Geological Features of Wisconsin

Cave of the Mounds

The most beautiful cave in Wisconsin and the upper Midwest, Cave of the Mounds lies just off U.S. Highways 18/151 in Blue Mounds, WI. The United States Department of the Interior and the National Park Service designated Cave of the Mounds a National Natural Landmark because the site possesses "exceptional value as an illustration of the nation’s natural heritage and contributes to a better understanding of man’s environment". Curious tourists and budding cave enthusiasts have been enjoying an introduction to caving in the midwest for years while visiting Cave of the Mounds.

Commonly referred to as the "jewel box" of America’s major caves for the variety and delicacy of its formations, Cave of the Mounds is recognized by the Chicago Academy of Sciences as "the significant cave of the upper Midwest".

A guided tour of the Cave takes you past a varied collection of colorful stalactites, stalagmites, columns and other formations. The main cavern began forming over a million years ago as acidic water dissolved the limestone bedrock far below the surface. As Cave of the Mounds staff like to point out, geologic time is mind-boggling. It is difficult to imagine the time it took for the large caverns to be dissolved within rock that is itself believed to be over 400 million years old!

A lower meandering portion of the Cave was formed by the rushing water of an underground stream. The contrast between the chemical and mechanical processes of cave formation is one of the geologic lessons illustrated on The Cave Tour.

People who come to the Cave of the Mounds in the summer appreciate the constant 50-degree temperature. They also enjoy the park-like grounds, with picnic areas, walking trails and rock gardens. Winter visitors can take advantage of the Cave’s comparative warmth. The Cave is open in the winter on weekends, and during the week by advance reservation.

History of the Cave: Cave of the Mounds takes its name from the Blue Mounds, two large hills which have long been Wisconsin landmark features. The West Mound, at 1716 feet, is the highest point in Southern Wisconsin; the East Mound reaches 1489 feet. Cave of the Mounds lies under the southern slope of the East Mound.

This area was settled by Ebenezer Brigham, a successful lead miner who became Dane County’s first permanent white settler in 1828. The West Mound is now a Wisconsin state park; part of the East Mound still belongs to the Brigham family. Brigham County Park lies along the wooded northern edge of this East Mound. Both parks afford magnificent vistas of southern Wisconsin.

Cave of the Mounds was accidentally discovered on August 4, 1939. Workers, who were removing high quality limestone from a quarry on the Brigham Farm, blasted into the Cave. The blast tore the face off the quarry and revealed a great underground cavern. All quarrying stopped and never resumed. The dynamite blast revealed a limestone cave more than twenty feet high opening into other rooms and galleries, all containing numerous mineral formations.

The excitement of the discovery brought so many curiosity seekers that the Cave had to be closed in order to preserve it. Soon, lights and wooden walkways were installed. And, in May 1940, Cave of the Mounds was opened to visitors. Millions of visitors later, the Cave’s wooden walkways were replaced with concrete; a large stone building replaced the original entry building; and theatrical lighting has been installed to dramatize the colors and shapes within the Cave. Picnic areas, walking trails, rock gardens, gift shops and a visitor center have all since been developed.

In 1988, Cave of the Mounds was designated a National Natural Landmark by the United States Department of the Interior and the National Park Service. In receiving this honor, Cave of the Mounds was recognized as "a site which possesses exceptional value as an illustration of the nation’s natural heritage and contributes to a better understanding of man’s environment."

Limestone, a sedimentary rock produced from hardened marine sediments and shells comprises the cave. This rock was formed during the Ordovician period, over 400 million years ago. During this time Wisconsin was submerged under warm shallow seas. Massive amounts of shell life lived in this sea and over many years numerous layers of calcium carbonate were laid down and slowly compacted into present day limestone. The sea retreated and erosion began to deteriorate these layers of limestone. Today’s exposed rock of the cave is a special kind of limestone named galena dolomite which contains over 20% magnesium.

