Geologic History of New Hope Cave, Manitowoc County, Wisconsin

John A. Luczaj (luczajj@uwgb.edu) and Ronald D. Stieglitz (stieglir@uwgb.edu)Department of Natural & Applied SciencesUniversity of Wisconsin - Green Bay2420 Nicolet DriveGreen Bay, WI 54311

INTRODUCTIONThis article presents the preliminary results of an ongoing study of the geologic

history of New Hope Cave in Cherney Maribel Caves County Park, Manitowoc County, Wisconsin. The sediments in the cave contain an important record of the events responsible for filling the cave, and may shed light on the geological processes operating on the surface. A brief introduction to the regional geology of northeastern Wisconsin is given here to help the reader place this research into a broader context.

GEOLOGIC SETTINGThe bedrock of Northeastern Wisconsin is primarily composed of the

sedimentary rocks dolostone, shale, and sandstone that were deposited during the first half of the Paleozoic Era between 540 and 350 million years ago (Figure 1). Most of these were deposited in warm shallow oceans when this part of North America was situated very near the equator, and at times when global sea level was episodically higher. The limestone deposited in those oceans was altered into a new type of magnesium-rich carbonate rock, known as dolostone (made from the mineral dolomite), by water-rock interaction processes early in their history (Luczaj, 2006). As a result, northeastern Wisconsin’s caves are hosted exclusively by dolostone.In total, up to 600 meters (~2,000 feet) of Paleozoic sedimentary rocks are preserved in the area above the Precambrian basement rocks (Figure 2). These Paleozoic rocks thickens to as much as 4 kilometers under the central part of

the lower peninsula of Michigan. These rock units have been tilted slightly to the east and thicken to as much as 4 kilometers in the ancestral Michigan basin, a folded

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Figure 1. Bedrock Geology of Wisconsin showing the distribution of Paleozoic rocks. Figure courtesy of WGNHS.

structure underlying lower Michigan. This is the reason that the well-known Niagara escarpment trends northeastward from the region south of Fond du Lac to the tip of Door County and beyond. The Niagara escarpment is essentially a prominent cliff or ridge of Silurian dolostone that stands high because it is more resistant to erosion than the softer underlying Maquoketa Shale.

The youngest rocks preserved in eastern Wisconsin are the Devonian (359-416 million years ago) sediments near Milwaukee. Rocks younger than this that were deposited in the region have since been removed after long periods of river erosion and, more recently, glacial erosion. As a result, there is a large gap in the geologic record of Wisconsin that stretches from about 360 million years ago until around 20,000 years ago when the glaciers began their most recent retreat from the area. Glacial sediments from at least three major advances of the Late Wisconsin lobes of the Laurentide Ice sheet are preserved in northeastern Wisconsin. They are interspersed with thick sequences of fine-grained sediments from Glacial Lake Oshkosh (Hooyer, 2007). These glacial deposits from the Pleistocene Epoch rest directly upon freshly scoured bedrock and conceal much of the underlying geology of the region, including much of the karst landscape.

The glacial till that covers the Cherney Maribel Caves County Park is mapped as the Valders Member of the Kewaunee Formation. This unit was deposited by the Lake Michigan Lobe of the Laurentide ice sheet between about 12,000 and 13,000 14C years before present (Clayton and others, 2006). Little is known about the age or character of glacial sediments that might have been present over the park before this event.

KARST DEVELOPMENT IN NORTHEAST WISCONSINCaves have been found throughout much of Wisconsin where carbonate bedrock

is present. They are best accessible in regions where no glacial materials are present or in areas with thin glacial drift. In northeastern Wisconsin, there are two principal bands of carbonate rocks (in this case dolostone) that are susceptible to the development of karst features such as caves, sinkholes, and solution-enlarged fractures. To the east of Lake Winnebago and the Fox River Valley, the Silurian dolostones extend all the way to the Lake Michigan shoreline. To the west of the Fox River Valley, two layers of Ordovician age dolostone bedrock of the Sinnipee and Prairie du Chien Groups are quite extensive and cover most of the region as far west as

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Figure 2. Stratigraphic column for northeastern Wisconsin. Modified from Mugel and Pratt, 1991; Harris and others, 1998.

