Flaming meteors, dark caves, and raging waters—Geological curiosities of western Wisconsin
Published:January 01, 2011
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Jean Cunningham, Holly A.S. Dolliver, William S. Cordua, 2011. "Flaming meteors, dark caves, and raging waters—Geological curiosities of western Wisconsin", Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America, James D. Miller, George J. Hudak, Chad Wittkop, Patrick I. McLaughlin
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On a regional geological map, western Wisconsin looks as if it has very simple, even boring, geology. It is dominated by flat-lying, layer-cake Ordovician sedimentary rocks thinly overlain by Pleistocene glacial drift, yet detailed investigation reveals many interesting geologic features, furnishing research projects and teaching examples for all levels of geological education. In this “simple” area around Spring Valley, Wisconsin, you will see a major cave, an old mine, a large earthen dam and a meteorite impact site. In addition to what these sites say about the area’s geologic and human history, they furnish insight into how geologists piece together evidence as well as illustrate relevant subjects such as groundwater supply, control of catastrophic flooding and intelligent land use. It is our hope that, while enjoying learning about the features in this scenic part of Wisconsin, many of you will be inspired to seek out the unique geology of your area, and bring that knowledge into your classrooms.
The goal of an earth science educator is to introduce students to the science. The easiest way to accomplish this is to take them into the field where geology comes to life. Each of the four sites visited on this trip offers the educator unique opportunities to introduce a class to stratigraphy, geomorphology, structural and karst geology, hydrology, and much more. Whether visited together or separately, the four sites will provide for stimulating discussion and thought-provoking questions that can be expanded upon back in the classroom.
Field Trip Sites
Site 1. Crystal Cave, Spring Valley, Wisconsin. Wisconsin’s longest cave offers a unique opportunity to study karst geology. This stop will show how caves can be used to expose students to a seldom seen, yet vitally important segment of our environment.
Site 2. History of Flooding and Flood Control along the Eau Galle River in Spring Valley, Wisconsin. The dam built by the U.S. Army Corps of Engineers is critical to controlling periodic flooding throughout the Eau Galle River Valley. Here you will learn why this dam is so important to the residents downstream.
Site 3. Gilman Iron Ore Mine. Spring Valley was a booming mining town from 1893 to 1907. The ore deposits, though small, were rich and locally plentiful. Learn about the geology of the “brown ore” and see a deposit closeup.
Site 4. Geologic discovery through anomaly investigation as shown by the Rock Elm Disturbance, Pierce County, Wisconsin. The presence of folded and faulted rocks in the normally undeformed Ordovician rocks of Pierce County lead to the discovery of a hypervelocity impact feature now called the Rock Elm disturbance. Follow the process used to investigate anomalies, develop theories and draw some interesting conclusions.
site 1. crystal cave, spring valley, pierce county, wisconsin
Crystal Cave is a multi-level maze cave located in Southwestern Wisconsin’s Pierce County (UTM: 559,174E/4,964,585N). The cave is ∼1.4 km (4600 ft) in length and has a depth of 21 m (69 ft). The entire cave has formed in the lower Ordovician Prairie du Chien Group and contains glaciofluvial deposits washed in during the final stages of the Wisconsin Glaciation.
Crystal Cave has been operated as a showcave since 1942 with current visitation averaging 35,000 per year. The cave is utilized by educators throughout the upper Midwest as an excellent teaching tool, exposing students to rocks and minerals, karst features, and hydrology.
Crystal Cave has formed in the early Ordovician Prairie du Chien Group, a shallow marine deposit of dolostone, and dolomitic sandstones. The Prairie du Chien is divided into the lower Oneota Dolomite and the upper Shakopee Formation. Both are part of transgressive/regressive sequences which began in the Cambrian and continued through at least the Silurian.
The Oneota Dolomite is the basal unit of the Prairie du Chien Group. It is a tan to gray, fine- to medium-grained dolostone ∼20 m (65 ft) thick. The Oneota exhibits massive bedding in the upper portion of the section, becoming more thinly bedded lower in the section as the top of the Cambrian Jordan Sandstone is reached. Fossils are rarely found in the Oneota, but occasionally cephalopods and stromatolites are seen. The most common fossils are gastropods. The lowest level of Crystal Cave is developed in the Oneota.
The New Richmond Sandstone Member is the lower member of the overlying Shakopee Formation. It is well-sorted sandstone composed of quartz grains and dolomite rhombohedrons. In the cave, the New Richmond is approximately 1.5–2 m (5–7 ft) in thickness and is thinly bedded with numerous burrowing trace fossils. The New Richmond forms the upper portion of the passageways of the lowest level of the cave. A clearly defined erosional unconformity separates the New Richmond from the underlying Oneota (Fig. 1). Schultz (2004) describes the contact with the overlying Shakopee as “a sharp, even contact.” The New Richmond represents the transition from deeper, quieter waters with little source material to a more active near-shore/beach environment.