Cave of the Mounds began to form 1 to 2 million years ago when it was still submerged in the sea. It was in the upper layers of the water that are often acidic due to the added carbon dioxide from run off. This carbon dioxide mixes with the water and forms a weak carbonic acid. This acid dissolved the limestone creating cavities within it. It was along a large crack, known today as the lifeline of the caves, that the Cave of the Mounds was created. Acidic water seeped into this crack and dissolved the rock.

As stream levels lowered due to the erosional action of the flowing water so did the water table. Once the water table was below the level of the cave it was filled with air allowing for new developments within the cave. Water percolating through the soil carried calcium carbonate down into the cave. Once the water enters the cave the calcium carbonate is precipitated in the form of calcite. Every droplet of water leaves calcite crystals on the surfaces of the cave. These crystals stick to floor, walls, ceiling and one another. Formations such as stalactites and stalagmites, along with sheets of flowstone on the walls are formed from these accumulated crystals. These formations known as speleotherms grow at a slow rate which is determined by the amount of calcium carbonate present and the rate at which the water flows. One cubic inch of speleotherm can take anywhere from 50 to 150 years to develop.

Stalactites hang down from the ceiling of the cave and begin as hollow circles. They first begin to grow when calcite crystals gather around a hanging water droplet. Each new water droplet leaves another ring and successive droplets gather until a thin hollow tube hangs from the ceiling. If the original tube is plugged it will develop into a cone.

These speleotherms come in many shapes and colors. The colors can be attributed to the various minerals in the dripping water. Blues and grays are created by manganese oxides while reds and browns are left by iron oxides. Cave raft or lily pads are one of the many forms that speleotherms take. They are formed when water in the bottom of the cave floats a light plate of calcite until it becomes too heavy and sinks. Multiple plates form on top of one another until they form a raft. Water may drop onto the pad until it forms a stalagmite which causes it to resemble a lily pad. Helectites are another unusual form of speleotherm, growing sideways and down, seeming to defy gravity. Also present are minute cave pearls, known as oolites. These pearls form when grains of sand, at the bottom of pools, supply catalysts for deposition of calcite.

Multiple searches to find links between tunnels of the Cave of the Mounds and other nearby caves have been attempted. However, as of late, no such effort has been a success. Cavers do continue to search in the hope of finding more.

Sources

1. Geology of the Cave, Cave of the Mounds National Natural Landmark webpage. http://www.caveofthemounds.com/geology.htm

2. Blue Mound Cave, Richard L. Dieterle. http://hotcakencyclopedia.com/hoBlueMoundCave.html

Copper Falls

Copper Falls located four miles north of Mellen, WI is part of the Bad River, which originates in east central Ashland County and meanders northward until it empties into Lake Superior. Copper Falls is a 29 foot drop. This first drop of the Bad River flows through high canyon walls for approximately 2 miles.

Many millions of years ago there were mountains in northern Wisconsin composed of granite and greenstone. Over many years these mountains were weathered down into a rolling, rock plain. In the area around Lake Superior this rock plain was downwarped and eventually covered by a shallow sea. The surrounding rivers and streams carried sediments into the sea where they created a thick layer of sea bottom sediments. Water from deep within the earth seeped outward and upward into these sediments. These waters carried with them large amounts of iron ore, which is why this area is filled with this valuable ore today.

The area in which Lake Superior now rests was once the site of volcanic activity. Out of deep fissures flowed thousands of cubic miles of lava. This lava spread creating thick layers, up to 60,000 feet deep in some areas. So much lava spewed from under the earth’s crust that it began to sag under its own weight creating the Lake Superior basin.

Surrounding rivers deposited sand and mud into the settling basin. Over time the sediments hardened to form sandstones, conglomerates and shales. The Lake Superior basin continued to slip downward causing these layers of rock and the surrounding lava rock to slant downward. Some of these layers broke apart while others slid on top of one another, this accounts for the present arrangement of rock layers standing on edge.