Shawano and New London. While they have essentially the same chemical composition, the difference in development of karst features in each group of rocks (or lack thereof) is striking. A wide range of karst landforms has been recognized and mapped during studies focusing on groundwater resources in the Silurian bedrock (e.g., Wiersma and others, 1984; Johnson and Stieglitz, 1990). These include small karren, dolines, solution-enlarged fractures, and dry valleys. In contrast, very few karst features have developed in the Ordovician dolostones of northeastern Wisconsin. In fact, extensive research conducted by Luczaj (e.g., Luczaj, 2006, ongoing) has shown that fractures within Ordovician dolostone still preserve relatively unweathered sulfide minerals along vertical fractures in the region. There may be several reasons for this

systematic difference, but perhaps the existence of the Maquoketa Shale has had the most profound effect. Prior to Pleistocene glaciation, it is likely that the Maquoketa Shale extended considerably farther west than its present location. This would have, in effect, isolated the underlying Ordovician dolostone from aggressive waters at the surface or in shallow groundwater. In addition, the topographic elevation of the Silurian

might have also amplified the karst development to the east because of high groundwater flow rates (Bloom, 1998). In any event, it seems clear that the Silurian rocks east of the Fox River Valley are host to the majority of the karst features of northeastern Wisconsin.

One of the most difficult questions to address while unraveling the karst history of northeastern Wisconsin is the question of its timing. Because the process of karst development is primarily one of bedrock dissolution, geologists are left with little evidence that records precisely when this takes place. To establish an age range for a particular event, geologists use cross-cutting relationships between rocks of known age as well as numerical age dates determined for certain materials such as isotopically-dateable materials.

Most of the karst developed in northeastern Wisconsin appears to predate the Pleistocene glaciation, which likely began about 2.4 million years ago in this part of North America (Clayton and others, 2006). A wide variety of debris has been observed in sinkholes, caves, and solution enlarged joints of northeastern Wisconsin. It appears likely that the upper portion of the pre-glacial karst landscape has been removed,

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Figure 3. Solution-enlarged fracture in Silurian dolostone from southern Brown County. Photo by John Luczaj.

leaving open or sediment filled features (Figure 3). Unfortunately, no isotopic age dates have been obtained from speleothems or cave decorations, and organic materials have yielded age dates that are too young to indicate much about the timing of cave development.

WHAT HAS BEEN FOUND IN EASTERN WISCONSIN CAVES?A wide variety of materials have actually been discovered in caves or other karst

features from Door County in the north to places as far south as Waukesha County, Wisconsin. This paper does not attempt to provide a comprehensive review of all Wisconsin cave discoveries. Instead, it is an attempt to illustrate the variety of materials that can be found and to provide examples of how this information can help geologists reconstruct the history of our region. Probably the oldest karst feature preserved in the Silurian bedrock of eastern Wisconsin was found in the Halquist Quarry in Waukesha County, Wisconsin. Early Devonian non-marine plant fossils were found atop an estimated 16 meter section of mudstone in a large, solution-enlarged fissure in the Silurian Waukesha and Racine formations. These non-marine plant fossils record the earliest plant fossils preserved in Wisconsin karst (Kuglitsch and others, 1998; Jahren and others, 2003).

When one thinks of caves, stalactites, stalagmites, and other cave decorations often come to mind. These precipitates of calcium carbonate (CaCO3) usually form when the CO2 is released from solution when water enters a cave (Bloom, 1998). In some cases, these “speleothems” can be age dated to reveal the timing of precipitation and have major implications for our understanding of past climate changes (e.g., Drysdale and others, 2007). Bones, teeth, wood, pollen, and other organic materials have revealed much about the history of caves throughout the world. In Door County, Wisconsin, the Brussels Hill Pit Cave preserves a sequence of cave sediments and flowstone that are interpreted to be approximately 10,000 years old. This cave, which is the deepest in Wisconsin at 28 meters, preserves numerous remains of wood, leaves, shrews, bats, deer, bear, beaver, muskrat, and otter. Two 14C dates on organic sediments at the 28 meter depth were dated at 671 and 1,820 14C years before present. While the record of organisms preserved at this location appears much younger than any glacial activity in the region, important information about Wisconsin’s pre-European settlement faunal assemblages can be learned (Brozowski and Day, 1994). A more striking example of how a surface faunal assemblage can be preserved in a karst feature is from the Pipe Creek Junior Sinkhole in Grant County, Indiana (Farlow and others, 2001). There, an assemblage of numerous plant and animal species from part of the Pliocene Epoch (~4 to 5 million years ago) were found in a sinkhole uncovered by quarrying operations. The animals, some of which are now extinct, included frogs, turtles, fishes, birds, snakes, as well as small and large mammals. The deposit is a unique window into the continental biota and the climate present in the continental interior of eastern North America during the Tertiary Period. The contents of most northeastern Wisconsin caves usually consist of sediments washed in from above, in the case of sinkholes and fissures, or less commonly sediments deposited by water flowing through cave passages. Sediments deposited by flowing water can preserve spectacular information about the direction, speed, and