The upper member of the Shakopee Formation is the Willow River Dolomite. In the cave, it is a dolostone ∼12–14 m (40 ft) thick with stromatolites, trace fossils and gray-white chert lenses common. This dolostone is more thickly bedded than the New Richmond and indicates a return to a deeper water environment. The upper 6 m (20 ft) exhibits intense weathering with deposits of bog or brown iron ore (limonite/goethite) and accompanying lenses of greenish gray illite found throughout. This iron ore is of the same origin as the Gilman Ore deposit discussed in the Site 3 section. The base of the Willow River Dolomite forms the ceiling of the lowest level of Crystal Cave.
Crystal Cave is a multi-level maze cave (Day, 2009; Fig. 2). The upper two levels consist of large, dome-shaped rooms connected by short passages. Ceilings extend into the weathered zone and, in one instance, have actually broken through to the surface, creating a sinkhole. The lowest level is the most extensive with cave development occurring along northeast-southwest–trending joints in the Prairie du Chien Group.
The development of Crystal Cave has traditionally been attributed to dissolution of the dolostone by dilute carbonic acid. In this process, as rainwater and snowmelt percolate through the soil horizon, biogenic carbon dioxide is picked up by the water, creating a weak solution of carbonic acid. The carbonic acid continues to seep deeper into the subsurface, dissolving bed rock along joints and fractures. As more bedrock is dissolved, the joints and fractures enlarge, eventually connecting. When the enlarged areas become the size a human could enter, the underground voids become known as caves.
Recent investigations by A.N. Palmer and M.V. Palmer (2009, personal commun.) suggest Crystal Cave may have a hypogenic origin. A hypogenic cave forms when water which has migrated up from below, often along a fault, dissolves the rock. Palmer cites passages in Crystal Cave with very irregular contours and profiles as typical of hypogenic caves such as Carlsbad Caverns, New Mexico. Because Crystal Cave has been modified by periodic flooding events related to glaciation, this invasion of surface streams may have removed most or all evidence of a hypogenic origin except the very irregular contours and profiles of the cave’s passages. Further study of the hydrology, geochemistry, and clay mineralogy, and regional mapping of the area are necessary to resolve the question of the cave’s origin.
It is not known when Crystal Cave began to develop, but karst development in Southwestern Wisconsin may have begun as early as the Paleozoic (Hedges and Alexander, 1985). The proximity of the terminus of the Superior Lobe of the Laurentide Ice Sheet to Crystal Cave would have provided copious amounts of water that periodically flooded the cave, most likely enlarging existing passageways while depositing glacially derived sediment throughout. These flooding events make dating the formation of Crystal Cave extremely difficult.
Glaciation and Crystal Cave
Crystal Cave is located in an area of Wisconsin that was glaciated during the last glacial event, the Wisconsin Glaciation (110,000–10,000 years ago) (Baker, 1984). Meltwater from the retreating glaciers flowed through the cave depositing sediment throughout the lower level. The impact on the cave can be seen today where the suspended material acted as an abrasive, enlarging passageways and scouring the walls. It was at this time that the cave was almost completely filled with outwash debris ranging from clay to pebble-sized particles. Examination of cave sediments shows horizons of coarse pebbles, sandstone, loess, and even charcoal. An ongoing study by Bellomo et al. (2011) shows depositional dates of the sediments from 24,000 to 16,000 yr B.P. which corresponds to the deposition of the Poskin and Sylvan Lake Members of the Copper Falls Formation (Clayton et al., 2006). Deposition of sediment in the cave appears to have ceased around 11,000–12,000 years ago when speleothem development began. Flow-stone deposited directly on the cave fill has been dated by Lively (1983) with dates ranging from 11,600 (±600) to 7000 (±400) before present. The sample dates are U/Th disequilibrium dates derived using Alpha Spectrometry. There was no correction for potential detrital Thorium, therefore dates are maximums and would represent the earliest possible speleothem deposition in Crystal Cave.
The southeast end of the lowest level of Crystal Cave has optimal conditions for speleothem development. Soda straws, stalactites, stalagmites, columns, flow stone, bacon strips, and ribbons are the most common, though helectites and cave pearls can also be found.
The speleothems are formed from calcite. Where the calcite is pure, the speleothems are a milky white. Occasionally, trace amounts of iron or organic acids will stain the calcite various shades of tan to dark brown.
Speleothems develop in the wet areas of Crystal Cave where joints and fractures connect to the surface soil zone. As rainwater or snowmelt percolate through the soil, it absorbs carbon dioxide resulting in weak carbonic acid. This carbonic acid, as it makes its way to the water table, is strong enough to dissolve the surrounding dolostone. Once the calcium-saturated carbonic acid reaches the void of the cave, it will either form a droplet on the ceiling, flow down the cave wall, or drip onto the floor of the cave. Several other reactions now occur simultaneously.
As the carbonic acid leaves pore spaces in the ceiling rock forming a droplet, pressure is reduced. This allows some of the dissolved carbon dioxide to escape the carbonic acid.
