Crystal Cave is a rare, unique geological feature in Western Wisconsin.  The Cave has a long, fascinating geological history, a unique biological environment and a delicate yet tenacious ecology.   Following is a discussion of Crystal Cave’s geology.

Cave ages are a tricky problem because it is difficult to pin a date on an empty space.  However, by using observations relating the cave to the history of the surrounding region along with radiometric dates (dating based on measuring the decay of certain radioactive minerals) and paleomagnetism (ancient magnetism measured in cave sediment), reasonably good estimates can be obtained.  Caves are rarely more than several million years old. 
 
Crystal Cave is estimated to be about one to one and one-half million years old.  This information is based on the history of the Mississippi River and the advances and retreats of the glaciers.  It is safe in saying “about a million years old, and perhaps older”.  Remember, the rocks in which Crystal Cave are found are about 450 million years old.  A cave is ALWAYS younger than the rock in which it is found!

One of the most easily dissolved rocks is LIMESTONE.  Limestone is composed of calcium carbonate which has precipitated on the ocean floor and mixed with fossil fragments that are also usually calcium carbonate.  As the deposit of calcium carbonate thickens, water is squeezed out and the material hardens into rock. Sometimes, after the limestone has been deposited,  an influx of groundwater from a new source area introduces the element magnesium to the rock.  The magnesium atoms will replace some or all of the calcium atoms creating the rock called DOLOMITE.  Dolomite looks just like the limestone but dissolves much more slowly.  A unit of rock may contain pure limestone, pure dolomite or any combination from dolomitic limestone to limy dolomite.  The rocks in which we find Crystal Cave are limy dolomites to dolomites.

The limestone/dolomite in which Crystal Cave was formed was deposited approximately 450-500 million years ago during a period of time geologists call the ORDOVICIAN.  During the Ordovician, animal and plant life was still confined to the seas.  Massive coral reefs were growing in oceans, and the seas teemed with animals such as trilobites, gastropods, cephalopods, crinoids, bryozoans, and algae mats called stromatolites.  Wisconsin sat near the equator and warm seas covered most of what we now know as North America.  Thick, massive sequences of limestone and dolomite were being deposited worldwide.
 
At Crystal Cave, there are three units of rock deposited during the Ordovician.  The lowermost and oldest rock unit is called the ONEOTA DOLOMITE.  It is a thick sequence of limestone/dolomite formed when the sea was deeper and relatively quiet.  The next higher rock unit is a sandstone called the NEW RICHMOND SANDSTONE.  It is about eight to ten feet thick and formed during a time when the sea has become more shallow and sediments from the land washed into the water.  The uppermost unit is the SHAKOPEE FORMATION.  By the time the Shakopee began to form, the sea had again become deeper and more quiet allowing for deposition of limestone/dolomite.  All three units make up what is called the PRAIRIE DU CHIEN GROUP.  It represents a sequence of rocks called a transgressive-regressive sequence.  What this means at Crystal Cave is that the seas would transgress, or move, onto the land as the water depth increased (deposition of the Oneota) then regress as the water depth became more shallow (deposition of the New Richmond).  Finally, the sea made a final transgression back onto the land when the Shakopee was deposited.

In order to create a cave, rock must be removed, leaving a void underground.  This is accomplished by dissolving away the minerals present in the limestone and dolomite.  The best dissolving agent is acid which seeps into the rock, either from the surface or from deep in the earth.  The most common acid found in nature is CARBONIC ACID.  (Carbonic acid is what give soda pop its fizz.) Carbonic acid is carbon dioxide and water mixed together.  The carbon dioxide (naturally occurring) is mainly found in the soil horizon where decomposing organic material release the gas.  A small amount of (naturally occurring) carbon dioxide is also found in the atmosphere. 
 
When precipitation falls, either as rain or snow, the water will combine with the carbon dioxide to create carbonic acid.  Eventually, the acid will seep through the soil zone and into the rocks.  If limestone or dolomite are present, the acid will begin to dissolve the calcium from the rock leaving voids or holes called caves.
 