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hydraulic characteristics of the system. It is this type of system that was responsible for sediment deposition in the New Hope Cave at Cherney Maribel Caves County Park.

NEW HOPE CAVECherney Maribel Caves County Park is located along the western side of the

West Twin River in northwestern Manitowoc County, Wisconsin. The New Hope Cave, located near the northern border of the park, was discovered by Norb Kox and Paul Pryes on February 5, 1984 (Figure 4). The cave contains over 400 feet of passageway,

which was filled or nearly filled with sediment. Although excavation began in 1984, more systematic excavation began in the past few years with floor to ceiling excavation beginning in 2006 (Henry Welch, written communication). Systematic excavation of the unconsolidated sediments in the cave is being performed by members of the Wisconsin Speleological Society and the Friends of Maribel Caves and is facilitated by the Manitowoc County Parks System. Once opened, passageways vary in height between about 5 feet and 13 feet. The sediments in the cave have a variety of grain sizes, indicating a series of events, some of which were high velocity flow events. Sedimentary strata in the cave are somewhat variable in extent and thickness, but a general picture of the stratigraphy shows three main packages of sediment that have been documented in the cave.

Stratigraphy of Cave SedimentsThe lowermost package is 0 to ~4 feet in thickness and is made mostly of fine to

medium grained sand, which overlies laminated greenish to red clay and sand intercalations near its base. The lowermost package of sediments is the most visually distinctive of the three because of its bright orange appearance (Figure 5). It exhibits

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Figure 4. Map of New Hope Cave courtesy of Letha Welch, slightly modified.

what is known as liesegang banding, a chemically precipitated banding of iron oxides that occurs in fluid saturated sediments or rock. The top of this orange unit shows a distinct erosion surface and pieces of dark orange flowstone, indicating that the erosion occurred after development of the iron oxide staining. Numerous pieces of collapsed ceiling rocks up to a few feet in length (~1 meter) are preserved at the interface between the lower and middle packages of sediment.

The middle package of sediments is present throughout most of the cave, and has a light buff to dark brown color. Overall the package was best preserved in the Southeast passage, where it was at least 6 feet thick. Two very coarse grained conglomerate layers were preserved in this package of rocks, each of which is about 1 foot thick. The first gravel layer occurs at the base, directly above the orange colored sediments of the lower Package (Figure 5). Overlying each of the two gravel layers is a thick sequence of well sorted, buff to brown colored sand that illustrates tabular cross-bedding and ripple cross-laminations.

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C

C

W

Lower Package

Middle Package

Do

losto

ne W

all

Ro

ck

S

S

Figure 5. Exposed cut showing the lower and middle sediment packages at profile 7 in the Southeast Passage. “C” indicates conglomerate layers, “S” indicates sandy layers of the middle package, and “W” indicates a water escape structure. Photo taken on 11/12/2006, courtesy of Letha Welch.

The conglomerate layers in the middle package of sediments consist of coarse gravels up to 5 inches (13 cm) in diameter, with a sandy matrix between the grains. These gravels are apparently the first record of glacially-derived sediment in the cave.