The removal of the carbon dioxide weakens the acid which reduces the ability of the calcite to remain in solution.
The calcite now comes out of solution, crystallizing around the outside of the droplet, along the wall or on the floor of the cave.
Where the calcite is deposited within the cave will determine what type of speleothem will develop.
Soda Straws and Stalactites
Soda straws form when calcite crystallizes around a droplet of carbonic acid suspended from the ceiling of the cave. The result is a hollow tube structure the diameter of a drop (Fig. 3). The soda straw will continue to grow as long as the supply of calcite is uninterrupted or until it breaks under its own weight. The longest soda straw in the world is 9.03 m (30.5 ft) and is found in an unnamed cave in Mexico (Bunnell, 2010), but most will break long before this length is reached. Most soda straws will develop into a stalactite when the central tube becomes plugged and carbonic acid no longer flows through to the tip. The acid starts flowing along the outside of the formation building layers of calcite. The result is a larger, more stable formation called a stalactite.
Stalagmites, Flowstone, and Columns
When the dripping of carbonic acid from the ceiling of a cave occurs too quickly for calcite to crystallize, the solution will fall to the floor or flow down the walls. Again, the carbon dioxide is released and the calcite will be deposited. But, depending on how the carbonic acid “landed,” different types of speleothems will grow. If the solution flows down the wall or across the floor of the cave, the calcite will deposit in sheets, a milky coating referred to as flowstone. Drops falling from the ceiling onto the floor will disperse in a circular pattern depositing calcite in a cone-shaped formation with the highest concentration of material directly under the source of the drip. The calcite will continue to build upward forming a stump-like formation called a stalagmite. When a stalactite and stalagmite meet and grow together, it is called a column.
If the ceiling of a cave slopes, the calcite will deposit in thick, wavy bands called ribbons or bacon strips. Both form the same way, but ribbons are milky white while bacon strips contain intervals of iron-staining giving it the appearance of bacon. Helectites are formations that start as a stalactite but soon begin growing in unusual, gravity-defying shapes. There are several theories regarding the method of growth of these speleothems, which often resemble a pile of spaghetti. One theory considers wind currents blowing through the cave; another implies a breakdown of the crystal structure which disrupts the flow of fluid through the formation. It is one of the mysteries yet to be solved.
Growth Rate of Speleothems
The rate of growth is dependent of several factors, including precipitation rate, the amount of vegetation on the surface, surface water temperature, and the temperature of the cave.
Water is critical to speleothem development, so precipitation rates obviously will determine whether or not speleothems will grow. During years of drought, growth can slow or even completely halt. Then again, during years of heavy precipitation, the increased flow will prevent droplets from developing on the ceiling or will wash down walls and off the floor too quickly to allow deposition to take place.
Heavy vegetation growth on the surface will impact the amount of water available in the subsurface. Agricultural activity pulls less water from the soil than a dense forest with large trees and thick underbrush. The amount of water entering the subsurface will determine the amount of carbonic acid available to the cave. The amount of decaying organic material also affects the amount of biogenic carbon dioxide available in the soil.
Temperature of the surface water as it percolates through the soil is critical for absorption of carbon dioxide. Warm rain will absorb more carbon dioxide than cold snowmelt. This in turn will affect how much calcium carbonate can be dissolved from the bedrock and carried in solution into the cave.
Cave temperature will determine how much carbon dioxide will be released from the carbonic acid and therefore, how much calcite will be deposited. Too warm or too cold and the amount of carbon dioxide released will be reduced.
That said, speleothem growth in Crystal Cave continues at a steady rate. Since commercialization in 1942, the cave has seen growth on formations broken during late 1941 and early 1942. This new growth shows ∼20 mm (0.75 in) over the past 70 years. Recent deposits of calcite are found on rock surfaces exposed during excavation and in some cases, even on steps and railings.
Discovery and Commercialization
Crystal Cave was discovered in 1881 by a local farm boy, William R. Vanasse, when the sixteen-year-old came across a small leaf-filled sink in a farm field. Returning the next day with his younger brother, George, to explore the cave, the boys saw only shallow entrances to partially clay-filled galleries on the upper level, and the existence of other levels and galleries was not suspected. Crystal Cave, at that time called Sander’s Corner Cave, remained in a semi-filled condition for several decades. A slight amount of collapse from the sink on the surface was the only alteration visible to the succession of adventurers who visited the cave.
The cave was developed—or commercialized—by Henry A. Friede, an advertising agency manager and amateur geologist from Eau Claire, Wisconsin. Friede had been interested in caves for some time and had studied many possible sites in the area, hoping for a discovery equal to that of Blue Mounds (now Cave of the Mounds) or the caves near Harmony, Minnesota (Mystery Cave and Niagara Cave). The most likely prospect was Sander’s Corner Cave, a quarter mile south of the junction of State Highway 29 and County “H” (now County CC). Work began during the week of 2 November 1941 on the first and second levels. By 20 November, clearing of the passageways on the lowest level had begun by workers using a drag line to remove the silt and debris filling the cave. It was during this same time that Alvin Peterson began developing plans for the entrance building.
