and abandoned rock-cored meanders are a fascinating aspect of valley evolution in the Paleozoic Plateau.
Another component of northeastern Iowa valleys are the thick alluvial deposits within them. In the case of the upper Mississippi, large meltwater floods during the waning phases of the Wisconsinan scoured the valley, removing or rearranging earlier records. Side valleys now store the geologic ledgers registering earlier fluctuations in river level and sediment load that occurred out in the master valley. The ledgers exist in the form of alluvial terraces, those level but elevated remnants of older floodplain surfaces that flank valley margins. Clear Creek and Village Creek valleys west and southwest of Lansing contain well preserved terrace remnants as high as 60 feet above the present floodplains. Terraces are important in documenting the region’s complex alluvial history and the response of tributaries to changing water levels associated with glacial melting and drainage diversions in the Mississippi Valley.
For many years the eastern half of the Paleozoic Plateau was thought to be untouched by Pleistocene glacial activity and was referred to as the “Driftless Area,” which included similarly rugged landscapes in the adjacent parts of Illinois, Wisconsin, and Minnesota. This concept is no longer valid, however, as patchy remains of Pre-Illinoian glacial drift over 500,000 years old are documented on stream divides in the Iowa portion of this area. Pre-Illinoian drift is even more widespread in the western half of the Paleozoic Plateau, although it still plays no significant role in the present appearance of the region’s landscapes. Regardless of the relative amounts of glacial drift, the region stands united by the strong influence of bedrock geology, by a marked contrast to the remainder of the state where landscapes are subdued by glacial deposits, and by its many topographic and ecologic similarities.
Our understanding of Iowa’s glacial-age landscape evolution has grown significantly in recent years. As specific geographic areas and stratigraphic deposits have been examined in detail, we have seen an emerging unity among the state’s diverse landform regions. The Paleozoic Plateau, for all of its spectacular differences, fits neatly into the erosional history we have been compiling since looking at the Southern Iowa Drift Plain. Though the sedimentary rocks of this northeast Iowa landscape are ancient in comparison to all other landscape-forming materials in Iowa, the “preglacial” antiquity previously assigned to the landscape features themselves is being challenged. Recent investigations suggest that much of this rugged terrain is actually quite young. Stream erosion and hillslope development since Pre-Illinoian glacial events have produced the deeply dissected landscape and in the process have removed the glacial deposits from all but isolated upland positions. The same episodes of erosion reflected in the changing hillslope gradients of the Southern Iowa Drift Plain and in the scoured Iowan Surface and Northwest Iowa Plains also had a significant impact on the Paleozoic Plateau. The effects, however, are magnified by the bedrock-controlled relief and complicated by the steep slopes. The isolated distribution of Pre-Illinoian glacial deposits found on stream divides high in the landscape shows that the Mississippi River evolved during the early to mid-Pleistocene. The river’s network of tributaries in Iowa became established after the last Pre-Illinoian glacial events in the region 500,000 years ago. Loess-mantled, rust-colored Late-Sangamon paleosols are found on these remnant glacial deposits, and in places they extend onto the bedrock surface in the form of waxy red clays. At other locations loess lies directly on bedrock without any hint of a paleosol, conditions typical of the Wisconsinan-age Iowan Surface. These relationships confirm that multiple erosional episodes have cut across the entire Paleozoic Plateau.
Karst conditions develop at or just above the zone of groundwater saturation. Caves and passageways thus are strongly linked in time to periods when stream downcutting lowers the water table in shallow carbonate bedrock. The speleothems (stalactites, stalagmites, and flowstone) that decorate cavern systems cannot form until the karst openings fill with air and are largely free of groundwater. Uranium-thorium series radiometric dating of speleothems in Iowa and Minnesota indicate that major valleys were carved deeply enough into the landscape to bring the water table down and foster a major period of speleothem growth between about 100,000 and 160,000 years ago.