Clasts of gabbro, granite, and other non-local rocks are found in these two layers, likely indicating glacial transport to the area from locations hundreds of kilometers to the north. The sand layers above each gravel layer show spectacular channel scours, fluid-escape structures, and angular blocks of dark brown coarse sand that were apparently transported while frozen (Figure 6). These sand layers preserve sedimentary structures such as cross bedding and climbing ripples that indicate flow was apparently moving

eastward during deposition in the cave, toward the bluff along the West Twin River valley. The sands of the middle package were likely deposited by water flowing at 10 to 20 cm/second, but the coarse gravels likely required flow rates upward of at least 1-2 meters/second (Bloom, 1998).

The upper package of sediment was best exposed in the West Passage at the time of this article. This package also rests above a substantial erosion surface developed on the top of the middle package of sediments. These sediments consist of laminated brown clays with interbedded poorly sorted gravel containing clasts up to an inch (~2 cm) in diameter. Overall, this package appears better preserved to the west, but most attempts to carve vertical exposures of this unit have not been successful due to collapse of the water-saturated sediment.

Above the three packages of sediment in the cave is a flowstone layer, usually one-half to one inch (1-2 cm) thick, but rarely as thick as 4 inches (J.D. Skattebo, personal communication). This layer was present in most parts of the cave where excavation has been completed. In some places, collapsed ceiling rock (Silurian dolostone) is preserved beneath the flowstone at or near the top of the upper sedimentary package. A loamy layer occurs in the Southeast passage above the collapsed ceiling rocks, but below the flowstone layer. Additional ceiling collapses have been found on top of the flowstone layer.

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Figure 6. Dark brown angular clasts of unconsolidated sand. The angular shape and lack of a cohesive matrix suggests that these blocks were frozen when deposited. Block on left is approximately 4 inches long. Photo by John Luczaj.

Water Escape StructuresAt numerous locations along the Southeast passage, vertical conical and cylindrical-shaped water escape structures have been identified. They appear to emanate from either the lowermost gravel layer in the middle package of sediments or from fractures at the floor of the cave. They range in size from a few inches wide and a foot high to as much as three feet in diameter and at least 6 feet high. The largest water-escape structure is over 3 feet wide and continues vertically from the bedrock floor through the middle package of sediments at the end of the southeast passage (J.D. Skattebo, personal communication). One of these was a cylinder of gravel that

was cored by sand (Figure 7; J.D. Skattebo, personal communication). Others are funnel shaped structures that widen upward, and some of these has graded bedding that displays a fining-upward trend in grain size.

These structures are interpreted as water-escape structures for two main reasons. First, they emanate from zones of high permeability that are capable of carrying significant amounts of water. In addition, they cut across less permeable, finer grained sediments toward the surface. One of these even shows evidence for at least two episodes of water escaping.

Flowing water along the surface of the cave sediments is unlikely to have generated these structures. Seasonal fluctuations of the water table are also not likely to have generated the differential pressures needed to cut through the upper layers. A more likely source for the high-pressure water is one of glacial origin. The interconnection between karst systems and subglacial drainage systems was reported by Sharp and others (1989, page 129) for a glacier in Switzerland. High-pressure water at the base of the glacier during transport or melting might produce the hydraulic head necessary for the formation of such structures.

Age and Origin of Sediments in the CaveIn an alcove north of West Dome #1 in the West passage, several disseminated

bones were found by J.D. Skattebo and Al Schema during January and February 2008 (Figure 4). The bones were found within sand and gravel layers of the upper package of sediments, about 2 inches (5 cm) below the flowstone layer. While bones of several animals have been found both above and below the flowstone throughout the cave, including bats and possibly a woodchuck, these bones appeared to be contained within the gravel. As a result, an age estimate for the upper sediment package can be made.

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Figure 7. A large gravel water escape structure in the Southeast passage. In this view, the cross-bedded sands have been excavated around the structure, which is about 1 foot in diameter. Photo courtesy of Letha Welch and J.D. Skattebo.