By early April 1942, much of the debris had been removed and construction was begun on an entrance building. Arthur Maher, from Durand, Wisconsin, was hired as stone mason. At that time, plans called for a building measuring 52’ × 30’, built from “loose fragments of dolomite removed from the cave, and will be of one story with a full basement. An easy series of stairways will lead from the basement to the first and to the second and third levels below. The basement will be used to display the numerous minerals, rocks, and fossils found at the cave site.…” (Taken from The Sun newspaper, 23 April 1942.) The cave itself had a reported 335 m (1101 ft) of passageway open to the public at a depth of 25 m (81 ft). The local Sun newspaper reported opening day as sometime in June of that year. In conversation with Mrs. Henry Friede, original plans were to have a “Memorial Day Weekend Grand Opening!”
There are no newspaper accounts of opening day. On Friday, 29 May and Saturday, 30 May 1942, the area received 1520 cm (6–8 inches) of rain in 30 hours causing massive flooding of Spring Valley (water depths of 2 m [6–7 ft] and more almost destroyed the newspaper office). Mr. Friede was forced to delay opening weekend until 7 June 1942.
For much of the period following opening of the cave in 1942, the length of known passageways remained ∼400 m (1300 ft). Side leads too narrow to explore discouraged future property owners until cavers and geologists Blaze and Jean Cunningham purchased the cave in 1986. Intrigued by the many small openings, the two cavers began digging loose sediment, and a new period of discovery began.
Members of the Minnesota Speleological Survey and the Wisconsin Speleological Society soon joined in the push to find more cave passages. In 1991, a tight lead was dug open and almost 300 m (1000 ft) of new cave was discovered. The excitement brought more cavers and more digging, resulting in more than 915 m (3000 ft) of new rooms and passages added over the next several years. These areas remain “wild cave,” meaning no commercial development is planned.
The 100 acres of property surrounding the Crystal Cave entrance continues to be explored for new caves. Recreational cavers gather during various events each year to work on new digs and explore the vast resources of this unique geological setting. A handful of caves with names like Can Coon, Fuzzy Critter, Fox Den, Tree Fork, Gravity Works, Gargoyles, and South Portal exist on the cave grounds, waiting for explorers to seek out their hidden treasures.
site 2. the history of flooding and flood control along the eau galle river in spring valley, wisconsin
Note: English units are used in this paper to be consistent with units of data measurement and collection. All historical information summarized from Blegen (1995) unless otherwise noted.
History of Flooding in Spring Valley
The first settlers arrived in Spring Valley, Wisconsin, in the late 1850s seeking timber to support sawmills and build railroads that would drive economic development in the region. Most of the timber was harvested in close proximity to the Eau Galle and other rivers to facilitate transportation and processing of the logs.
As a result, the land surrounding the river became eroded and the river channel was often jammed with logs—a recipe for disaster. On 15 May 1894, the skies unleashed three days of continuous rain, and Spring Valley’s historic battle with flooding began.
As if the environmental concerns with erosion and accelerated runoff weren’t enough, poor building and design of railroad bridges exacerbated the problem. By the late 1800s, Spring Valley boasted of having more than 12,000 ft of railroad track. With the rapid pace of development (mostly a result of iron mining by this time), the railroad tracks and bridges needed to be constructed quickly. This meant that the design had to be simple (a lot of pilings in the channel) with limited earth moving and grading, resulting in bridges with minimal clearance over the river. As flood waters poured down the Eau Galle, the bridges acted like dams, collecting debris and backing flood waters into downtown Spring Valley, not only destroying homes and businesses, but often wiping out the bridges (Fig. 1). One bridge in particular, on the south side of town, became affectionately known to residents as the “dam bridge.” In 1896, as flood waters were again rising, the bridge was “struck by several heavy discharges of lightning,” destroying the bridge, but quickly causing the flood waters to recede. It was later discovered that the “lightning” was actually dynamite used by inventive local citizens desperate to find a flood control solution.
The railroad bridges were rebuilt, and floods continued to be a way of life for the residents of Spring Valley. Each time it flooded, residents cleaned up and hoped they had seen the worse. Major flooding occurred in 1894, 1896, 1903, 1907, 1934, and 1938. And then came 1942. A multi-day rainfall event the end of May brought more than 6 in of rain, again washing out the railroad bridge and flooding downtown. Impressively, by 24 August, train service had been restored—for ∼3 weeks anyway. After a relatively dry summer, nearly 7 in of rain fell between 1 and 16 September. On 17 September, the temperature dropped more than 40 °F (a bad sign) and the skies unleashed. The local newspaper stated the following: “.if such sheets of water had fallen in Noah’s time, no forty days and night would have been needed for his flood to come” (Fig. 2).
