscape has isolated the valley from much disturbance. It hosts an array of natural habitats and ecological niches, reflecting the various geologic substrates, slope aspects and moisture conditions that are present.

Small prairie glades and gnarled cedars thrive along exposed bluff tops. Harebells and lichens cling to sheer bedrock walls. Canada yews and ferns line moist rocky recesses. Mosses coat the rock rubble at the base of cliffs. Oaks and hickories thrive in the upland forests and tributary ravines. Cottonwoods and bluebells grow amid bottomland deposits of sand and gravel along the creek channel and within the floodplain corridor.

Whitewater Canyon is a valuable natural and scientific asset, worthy of further inventory and interpretation by geologists, botanists, biologists and archaeologists. The canyon also preserves for future generations some fascinating and instructive geological chapters in our state’s natural history.

Additional resources:

Mycountyparks.com offers visitor information for Whitewater Canyon. Resources are available under Dubuque, Jackson and Jones counties. Learn park hours, recreational activities, features and history. 

Jean C. Prior. Landforms of Iowa. University of Iowa Press, 1991. This full-color, readable account of Iowa’s landscape features describes their diversity across the state, their underlying earth materials, and the geologic processes responsible for their formation. Abundant photos, maps, diagrams, references, and places to visit.

Wayne I. Anderson. Iowa’s Geological Past: Three Billion Years of Change. University of Iowa Press, 1998. This books offers a comprehensive look at Iowa’s fascinating geologic past as preserved in its rock record, from ancient volcanic lavas to tropical seas teeming with marine life to massive ice sheets. Numerous photos, maps, diagrams and references.

Iowa Geological Survey website. This site contains a wealth of information, including a huge variety of maps and satellite images, educational materials, and a “browse area” with links to dozens of diverse topics ranging from the geology of specific state parks to Iowa’s statewide land cover inventory to “the age of dinosaurs in Iowa.”

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A landscape perspective

Whitewater Canyon offers clues to bedrock and glacial features that affect landscapes elsewhere in the state.

For example, the Silurian-age dolomite that distinguishes Whitewater Canyon is seen again in the continuous, usually forested, line of bluffs known as “the Silurian Escarpment” (see map above). This prominent terrain feature can be traced diagonally across northeastern Iowa from central Fayette County to southeast Jackson County, where even the course of the Mississippi River is deflected eastward by the resistant dolomite outcrops.

The Silurian Escarpment also marks the boundary between two of the state’s major landform regions. While Whitewater Canyon actually lies within the Iowan Surface region, it is in a transitional area where bedrock exerts more local control on the landscape, as is characteristic of the Paleozoic Plateau region.

Glaciers also produced large-scale landscape changes throughout the state—with ripple effects in Whitewater Canyon.

Massive ice sheets last covered this part of Iowa about 500,000 years ago, leaving behind deposits of pebbly clay, of which only thin, uneroded remnants remain in the well-drained Whitewater Canyon area. This was followed by deposition of wind-blown silt (loess) and fine sand, from roughly 30,000 to 14,000 years ago, which contributed to the thin topsoil on the canyon’s uplands.

Near the end of this period, roughly 21,000 to 16,000 years ago, Iowa and the upper Midwest were plunged into extreme glacial cold. Large areas of permafrost developed in Iowa’s ice-free, tundra-covered landscapes, causing accelerated erosion. Frost action shattered exposed bedrock—including the walls of Whitewater Canyon, leaving slopes of rock rubble still found at its base.

As climates moderated, one last push of glacial ice surged into north-central Iowa between 15,000 and 12,000 years ago. Melting ice revealed the relatively flat, poorly drained landscape that characterizes the Des Moines Lobe region—a sharp contrast to the exposed bedrock and dramatic landscape of Whitewater Canyon. Together they illustrate the remarkable diversity of Iowa’s landscapes.