Some caves are formed by SULFURIC ACID.  The sulfuric acid is a combination of hydrogen sulfide gas and water found deep in the earth.  The acid, again, dissolves the limestone creating caves.  Geologists are discovering that most large cave systems such as Carlsbad Caverns in New Mexico were formed by sulfuric acid.  These caves often contain massive deposits of gypsum and other sulfur-related mineral.
 
Crystal Cave is assumed to have been formed by dissolving the rock by carbonic acid but evidence also exists which seems to point to sulfuric acid playing a part in the Cave’s formation.  More scientific study is necessary to determine which method has dominated in development of this cave. 

The limestone/dolomite where we find Crystal Cave has been deposited one layer at a time.  The result is that it looks like a layer cake.  Geologists call these horizontal layers BEDS or BEDDING PLANES.  Each bed is separated from the overlying and underlying beds by some sort of change in composition, deposition, or erosion.  In Crystal Cave the rocks on First Level show little or no bedding.  This is because the rocks have been weathered and the visible beds have been disturbed and destroyed.  The beds on the Second Level vary from a foot or more down to mere inches or less.  In the Ballroom you can see very thinly bedded rocks overlying massively bedded rocks.  It is possible to observe how the rocks have been eroded at different rates.
 
In addition to the horizontal bedding, are vertical cracks called JOINTS or FRACTURES.  These breaks in the rocks formed when the limestone/dolomite underwent movement (in Wisconsin, this was an uplifting movement, or doming).  There are usually two sets of joints in the rocks, one being the major, or most pronounced, and the other being secondary or less obvious.  The ceiling in Crystal Cave, especially in the Ballroom or the New Section, show excellent examples of joints.
 
Caves will tend to form along the bedding planes and joints found in the limestone/dolomite as these are zones of weakness in the earth.  The carbonic acid, being a liquid, will “search” for the path of least resistance when percolating through the rocks, i.e.; joints.  As it migrates downward, it is dissolving the rock, enlarging the joints creating even larger channels.  Eventually, the joints are enlarged to the point where they interconnect creating a network of passageways.  The points of intersection are often the “rooms” we see in the cave.  The individual joints are the passageways.  Once the “rooms” develop, dissolution often occurs along the bedding planes, enlarging them even more.

Most caves are formed by the dissolving of bedrock by underground water (groundwater).  Nearly all caves open to the public are of this type.  These caves are called SOLUTION CAVES formed in rocks such as limestone or dolomite.  They are part of a unique kind of landscape containing sinkholes, sinking streams, and springs.  This landform type is called KARST, named for a plateau in western Slovenia.  About 10-15% of the world’s surface is karst.  Water that seeps through the soil enlarges the widest cracks in the underlying bedrock.  As the soil subsides into the largest of the growing openings, surface depressions gradually form, which are called SINKHOLES.  As sinkholes enlarge, greater amounts of water are funneled into the widening cracks below them.  With time, the fastest-growing cracks become caves.  Most underground water emerges as springs in nearby valleys.  If a cave grows large enough, its ceiling may collapse, forming additional sinkholes.  The most extensive karst landscapes in the U.S. are located in Kentucky, Missouri, Tennessee, southern Indiana, northern Alabama, and southeastern West Virginia. 
 
The area around Spring Valley is also a karst landscape.  Many farm fields are dotted with sinkholes, especially in the springtime when snowmelt is seeping into the ground.  The village of Spring Valley was named for the several springs which emerge in and around the community.
 
There are several other types of caves, most of which are small.  Shelter caves are formed by weathering of weak rock layers overlain by stronger layers.  Many, like at Mesa Verde, have provided shelter for humans.  Lava caves are formed during volcanic eruptions, when the surface of a lava flow solidifies and the interior molten material flows out from beneath creating a void.  Many lava tubes are found in Hawaii.  Sea caves are formed by the erosional action of waves along steep seacoasts as in California, Oregon and Washington.
 