The U.S. Army Corps of Engineers (USACE) later estimated that the peak discharge of the Eau Galle was an unimaginable 33,000 ft3 sec–1 (USACE, 2011a). It took only 15 min for the Eau Galle to overfill its banks and reach impressive depths of 20 ft near the south end of town (Figs. 3 and 4). Residents were trapped in their houses and businesses while trying to save property and possessions. Hardly a building was standing and there was no drinking water or food supply. The state of Wisconsin quickly dispatched the Red Cross and state guardsmen. Damage was estimated at more than $1.5 million. It was a miracle that no lives were lost.
Immediately after 1942 flood, residents who were now terrified of what future floods could do needed a “cure.” Two plans were up for debate: building a flood control structure or relocating the town. On 1 October 1942, less than 15 days after the historic flood, the headline on the front page of the local newspaper read “A New Spring Valley.” Stock was sold to purchase the land, and famed architect Frank Lloyd Wright visited the town and offered to design the new community. Despite momentum and enthusiasm, residents soon lost interest as homes and business were cleaned and rebuilt. With the successful completion of Boulder (Hoover) Dam, Americans and especially the residents of Spring Valley were sold on the idea of constructing a dam. By 1943, the U.S. Geological Survey began detailed topographic mapping and soon after the U.S. Army Corps of Engineers began their assessments. In 1956, the House and Senate passed the “Eau Galle Project,” only to receive a veto from President Eisenhower. The project was eventually approved two years later after modification. After a long wait, $4.18 million of funding for the project was finally made available in 1964, and construction began in 1965.
Construction of the dam was no simple task. The predominant bedrock in the region is of the Prairie du Chien Group, consisting largely of Ordovician dolostones. Dissolution created an underground network of caverns, including Crystal Cave. Engineers were concerned that the water level in the reservoir during flooding could cause enough pressure to force water through the subterranean passageways under the foundation of the dam and create artesian conditions downstream. To prevent this, the core of the dam was excavated nearly 45 ft to the bedrock surface. Holes were drilled into the bedrock along the length of the core and concrete was pumped at high pressures into the bedrock to fill the voids and cracks. The core was then back-filled with clay and an earthen dam was built (Fig. 5). It took an estimated 56,700,000 ft3 of material to complete the dam. The normal pool elevation is 940.0 ft above sea-level, ∼20 ft above the river valley floor. An emergency spillway (which serves as the road to the Eau Galle Recreational Area) was constructed to take floodwaters in excess of 1020.0 ft Construction of the spillway required excavation into the Prairie du Chien, exposing a nice bedrock section and serving as rip-rap on the face of the dam.
The dam was dedicated on 21 September 1968. Ironically, just days before the dedication, more than 3 in of rain fell, filling the reservoir in a single day (it was estimated to take several years normally). Since the construction, major storms have caused the water surface in the reservoir to rise more than 10 ft on three occasions (Fig. 6). August 2010 again brought historic rainfall to the region. Between May and August, more than 25 in of precipitation fell, with 8 in occurring in a one-week period in the middle of August (MRCC, 2011). The water level in the Eau Galle reservoir rose a record 22 ft to an elevation of 962.5 ft in less than a half-day, although this was still more than 50 ft below the elevation of the emergency spillway (USACE, 2011a) (Fig. 6). While the town of Spring Valley was spared, other areas within the region sustained major property and infrastructure damage.
Today, the Eau Galle Recreational Area is a popular destination for picnicking, hiking, fishing, horseback riding, and camping. In addition to flood protection and recreation, the U.S. Army Corps of Engineers operates the Eau Galle Aquatic Ecology
Laboratory (EGAEL) at the site. This cutting-edge facility researches complex ecological processes and water quality problems including watershed-scale nutrient transport, sedimentation, and rehabilitation of aquatic ecosystems (USACE, 2011b).
Site 3. The Gilman Iron Mine, Spring Valley, Wisconsin, (Utm: 557,447E/4,966,143N) West Of Spring Valley, Pierce County, Wisconsin
The Gilman Iron Mine near Spring Valley, Wisconsin, was one of a number of “brown iron ore” deposits discovered in this area in the 1870s. The mine was opened in 1893 and closed in 1907. A smelter was brought in, and processed not only local ore, but ore from the Gogebic and Mesabi iron ranges. (Allen, 1909; Van Hise and Leith, 1911). At its peak, it produced 100–125 tons of pig iron per day. Lime for flux and charcoal for fuel were produced nearby (Blegen, 1995). The mine and the smelter sites are still accessible. Although the outcrops are overgrown, we will be able to collect typical iron ore samples and the underlying dolostone at this stop (Figs. 1–3).
The iron ore is limonite, mostly made of nodules and ledges of massive earthy goethite. Local dark brown lustrous goethite occurs as botryoidal masses in vugs. It forms massive to weakly stratified blankets lying unconformably on Ordovician Prairie du Chien Group dolostones. It rests underneath a pre-Wisconsin drift cap and a layer of clay of unknown age. The ores are considered to be a weathering residue with the iron redeposited by surface and ground water in stagnant conditions on the surface and along joints penetrating into the underlying dolostones (Allen, 1909; Van Hise and Leith, 1911).