The bones found have not yet been precisely identified, but they appear to include thin bat bones, two bat mandibles, and two larger vertebrae (Figure 8). The two vertebrae were selected by Beta Analytic, Inc. of Miami, Florida as ideal for radiocarbon dating. According to the laboratory, the two bones contained plenty of carbon for an accurate measurement of carbon-14 (14C) content. The bones were prepared for analysis by standard methods, which involved bone collagen extraction with alkali treatment. The isotopic composition of the collagen was analyzed to determine whether alteration had likely occurred. The 13C/12C ratio for the collagen was -20.2‰, which indicates that the bone was likely not altered and that it is likely to yield an accurate age date. The remaining carbon was analyzed to determine the radiocarbon age using standard accelerator mass spectrometry (AMS) methods.

The Conventional Radiocarbon Age reported by Beta Analytic, Inc. is 4,930 ± 40 years before present (BP). Because the percentage of 14C relative to other isotopes of carbon in the atmosphere has not been constant through time, the radiocarbon age needs to be calibrated. Using the INTCAL04 Radiocarbon Age Calibration, Beta Analytic determined the 2! calibrated result to be 5,590 to 5,740 Calendar years BP. As a result, the age of this material is one of the oldest, if not the oldest, bone recovered from a Wisconsin cave.

Although the bones were transported and deposited along with the sediments, they showed little or no abrasion. The gravel layer in the upper sediment package in which the bones were discovered is less than or equal to 5,590 to 5,740 years old. The nature of the non-abraded vertebrae and the unbroken thin bat bones suggests that these bones were reworked only by a relatively mild event that modified the uppermost portion of the upper sedimentary package. The middle and lower sediment packages in the cave are likely considerably older than 5,590 to 5,740 years. While it is not presently possible to interpret the exact origin of each of the layers, certain constraints can be placed on their formation.

The middle sedimentary package contains sands and gravels deposited under high enough flow rates to indicate very rapid flow of water in the cave. The gravel layers contain small glacial erratics that indicate a glacial origin for the sedimentary debris. After deposition of the middle sedimentary package, high-pressure water must have escaped from high permeability fractures and gravel layers in communication with those fractures. It seems likely that these sediments were deposited during or before

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Figure 8. One of two vertebrae found near the top of the upper sediment package, about 2 inches beneath the flowstone layer. Vertebra is approximately 1 cm long, and is of an unknown animal, likely a mammal larger than a bat, but smaller than a woodchuck. Photo by Dan Meinhardt.

the final melting of the last ice sheet that occupied the area. The presence of apparently frozen blocks of sand in the cave indicates a significantly colder climate, given that the temperature inside the cave has not been observed below 42°F (5.5°C), despite exterior temperatures of -21°F (-29.5°C) (J.D. Skattebo, personal communication).

The lowermost package is more difficult to interpret. The collapsed ceiling interval and the liesegang banding preserved in the lowermost sedimentary package occurred before the deposition of the middle package of sediments. While it is not easy to determine when deposition of this package occurred, it is possible that the sediment was deposited either during an interglacial period or perhaps even before the first glaciers reached the region during the Late Pliocene or Early Pleistocene epochs. Non-local pebbles and grains have not been documented from this lower sedimentary package, and a substantial amount of time likely elapsed to record the thick flowstones, ceiling collapse blocks, and the liesegang banding of iron oxides. It is important to note that the chemical and hydrologic conditions responsible for forming the liesegang banding have not reoccurred for at least as long as the middle sedimentary package has existed (> 5,590 years).

SUMMARY AND FUTURE WORKThe New Hope Cave in Cherney Maribel Caves County Park contains a

spectacular record of water-transported sediments that preserve a wide array of sedimentary structures. Three main sedimentary packages are recognized in the cave require that at least some of the sediment to be at least 5,590 to 5,740 years old. Water-escape structures and high flow regime sedimentary structures are consistent with glacial activity in the region. The lower unit in the cave displays an unusual orange iron oxide banding, and is likely considerably older than the other two units, possibly even pre-Pleistocene.

Work continues on the excavation along the West passage and a new branch from the Southeast Passage, deemed “Hidden Passage” (Figure 4). Further grain size and composition analyses will be performed, along with a search for more datable materials such as bone, wood, or pollen to help constrain the age of sediments in the cave.