A generally accepted definition of a cave is as follows;
            1.  Must be large enough for a person to enter.
            2.  Must be able to go in far enough to enter total darkness.
 
The question is often asked “What is the difference between a cave and a cavern?”.  Technically, there is no difference.  Oftentimes, people use cavern to indicate a very large cave but they are still the same.  A comparison would be “cave is to cavern” as “man is to male”.

Many caves have, or have had, water flowing through the passages either on a permanent basis or periodically such as during rains.  These caves are often flooded and can fill with water or mud very rapidly.  This rapid movement of water carrying mud, silt and sand, causes enlargement of the cave by erosion  Other caves, such as Crystal Cave do not have active water flows.  From about 27,000 to 10,000 years ago, as the glaciers moved through the area, water did enter Crystal Cave, depositing the mud which fills the passages.

Crystal Cave is a series of interconnecting passages, rooms, and domes.  Each forms from the rock being dissolved.  The passages are called FISSURE PASSAGES or simply fissures.  They are straight, narrow and have formed along the joints in the rocks.  They are usually much deeper that wide.  In Crystal Cave, the fissure passages have been filled with silt and mud.  As you walk through, in most cases, you are not walking on the floor of the cave.  You are walking on a silt or mud which fills the lower portion of the passage.  This mud is known to be several feet deep in many places.
 
ROOMS are the large areas where joints intersect, or simply put, where the passages have joined together.  Some rooms are simply areas of large passages that look like rooms because they are larger than the surrounding passages.  An example of this would be Sheep’s Pen or First Stalactites.

All but the simplest caves consist of arrays of intersecting passageways that form distinctive patterns.  Different parts of the same cave may exhibit different patterns, and more than one pattern can be superimposed in a single location.  Crystal Cave is a NETWORK cave.  It is a “maze” of intersecting passages and rooms formed by the widening of nearly all major joints over a large area.  The straight, high and narrow passages form a pattern like city streets.  Look at a map of Crystal Cave and notice how the cave forms a “boxy” pattern. 
 
There are many other types of solution cave patterns including branching (60% of all solution caves), anastomotic (5-10% of caves), spongework (5%), ramifying (5-10%), and single-passage (>2%).  Network caves include about 15-20% of all caves formed by solution.

After a cave forms, it often has a variety of minerals deposited throughout.   Mineral deposits in caves are called SPELEOTHEMS (popularly known as “cave formations).  Speleothem shapes and mineral types are classified according to  the environment in which they formed.  Most of these deposits are made of calcium carbonate that precipitates from water (carbonic acid).  This is the same calcium carbonate that makes up the  limestone bedrock in which the cave is located. 
 
When the carbonic acid seeps downward from the surface, it dissolves the limestone bedrock, becoming saturated with calcium.  When the carbonic acid enters the cave on the ceiling or along the walls, it enters an air-filled void which has much less carbon dioxide than the acid.  The droplet of carbonic acid will begin to lose its carbon dioxide into the air just like soda pop loses carbon dioxide once it is opened.  This reduces the acidity of the liquid and also the amount of dissolved calcium the droplet can hold in solution.  The result is the calcium precipitates as crystals of calcite.

Crystal Cave has a variety of speleothems which decorate the passages and rooms.  Each forms in its own unique way.
 
STALACTITES are deposits which hang from the roof of a cave, looking like icicles.  They begin as a ring of crystals, deposited around a droplet of carbonic acid hanging suspended from the ceiling.  Slowly, the crystals build a small tube with the droplets moving through the center.  The stalactite, when hollow and only one drop wide, is called a SODA STRAW STALACTITE because of the resemblance to a soda straw.  Most soda straws have a short life span.  The central tube tends to plug causing the carbonic acid to run in a thin film down the outside of the soda straw.  Calcite continues to be deposited but now the deposition is on the outside of the speleothem, causing the soda straw to become larger in diameter; fat and longer, not skinny and longer.  Those soda straws that continue to grow as narrow tubes eventually break under their own weight.  One of the longest known soda straws is in Kartchner Caverns in Arizona.  It is 23 feet long, still growing, and only as wide as a drop of water and hollow.
 