Over the past 200 million years the Bad River has cut a course through these rock layers. It continued to cut its course until it was interrupted approximately one million years ago. It is during this time that the Wisconsin Glaciations occurred. Glaciers brought loads of sediment down from Canada and as they melted they dropped granite boulders and other debris. A layer of sand, mud, and rock was deposited over this area. The thick red clays that can be seen in surrounding areas were also left behind by the retreating glaciers.

The Bad River begins its course over black lava where Copper Falls is located then continues onward until it links with Tyler’s Fork at Brownstone Falls. The Bad River flows from this point over red lava in which it has carved a deep gorge. It then cuts into the conglomerate rocks of Devil’s Gate until it makes its way through a thin strip of black shale. The river then encounters thick layers of shale and sandstone, after which it makes a 180 degree turn through red clays to cut back through the shale and sandstone.

Sources

1. Copper Falls Geology, Wisconsin Department of Natural Resources. http://dnr.wi.gov/org/land/parks/specific/copperfalls/geology/

Devil's Lake

Devil’s Lake ranges from 40 to 50 feet deep and is fed by two small creeks. It does not have any outward draining bodies of water so its water losses are entirely made up from evaporation and little ground water flow toward the north. It sits approximately 500 feet below the western bluff of Devils’ Gorge and sits on top of 380 feet of sediment, making the bedrock relief in this area at least 850 feet. The lake is located three miles south of Baraboo in the southern half of the Baraboo Range. This range can be found in south central Wisconsin and formed over 1.6 billion years ago. It is one of the oldest rock outcrops in North America. This range of hills is approximately 30 miles long and 10 miles wide and surrounds an oblong depression known as the Baraboo Valley.

The ancient hills are formed from a ridge of metamorphic rock known as the Baraboo Quartzite. Over a mile thick, this quartzite has been formed from grains of sand that were deposited here as they flowed out of rivers into shallow seas which covered Wisconsin at this time. The sand particles were first compacted into sand stone and then under immense pressure and heat the sandstone became quartzite. The quartzite and underlying layers of rock are in their current arrangement because they have been formed into a syncline. Each end of this syncline forms a limb of the basin. The southern limb dips more gently which exposes more of the quartzite at the surface. The eastern edges of the syncline are rigidly joined and in the west they curve smoothly together as they descend inward toward the center of the basin. Weathered rock debris known as talus shroud the edges of Devil’s Gorge. The spacing of the fractures within the quartzite determines the size and shape of this rock debris. Commonly, they range in size from 4 to 5 feet long and normally are purple but appear grey in color because of the lichen that grows on their surface. They sit at a 20 degree angle. This is known as the angle of repose and is created by the pull of gravity and the rocks resistance to that pull because of its angular shape. On the surface of the quartzite are ripples that were left by the water that once covered Wisconsin. The eastern Bluff of Devil’s lake presents four layers of quartzite. Each layer’s ripples are in a different direction, representing the winds and currents on each day the layers were laid down.

Rocks younger than the Baraboo Quartzite, fewer than 600 million years old, do not have ripples on them. This is because bacteria were the only form of life present when the Baraboo quartzite was in its sand form. After the development of large more complex organisms ripple marks were stirred up and washed away by the movement of these organisms.

After the shallow seas withdrew the Baraboo range was pushed upward by converging continents into a ridge. In the center of this ridge was a depression that was eventually filled with softer rocks. This area was then dry for centuries. As time passed the softer rock was eroded away leaving gorges in the center of the Baraboo Range. Parfrey’s Glen is an example of one of these gorges. The rock formation there is a combination of Cambrian sandstone and quartzite so was easily washed away by flowing water.

Eventually water invaded this area again. Sand was deposited in the gorges and eventually on top of the hills. The entire range was eventually buried under sandy and limey deposits. Once again the seas retreated and an ancient river flowed through the area. This river removed the sand and exposed the quartzite and reopened the Lower Narrows Gap and the Devil’s Lake Gap, which were partially cut when the Baraboo valley and the gorges were being formed.