ACKNOWLEDGEMENTS The authors thank members of the Wisconsin Speleological Society and Friends of Maribel Caves for access to the cave, collection of materials, and excavation work. We especially thank J.D. Skattebo, Letha Welch, and Henry Welch for their reviews and efforts, without which this article would not have been published. The radiocarbon age dating was paid for by the UW-Green Bay Research Council, Friends of Maribel Caves, and the Wisconsin Speleological Society. Undergraduate student Jen Wessel also participated in sample collection, and interpretation of sedimentary structures.

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REFERENCES CITEDBloom, A. L., 1998, Geomorphology: a systematic analysis of late Cenozoic landforms. Prentice Hall,

Upper Saddle River, New Jersey. Third Edition, 482 pages.Brozowski, J. and Day, M. J., 1994, Development of Brussels Hill Pit Cave, Door County, Wisconsin:

Evidence from flowstone and sediment., Transactions of the Wisconsin Academy of Sciences, Arts, and Letters, v. 80, p. 1-16.

Clayton, L., Attig, J.W., Mickelson, D.M., Johnson, M.D., and Syverson, K.M., 2006, Glaciation of Wisconsin. Wisconsin Geological & Natural History Survey Educational Series, no. 36, 4 pages.

Drysdale, R. N, Zanchetta, G., Hellstrom, J. C., Fallick, A. E., McDonald, J., and Cartwright, I, 2007, Stalagmite evidence for the precise timing of North Atlantic cold events during the early last glacial. Geology, v. 35, p. 77-80.

Farlow, J.O., Sunderman, J.A., Havens, J.J., Swinehart, A.L., Holman, J.A., Richards, R.L., Miller, N.G., Martin, R.A., Hunt, R.M.Jr., Storrs, G.W., Curry, B.B., Fluegeman, R.H., Dawson, M.R., and Flint, M.E.T., 2001, The Pipe Creek Sinkhole Biota, a Diverse Late Tertiary Continental Fossil Assemblage from Grant County, Indiana. The American Midland Naturalist, v. 145, p. 367-378.

Harris, M. T., Kuglitsch, J. J., Watkins, R., Hegrenes, D. P., and Waldhuetter, K. R., 1998, Early Silurian stratigraphic sequences of eastern Wisconsin. in, E. Landing and M. Johnson eds., Silurian Cycles: New York State Museum Bulletin, v. 491, p. 39-49.

Hooyer, T.S. 2007. Late-glacial history of east-central Wisconsin. Guide book for the 53rd Midwest Friends of the Pleistocene Field Conference, May 18-20, 2007, Oshkosh, Wisconsin. 2007-01 Open-file report, 87 pages.

Jahren, A. H., Porter, S., and Kuglitsch, J. J., 2003, Lichen metabolism identified in Early Devonian terrestrial organisms. Geology, v. 31, p. 99-102.

Johnson, S. B., and Stieglitz, R. D., 1990, Karst features of a glaciated dolomite peninsula, Door County, Wisconsin. Geomorphology, v. 4, p. 37-54.

Kuglitsch, J. J., Taylor, W. A., and Gray, J., 1998, Non-Marine Devonian Plants from Southeastern Wisconsin. Geological Society of America, Abstracts with Programs, v. 30, no. 7, p. 31.

Luczaj, J. A., 2006, Evidence against the Dorag (Mixing-Zone) model for dolomitization along the Wisconsin arch – A case for hydrothermal diagenesis. AAPG Bulletin, v. 90, p. 1719-1738.

Mugel, D. N., and Pratt, W. P., 1991, Lithologic cross sections of Phanerozoic rocks in the northern Midcontinent, U.S.A. United States Geological Survey, Miscellaneous Field Studies Map, MF 1835-G.

Sharp, M. J., Gemmell, J. C., and Tison, J. L., 1989, Structure and stability of the former subglacial drainage system of the Glacier de Tsanfleuron, Switzerland. Earth Surface Processes and Landforms, v. 14, p. 119-134.

Wiersma, J. H., Stieglitz, R. D., Cecil, D. L., and Metzler, G. M, 1984, Characterization of the shallow groundwater system in an area with thin soils and sinkholes (Door Co., WI). in, B. Beck ed., Sinkholes: Their Geology, engineering, and environmental impact - Proceedings of the First Multidisciplinary Conference on Sinkholes, Orlando, Florida, October 15-17, 1984, p. 305-310.

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