Most of the deep entrenchment of the region’s valleys, as well as formation of the prominent rock-cored meanders, occurred during the Wisconsinan, before 30,000 years ago. The ancestral valley meanders filled with stream deposits and were abandoned between 30,000 and 20,000 years ago. Intense frost action, producing unstable slopes and massive mechanical weathering and erosion, was brought on by the severe cold of glacial climates about 21 ,000 to 16,500 years ago. Locally, these periglacial conditions produced massive colluvial slopes of angular, blocky rock fragments (talus) along the sides of entrenched valleys. Karst conduits near the land surface also collapsed into large, blocky talus. The small, narrow gorges, described earlier emerging from the Silurian Escarpment, often mark collapsed sections of former cavern systems that were exhumed by the intense late-Wisconsinan erosional episode.
From about 18,000 to 9,500 years ago, the waning Wisconsinan ice front fluctuated on either side of the continental divide in Minnesota, separating drainage to the Mississippi from that to Hudson Bay. Catastrophic meltwater floods periodically surged down through the Mississippi Valley. Left behind are the series of elevated terrace deposits mentioned earlier, composed of silts and clays deposited in the slack backwaters of the Mississippi’s tributaries. For the most part, slower-paced sedimentation, minor downcutting, and lateral migration of river channels have been the dominant activities in the region’s valleys since then. It is important to realize that the progression of deposits from these floodplains and terraces up to the steep colluvial slopes, to the filled and abandoned meanders, to the ornamentation in caves, and to the rusty red, clay-rich upland paleosols are important linkages in time that verify the surprising geologic youthfulness of these rugged landscapes, even though they are developed on ancient Paleozoic bedrock. The deposits also have important geographic links, demonstrating that many of this region’s significant features were affected by the deepening presence of the Mississippi Valley.
Quarries are common in the limestone and dolomite deposits throughout the Paleozoic Plateau. The easily accessible bedrock is used for road construction and maintenance, though some is quarried for building stone. Bedrock deposits of the region also have a colorful mining history. Lead (galena) ore was mined for at least 300 years from veins occurring along crevices in the Ordovician (Galena Group) dolomites exposed along the Mississippi Valley and its tributaries. The hills around Dubuque are honeycombed with shafts and adits of this lead-mining activity. Even iron ore was mined in the 1890’s from a small, unusual occurrence of Cretaceous rocks found northeast of Waukon. This area, known as Iron Hill, is one of the highest points of land (1 ,345 feet) along this segment of the upper Mississippi Valley.
In addition to the economic value of the bedrock materials, many water wells have been drilled into the shallow creviced limestones and porous sandstones for drinking water. The karst characteristics of the area are a vivid reminder that contaminants from the land surface have direct access to these groundwater zones. In this vulnerable geologic setting, care must be taken to manage waste disposal and land-applied farm chemicals to prevent pollution of underground aquifers. Widespread contamination of the region’s shallow aquifers by nitrate and bacteria already has caused many wells to be abandoned and new wells to be drilled to deeper groundwater sources.
The Paleozoic Plateau also contains a large share of the state’s native woodlands. The steep, rocky slopes are unsuited to cultivation and remain forested. Some of this timber cover actually has developed since the time of Euro-American settlement when naturally occurring fires were suppressed. Along steep, dry slopes above major valleys, remnant prairies called goat prairies often can be spotted among the forested slopes.
With cropland at a premium in many areas, the small but productive floodplains and terraces along valleys are usually farmed. Areas of level uplands are also cultivated or used as pasture, especially for dairy cattle. The rugged topography lends itself to many types of outdoor recreation in both winter and summer. Those fishing for trout seek the deep, quiet pools below rocky riffles along the numerous spring-fed streams. Canoe enthusiasts can travel many miles of wooded and rock-bound waterways. Collectors can find abundant fossils in many of the exposed bedrock formations. The heavily timbered landscape supports a colorful autumn foliage, and the scenic region attracts many visitors each year.
Author: Jean C. Prior
Edited by: Drew Hutchinson