Approximately 15,000 years ago a sheet of ice known today as the Wisconsin Glacier flowed over the Baraboo Range. The glacier flowed over only the eastern half of the range as can bee seen by the terminal moraine (rocks and gravel deposited by the glacier when it became stagnant and melted at its end) that marks the glaciers outermost boundary. To the west of this moraine is Wisconsin’s driftless area, an area that had been left unglaciated throughout the ice age. The ancient river that flowed through this area was then rerouted when the glacier plugged either end of the Devil’s Lake gap. Devil’s Lake is situated between these two plugs; it is a deserted valley of the ancient river.

Had the Wisconsin Glacier not flowed through the Baraboo range Devil’s Lake would most likely not exist and the ancient river would still flow through this region.

Sources

1. Land Before Time, Paul Herr, Devil’s Lake State Park. http://www.devilslakewisconsin.com/geo.html

2. Rocks and Water through the Ages, Devil’s Lake State Park, Wisconsin Department of Natural Resources. http://dnr.wi.gov/org/land/parks/specific/devilslake/nature/geology.html

3. The Baraboo Ranges and Devil’s Lake Gorge, A Geologic Tour: Keith Montgomery, Dept. of Geography/Geology UW Marathon County. http://www.uwmc.uwc.edu/geography/baraboo/baraboo.htm#A%20description%20of%20the

VanHise Rock

Located in Rock Springs, Sauk County, WI along state hwy 136, Van Hise Rock was named for the preeminent geologist of the 19th century Charles Richard VanHise. VanHise used this piece of rock to show the major structural features of metamorphic Precambrian rock and also to demonstrate the kinds of changes that occur in rocks through mountain building periods. He used this monolith to develop his groundbreaking conclusions of structural metamorphic geology.

This remnant continues to serve as a hands on learning device for professional geologists and students. The rock was recognized as a national historical landmark in 1999 with a formal dedication and roadside plaque.

Van Hise Rock is a 14 foot high and 6 foot wide erosional remnant, composed of pink quartzite and phyllitic quartzite, showing cross bedding and ripple marks which indicate the top of the original bedding. It was initially part of the Baraboo Range, a 700 to 800 feet high portion of an ancient mountain range which at its height may have towered between 1000 and 1600 feet above the plain below.

The range is east west trending and composed of weather resistant Precambrian quartzite, a metamorphic rock. One billion, seven hundred million years old, the Baraboo range is 1500 meters thick and crops out in a doubly plunging syncline in south central Wisconsin. It is one of several similarly exposed outcrops in the northern portion of the Midwest United States. Two general depositional environments are present within the Baraboo range. The first is a lower layer of braided stream complex with pebble conglomerate at its base. The second is a top layer of tidal influence. This range represents one of the oldest tidally influenced rock formations on earth.

The history of the Baraboo ranges formation is as follows: Over a billion years ago Wisconsin was covered by shallow seas. Rivers and streams that surrounded these ancient seas deposited sediments of sand into the seas. Over time the deposited sand layers formed sandstone, then after much time and pressure the sandstone was converted into quartzite.

Approximately 1.6 billion years ago during the Middle Proterozoic episodes of mountain building were taking place south of the Baraboo Range. This provided force to fold and metamorphose the quartzite. The quartzite was pushed upwards and a depression was left between the northern and southern halves of the feature. This depression was filled with softer sediments until the seas withdrew.

There was a long period of drier climate in which the softer sediments were eroded away, forming the Baraboo Valley. New seas covered the area and sediments were deposited all around the area covering the Baraboo range with sand and lime. The seas then retreated for the final time and an ancient river, that would one day become the Wisconsin River, washed away the sediments, once again exposing the quartzite range.

Fifteen thousand years ago ice from the Wisconsin Glacier flowed over the range covering the eastern half, consequentially smoothing its surface creating the glassy shine that can be seen toady. A terminal moraine was left at the outer most edge of the glacier, where the ice had become stagnant and as it melted sediment was dropped out. This line of deposited sediment rerouted the Wisconsin River allowing it to flow along today’s current path.