If the flow of carbonic acid from roof of a cave is too rapid for growth of a stalactite or soda straw, the liquid will drop to the floor of the cave where a STALAGMITE will form.  Stalagmites tend to be much broader, flatter and more round than stalactites because the drop will disperse when it hits the floor.  This spreads the carbonic acid over a much greater area.  Often, only a stalagmite will be found because the flow is always too great to allow formation of a stalactite.  If both a stalactite and stalagmite form, they will eventually join together to create a COLUMN.
 
Crystal Cave has many small HELICTITES throughout the cave.  Helictites are finger-like growths of calcite that look like clusters of spaghetti, branching and twisting without regard for what is up or down.  They are formed by water that seeps into the cave in amounts so small that it moves by capillary forces, rather than by gravity.  Water covers the growing helictites in a thin film, depositing most of its dissolved calcite on bulges and outward projections, where carbon dioxide is rapidly lost to the cave air.  Any bumps on the original helictite grow in this irregular manner, lengthening into branches. Most helictites have microscopic tubes running through their centers which supply moisture from the bedrock to the growing tips.
 
CAVE PEARLS are found near several of the small pools in the cave.  They form when the calcite is deposited around tiny pebbles or sand grains.  If the drop of acid falls from a great distance, the grains may roll around and remain unattached.  As they grow, the grains become more rounded until they look just like pearls.

Speleothems have many different ages and grow at different rates.  The growth rate depends on several conditions including 1) the temperature both inside and outside the cave, 2) the amount of precipitation,  3) the amount of water moving through the cave (rivers, flooding, etc.), 4) the abundance or lack of plants on the surface and 5) the amount of carbon dioxide available in the soil.  All these factors affect the amount of dissolved calcite carried into the cave.  Which in turn affects the growth rate of the speleothems.  Some speleothems can be dated by measuring tiny amounts of unstable or radioactive elements present within the calcite.  But if the formation is too old (>350,000), there will be too little datable material.  Also, to date a speleothem, it is necessary to destroy part of it.  This dating is done only when it is absolutely necessary.
 
The formations in Crystal Cave are relatively young, less than 3,000 years old for the largest.  Most of the small speleothems are under 500 years old.  Pictures taken just after development of the cave began (1941), show formations in Trails End that have since been broken and regrown.  The regrown portion is about 1” long.  We can TENTATIVELY place a growth rate on our speleothems of about 1.5” per 100 years.

Animal life was abundant during the Ordovician (450-500 million years ago) when the limestone/dolomite of the cave was deposited.  We find relatively few fossils here because the sediment was reworked by various bottom feeders present at that time.  In essence, the bugs and worms pretty much ate everything organic.  In other area close to the cave (Punkin Center, Ellsworth, River Falls, Eau Galle Lake) fossils are abundant and give us a picture of life at that time. 
 
Some of the most common fossils are discussed here.

A.  Trilobite.     The trilobite was the most abundant creature on earth during the Ordovician.  There were literally hundreds of species, found throughout the world, ranging in size from a few millimeters to 1/2 to 1 meter in length.  Trilobite fossils are so common in Wisconsin, the legislature made it our state fossil.  They looked like a common-day pill bug and were able to roll into a ball to protect themselves.  Trilobites had excellent eyesight and very complex eye structure for that time.  Trilobites were bottom feeders and crawled along the floor of the sea eating organic material.
 
B.  Crinoids.  Crinoids were a common filter feeder.  They resembled a lily with a stem and flower-like head.  Crinoids were anchored to the seafloor and lived close to the shore but in water deep enough so intense wave action would not break them off.  They filtered water through the “flower head” or calyx eating microscopic organisms.  Whole crinoid fossils are rare but the small cheerio-like segments of the stem are very abundant.
 