There are similar deposits elsewhere in Wisconsin as well in adjacent Iowa and Minnesota. Andrews (1958) includes the site as an exposure of the Iron Hill Member of the Cretaceous Windrow Formation. Iron oxide cemented sandstones containing leaf fossils of Cretaceous age are found several miles to the east (Keen, et. al., 2008). Bleifuss (1972) argues that similar deposits in Minnesota represent Tertiary weathering of siderite in Devonian sedimentary rocks. However, there is no evidence that Devonian deposits extended to this part of Wisconsin, thus it is likely this is a Windrow equivalent unit.
We thank Jay and Wendy Arneson of Spring Valley, Wisconsin, for allowing access to this property.
Site 4. Geologic Discovery Through Anomaly Investigation As Shown By The Rock Elm Disturbance, Pierce County, Wisconsin
Introduction to the Rock Elm Stops
The Rock Elm Disturbance is a nearly circular region of anomalous rocks and structures in west-central Wisconsin (UTM: 560,738E/4,951,549N). The disturbance has a diameter of ∼6.4 km (4 mi) and its center is ∼2.5 km south-southwest of the small town of Rock Elm, Pierce County, Wisconsin (Nelson, 1942; Cordua, 1985, 1987; Cordua and Evans, 2007). The feature is visible from space on Google Earth, as the different bedrock within the structure results in different soils and land uses. The finding of shocked quartz in the feature establishes it as the product of a hypervelocity impact of an extraterrestrial body. Paleontologic and stratigraphic evidence indicate a middle Ordovician time for this impact.
Discovery of this feature represents a small-scale application of an important principle in science: the investigation of anomalies. Anomalies are what help lead science into revising old theories and developing new ones. This trip will show how anomalies from the expected regional geology led to the recognition of this intriguing feature.
Figure RE-1 shows the basic geology and the sites we will visit. The stop at Crystal Cave has shown the geology normally expected in the region. The next stops visit several anomalous outcrops that do not fit into the regional picture. These represent pieces of the puzzle that are needed to understand the whole. A concluding overview stop will put these together for an interpretation of the structure. The trip participant should keep in mind in visiting the stops that this interpretation only emerged from the piecing together of numerous geological clues.
Western Wisconsin is part of the stable continental interior platform and is generally underlain by little deformed Paleozoic strata. These rocks were deposited from the Late Cambrian to the Middle Ordovician under shallow marine conditions. Regionally, the sedimentary rocks dip westerly at <1°. The normal Paleozoic units in the vicinity of the Rock Elm Disturbance are the Prairie du Chien Group, the St. Peter Sandstone, the Glenwood Shale, and the Platteville Formation. The bedrock of the region is partly obscured by glacial drift, alluvium and colluvium, which can locally be over 15 m thick (Brown, 1981). The bedrock is well displayed in Crystal Cave, but near Rock Elm the bedrock geology is full of anomalies.
Stop Rock Elm 1. Anomaly #1: Blue Rock Syncline, as Seen at the Blue Rock “Underlook”
Nugget Lake County Park. NW SW sec. 32, T26N RI5W (UTM: 561,336E/4,948,870N).
Within the Rock Elm Disturbance are several blocks of folded and faulted Prairie du Chien Group rocks. The largest of these, of which this outcrop is part, is informally called the Southern Fault Block. Here Plum Creek has cut through an elegant syncline in Prairie du Chien Group rocks. The contact between the Oneota Formation and overlying Shakopee Formation is taken to be the sandstone bed midway up the outcrop. The limbs dip from 20° to 30°. The fold plunges gently to the east. Joints here are not vertical, as is usually the case in this formation, but instead fan the fold. The outcrop is dominated by massive to thick-bedded dolostone, some with isolated vertical burrows. The dolostone is internally brecciated, and bedding planes locally are disrupted along nearly bedding parallel faults.
These rocks are obviously structurally anomalous compared to the flat-lying strata seen elsewhere throughout the region. When faced with this, the geologist must ask what forces could do this in such a local area. We are far from any plate boundaries, even as far back as the Ordovician. There are no signs of igneous materials, or recrystallization, which adds to the mystery. Field mapping, best done in the spring when both foliage and hunters are sparse, helped lay out the extent and orientation of these structures.
This fold is part of what is informally called the Southern Fault Block which forms a crescent shaped region at the south end of the disturbance with many mapped folds in Prairie du Chien Group dolostones. The folds are open and upright, with dips in the limbs ranging from 20° to 70°. The fold axes have a variety of orientations, but many trend north-northwest, and plunge gently south-southeast. The Southern Fault Block has a complex history. It was initially displaced downward relative to the normal section outside the disturbance. It was later uplifted, as shown in Stop Rock Elm 3.