Sources

1. Wisconsin National Register of Historic Places, VanHise Rock, Wisconsin Historical Society. http://www.wisconsinhistory.org/hp/register/viewSummery.asp?refnom=97001267

2. Baraboo Range, National Atlas .Gov. http://nationalatlas.gov/articles/geology/features/Baraboo.html

3. Precambrian Tidalites from the Baraboo Quartzite, Wisconsin, U.S.A., Richard A. Davis, Science Direct. http://www.sciencedirect.com/science?_ob=ArticleListURL&_method=list&_ArticleListID=641265090&_sort=d&_acct=C000032124&_version=1&_urlVersion=0&_userid=613910&md5=610dc719c392ded8903648693f648d21&view=f

4. Rock and Water Through the Ages, Wisconsin Department of Natural Resources. http://Dnr.wi.gov/org/land/parks/specific/devilslake/nature/geology.html

Kettle Moraine

Kettle Moraine, often referred to as the Kettle Range or Kettle Interlobate Moraine, runs from Kewaunee County south into Walworth County. Many prominent glacial features can be easily seen in the Northern Kettle Moraine State Forest which extends from Glenbeulah in Sheboygan County southwest then south 20 miles to County Highway H approximately 3 miles south of Kewaskum in Washington County.

Created when the Lake Michigan and Green Bay Lobes collided causing the deposition of sediments the Kettle Moraine exhibits classic examples of glacial geography. The Green Bay Lobe from the west created the bay of Green Bay. Lake Winnebago and the Horicon Marsh and the Lake Michigan lobe from the east formed Lake Michigan.

Northern Kettle Interlobate Moraine reaches its maximum elevation of 1,311 feet at the base of Parnell tower and has a general relief of 100 – 200 feet. Major lowlands can be found between 950 and 1000 feet occupied by Long Lake and the east branch of the Milwaukee River.

The most obvious features of this moraine, the ones for which it has been named, are the many kettles which dot its surface. Ranging in size from oversized lakes and enclosed valleys to miniature ponds, these depressions were left by buried ice that eventually melted. Greenbush Kettle, located 2 miles south of Greenbush along Kettle Moraine Drive is a favorite example of these depressions. It is one of the deepest and most symmetrical kettles that can be easily seen from the road. Depending upon the season Greenbush Kettle may be filled with or void of water as is the case with many of the kettles scattered over the moraine. There are many other classic examples of glacial landforms which may be found on the Kettle Interlobate Moraine. Parnell Esker is an example of one of these landforms. An esker is a winding ridge that has been formed from glacial drift that was deposited into a tunnel within the glacier. When the ice melted away the esker was left behind. The cores of eskers are filled with poorly sorted sand and gravel, this core is surrounded by a crust of well rounded and sorted sands and gravels. Often eskers are mined for this sand and gravel as is the case for a portion of Parnell Esker. Located near the town of Dundee, this esker is between 5 and 35 feet tall and runs northeast to southwest for approximately four miles.

Scattered throughout portions of Kettle Moraine are two different types of glacially formed hills. The type most identifiable are drumlins. These hills are smooth and elongated, composed of glacial drift. Often drumlins can be found in swarms or large groupings of multiple drumlins. From an aerial view drumlins are shaped like a tear drop with a rounded end and a tapered end, which points in the direction of glacial advance. Drumlins can only be found behind the end moraines of the glaciers that created them. The other type of hill is a kame. Kames are mounded hills of glacial drift that are formed when the drift flows into a hole in the ice. When the ice recedes the conically shaped mound is left behind.

Sources

1. Butler Lake Flynn’s Spring, Wisconsin Department of Natural Resources. http://dnr.wi.gov/org/land/er/sna/sna257.htm

2. Glossary, Ice Age Park and Trail Foundation.http://www.iceagetrail.org/glossary.htm

3. Dr. Karen Lemke, Glacial Geology web page. Department of Geography and Geology, University of Wisconsin Stevens Point. http://www.uwsp.edu/geo/faculty/lemke/geol370/index.html

4. Carlson, A.E., D.M. Mickelson, S.M. Principato, and D.M. Chapel (2005) The genesis of the northern Kettle Moraine, Wisconsin. Geomorphology, 67(3-4):365-374.