Stop Rock Elm 2. Anomaly #2: Rock Elm Shale in Road Cuts near Shale Pit
490th Street, east of crossing of Rock Elm Creek. SE SW sec. 21, T26N, RI5W (UTM: 562,875E/4,95I26IN).
At this elevation, one should be in the dolostones of the Shakopee Formation of the Prairie du Chien Group. Yet no dolostone is found here. Instead one finds shale interbedded with siltstone and fine-grained sandstone. This is informally called the Rock Elm shale (Cordua, 1985, 1987; Cordua and Evans, 2007). Its presence here represents both a lithologic and stratigraphic anomaly. We can examine typical examples of it in the cuts along both sides of the road. We will not climb back to walls of the water-filled pit, as access is difficult and the walls are treacherous.
The Rock Elm Shale is a minimum of 35 m thick and consists of gray, green, and brown shale interbedded with finegrained silty feldspathic wacke. The shales are non-calcareous and locally have pyrite and iron oxide concretions up to 5 cm. across. The sandstone interbeds average 2–4 cm in thickness and are internally laminated. Crawling tracks mark bedding plane surfaces. The shale-sandstone interbeds are well shown in the exposure, as is the essentially horizontal nature of the basin fill.
A paleontologic survey of outcrops and drill cores through the Rock Elm Shale (Nelson, 1942; Peters et al., 2002) found gastropods, conodonts, and scolecodonts, which suggest the Rock Elm Shale is Middle Ordovician in age deposited between 430 and 470 Ma. The fossils establish a marine origin for the Rock Elm Shale.
Mapping the distribution of this shale shows it as restricted to a relatively circular outcrop area (Fig. RE-2). The weathering of this rock provides a distinctive poor soil called the “Derinda acid variant.” Soil maps showed the approximate limits of the disturbance long before the geological cause of the bedrock anomaly was delineated (Haszel, 1968). This soil and its accompanying contrasting land use relative to the surroundings is the reason the disturbance shows up so well on satellite imagery (Fig. RE-1).
Stop Rock Elm 3. Anomaly #3: Boundary of the Southern Fault Block and Basin Fill
County HH, east of the junction with Highway CC. NE NE Sec. 31 T. 26N. RI5W(UTM: 560,I75E/4,949,662N).
This stop features two exposures. The outcrop in the trees south of the road shows deformed Prairie du Chien Group dolostones. On the north side of the road is undeformed Rock Elm shale. A fault, called the Highway HH fault, separates the two. These age relationships are critical in interpreting the geological history of the feature. The shale is clearly post-folding, but pre-faulting. Deformational relationships as complex as these are unknown in the Paleozoic rocks of western Wisconsin outside the Rock Elm Disturbance. Outcrops farther to the north, and drill core data both show that an angular unconformity separates the less deformed shale from underlying deformed Prairie du Chien Group dolostones.
Stop Rock Elm 4. Overview of the Rock Elm Disturbance—South Rim
2I0th Street, near 4I0th Avenue, 2.5 miles north of Ono, Pierce County, Wisconsin. NW SW sec. 32 T26N RI5W (UTM: 560,525E/4,948,492N).
From this vantage point, we are looking north into the Rock Elm Disturbance from the south rim. The farm on the farthest northwest horizon is on the northern rim of the disturbance. We can see most major features of the disturbance and put together the evidence telling its story (Fig. RE 2).
The break in slope before us is the trace of the ring boundary fault. It separates normal rocks from the anomalous rocks within the disturbance. The fault is marked topographically on the south by an arcuate ridge resulting from the juxtaposition of resistant Prairie du Chien Group dolostones and softer basin fill shales Here the fault trends nearly east-west and continues into the trees on the east, where it passes through Nugget Lake Park. Its curving nature is reflected in the bend of the escarpment visible on topographic maps. The fault then turns to the north past the eastern horizon, swinging back east-west, passing behind the wooded hills on the near skyline and south of the farm on the far horizon. On the northwest side of the disturbance, its trace is obscured by glacial drift, but it trends north-south again, west of the junction of County CC and County HH before returning to our present position. The elevations of Prairie du Chien Group rocks (in outcrop and drill cores) show that the area just inside the ring boundary fault has been displaced downward at least 45 m relative to outside of the disturbance.
The wooded hills to the north, forming the near skyline, are the central uplift Near the geographic center of the disturbance is an oval area ∼1 km by 2 km containing outcrops of sandstone and conglomerate that generally dip 20° to 40° away from the center. On the basis of lithologic similarities, the presence of brachiopods and the absence of post-Precambrian clasts, this sandstone is identified as the Upper Cambrian Mount Simon Formation, the basal Cambrian unit in the region. If this correlation is correct, then the rocks of the central uplift have been raised 300–350 m from their normal stratigraphic position (Thwaites, 1957; Mossler, 2001). Shocked quartz grains have been found in these rocks (French et al., 2004).
The low areas drained by Rock Elm Creek and Plum Creek are underlain by Rock Elm Shale (Stops Rock Elm 2 and 3). The flat-topped hills, especially visible to the east, are held up by the Washington Road sandstone, a feldspathic sandstone unit at least 15 m thick that conformably overlies the Rock Elm Shale.
The hummocky area between this vantage point and County HH is the Southern Fault Block. This is underlain by folded and faulted Prairie du Chien Group rocks (Stops Rock Elm 1 and 3). This block is bounded on the north by the Highway HH fault, which juxtaposes the deformed dolostones with little-deformed Rock Elm Shale.
Evidence for the Impact Origin of the Rock Elm Disturbance and an Interpretation of the Anomalies
Melosh (1989), Koeberl and Anderson (1996), French (1998), French and Koeberl (2010), and many others document the geologic features associated with meteorite impact craters. Many of these features occur at Rock Elm. Impact craters are often noticed by their circular morphology and presence of anomalous deformed rock in an area otherwise lacking significant deformation, as is the case with the Rock Elm Disturbance. The ring boundary fault can be interpreted as marking the edge of the modified crater. Various cores taken through the Rock Elm shale intersect underlying breccia with a variety of deformed and jumbled clasts of Paleozoic strata. This could be interpreted as impact breccia. The Southern Fault Block is interpreted as one of several large blocks that slumped into the crater as its walls collapsed. The central uplift is equivalent to those mapped in many other verified impact structures. The lack of any hydrothermal or igneous materials is consistent with an exogenous formation of the disturbance.
Quartz grains in rocks of the central uplift contain two types of shock-produced microstructures identical to known planar fractures typical of shock metamorphism (Fig. RE-3; French et al., 2004). These allowed tentative estimates of shock pressures of at least 5 and perhaps greater than 10 Gigapascals (French et al., 2004). No known terrestrial process is capable of such conditions in crustal rocks (French, 1998). Accepting this as an impact structure, one can do further modeling. Apparently an extraterrestrial impact occurred in the Ordovician ∼440–505 million years ago, at the end of or soon after deposition of the Prairie du Chien Group. French, et al. (2004) suggests that the impacting object to have a diameter of perhaps 200 m, producing a transient cavity 4 km in diameter and 1.3 km. deep. Almost immediately, during the modification stage, the crater rapidly widened and the central uplift rose. Fallback debris, impact glass, and slumped and backwashed megablocks of deformed Cambrian and Ordovician strata would rapidly fill much of the crater. The Southern Fault Block is interpreted as one of these slump blocks (Stops Rock Elm 1 and 3).
Later marine deposition filled the crater with a coarsening-upward sequence of the Rock Elm Shale and Washington Road Sandstone ∼440–470 million years ago (Stops Rock Elm 3 and 4). Minor reactivation of the structure, possibly due to continuing isostatic adjustment, caused minor faulting and slight dips in basin fill strata. These locally juxtapose basin fill strata against rocks of the central uplift and Southern Fault Block (Stop Rock Elm 3). Pleistocene and Quaternary fluvial and glacial processes complete the story.
Rock Elm “Gee-Whiz” Notes
(Adapted from Dr. Allen Scott, University of Wisconsin–Stout)
The shock features suggest the impactor (asteroid or comet) would have been about the length of 2 football fields (= Lambeau Field plus the surrounding stadium). The velocity of impact was likely ∼30 km/sec (65,000 mph). The impact energy approximately = 1 billion tons of TNT = 6300 Hiroshima-sized A-bombs. It formed a crater ∼3/4 mi deep and 4 mi across. The shockwave would have flattened anything within 70 km radius, with hurricane-force winds blowing 120 km away. Talk about a bad day in the Ordovician!
This work was partly funded with grants from the University of Wisconsin-River Falls Foundation and the Wisconsin Geological and Natural History Survey, and by STATEMAP, the state component of the National Cooperative Mapping Program of the U.S. Geological Survey.
Figures & Tables
Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America
This volume of 25 field guides plus one paper on field instruction was prepared in conjunction with the 2011 GSA Annual Meeting in Minneapolis, Minnesota. The diverse slate of field trips spans a geologically broad range of topics, including the Precambrian geology of the southern Canadian Shield; the economic geology of the Lake Superior region; Phanerozoic strata in Minnesota, Wisconsin, Iowa, and North Dakota; glacial geology; hydrogeology and limnology; undergraduate and K12 geoscience field education; archaeological investigations in the upper Mississippi River valley; and geology by bicycle.
- areal geology
- educational resources
- field trips
- geologic hazards
- glaciated terrains
- impact craters
- impact features
- iron ores
- metal ores
- natural curiosities
- natural hazards
- Pierce County Wisconsin
- rivers and streams
- road log
- solution features
- United States
- western Wisconsin
- Spring Valley
- Crystal Cave
- Rock Elm Disturbance
- Eau Galle River
- Eau Galle Dam
- Gilman Mine