Niagara Escarpment

The Niagara Escarpment is a ridge that runs across North America from southeast Wisconsin north through the Door Peninsula to the Manitoulin Islands of Ontario in northern Lake Huron, south across the Bruce Peninsula, and then east around the southwest end of Lake Ontario.

During the Ordovician era, 445 million years ago, North America was located in the tropical zone and much of the present land surface was covered by the Michigan Sea. Layers of sediments accumulated at the bottom of this sea. Some sediment flowed in from rivers as sand and clay, while others were deposits of calcium carbonate from the skeletal remains of sea organisms. Through processes of heat, pressure, and chemical reactions these sediments became stone. The calcium carbonate became limestone, and when magnesium was present it mixed to become dolostone. Layers of sediment were deposited over what was to become the Niagara Escarpment for 25 million years, from the Ordovician era (445 million years ago) to the lower Silurian era (420 million years ago). The rock layers of the escarpment and to its west are mostly Silurian dolostone while the layers to the east of and below it are mostly Ordovician limestone.

Multiple layers of sediment were laid down throughout the historical formations and disappearances of the Michigan Sea. However, after the Silurian ear, layers of dolostone and limestone were no longer formed. Approximately 250 million years ago a long period of erosion began when the Michigan Sea vanished for the final time.

The formation of the Niagara Escarpment can be accredited to two factors, the first being that layers of soft rock underlie layers of harder rock and the second that these layers are not horizontal, instead they are slightly inclined. The weaker layers of limestone, shale and sandstone weathered rapidly relative to the overlying layer of Silurian dolostone. As the erosion undercut the top layer pieces broke off due to their unsupported weight. This resulted in the retreat of the escarpment to the west and the increase in its height.

The erosion process is increased with the presence of water. Niagara Falls is a clear example of this occurrence. The Niagara River is constantly cutting into the escarpment, at a rate of approximately one meter per year. Today, between the Falls and Queenstown there are high cliff walls on either side. In the last 10 thousand years the falls have moved over 11 kilometers away from Lake Ontario.

The most recent ice age and its associated glacial activity have also made an impact on the escarpment. The Wisconsin ice age, lasting from 23 thousand to 10 thousand years ago covered Canada and the northern United States with a layer of ice 2 to 3 kilometers thick. The flowing glaciers of North America carried and subsequentially dropped sediments of all sizes over there extent. In areas where the glaciers melted and dropped their sediment load the Niagara Escarpment is not visible, instead, only sharp slopes can be viewed.

When the Wisconsin ice age ended and the final glaciers melted, lakes and rivers were left behind. These rivers cut valleys out of the escarpment which is why it no longer pursues a strait line but crisscrosses over its range.

Within Wisconsin the escarpment is often referred to as the ledge, a site for many natural points of interest: crevasses, caves, rock falls and many other rock formations. Massive fissures and crevasses were created by the glacial ice that flowed over Wisconsin. It is a poor site for farming due to the thin layer of soil at its surface and the fallen rocks at its base. This is why most of the escarpment is wooded. Also, water is often present at the base. Dolomite, although it is hard, can become fractured allowing water to seep through. This water is filtered through the ground until it reaches an impermeable layer of shale. The water is then forced up through springs which feed streams, marshes and lakes.

Sources

1. Geology and Origins of Niagara Escarpment, Cleve 1999. http://cc.msnscache.com/cache.aspx?q=72262772554418&mkt=en-US&lang=en-US&w=e2954f52&FORM=CVRE

2. Exploring Wisconsin’s Great Cliff, Wisconsin Natural Resources magazine online. http://www.wnrmag.com/stories/2000/aug00/niagar.htm

3. Niagara Escarpment, Encyclopedia Britannic online. http://www.britanica.com/eb/article-9055670/Niagara-Escarpment