C.  Cephalopod.  Cephalopods swam through the oceans and seas much like present-day squids.  Their shells were long and cone-shaped with one end open.  This is the end where the animal’s body could be seen.  Cephalopods grew to a length of 20-30 feet.  The name means “head-foot”, cephal=head, pod-foot.  In other words, the head and feet were on the same end.  Cephalopods were carnivores.
 
D.  Gastropod.  Gastropods were also bottom dwellers.  They looked like today’s snails.  They also fed off decaying organic matter or were filter feeders.  Their name means “stomach-foot”, gastro=stomach, pod=foot.  Their stomach was located in their foot.
 
E.  Trace fossils.  Throughout the cave are found trace fossils.  These are not fossils of the animals themselves but are traces that the critters left behind, sort of like a footprint.  The trace fossils found in the cave were made by small worms living just below the surface of the seafloor.  They were analogous to today’s earthworms in that they burrowed into the seafloor filtering out organic material.  Because the bodies were soft and without shells, fossils of the worms are extremely rare.  The burrows though are very common.  Most of the burrows found in Crystal Cave are found in the New Richmond Sandstone.
 
F.  Stromatolites.  Stromatolites are essentially piles of algae.  It is a very ancient form of life and has been found in rocks over a billion years old.  Algae will grow in layers, one on top of the other, building mounds several feet high.  The fossils show how the algae would drape over the older material as it grew.  Stromatolites can be seen growing along the northern coast of Australia even today.  Just imagine acres of green “toadstool-like” mounds and you have imagined stromatolites. 

Karst regions are notorious for environmental problems, and a knowledge of caves is one of the most important assets in being able to solve them.  The most serious are described here:  1) land instability -- as caves enlarge, the overlying land tends to subside into them.  This causes sinkholes, cracking of foundations and roads, disruption of pipelines, and diminished property values.  2)  Problems of water supply -- most of the groundwater flow in a karst area is through caves.  and unless these cave locations are known, the patterns of groundwater flow cannot be predicted.  Well yield tends to be high (if solutional openings are encountered) or rather low.  3)  Poor water quality -- the unfortunate thing about wells with high yield in cave areas is that their water quality is almost always low.  Wells with smaller yield are generally of higher quality.  Contaminants travel rapidly through caves and undergo very little filtering.  Every effort should be made to avoid spills, leakage, or dumping of wastes in sinkholes.  On a more positive note, caves give us information about the distribution of certain oil reservoirs and ore deposits.
 
It is clear that a knowledge of caves is essential for a proper assessment of any of these phenomena.  The study of caves is rapidly growing in importance, and only recently has it been recognized as a truly significant science of its own.

MINERALIZATION IN THE CAVE

Veins of mineralization can be seen on the first and second levels of the cave.  The most prominent vein is the brown-brownish red hematite (Fe2O3) and limonite (FeO(OH).nH2O), oxides of the element iron.  The iron in the rock has combined with oxygen (oxidized or rusted) to create the iron oxides.  During the years 1889-1910, iron ore was mined near the town of Spring Valley.  The ore was dug from open pit mines, transported to town by railcar and processed at a smelter located on the north end of the village.  The ore was very rich but deposits were small and localized.  Mining ended around 1910 when large deposits of iron were discovered along the shores of Lake Superior.  The tower from the smelter still stands in the local athletic field, the only reminder of the mining operations.
 
Another mineral present in the cave is a clay mineral called illite.  Illite can be seen in the same vein as the hematite and limonite.  It is a greenish-gray, soft, sticky clay formed from mica which has water incorporated into the crystal structure.  It also contains aluminum, silica, potassium, calcium and magnesium (calcium and magnesium come from weathering of the dolomite).  Illite is common in shales and has no significant economic use.  Other clays similar to illite are used in pottery making, construction, and in facial makeup.

CHERT STRINGERS

On the second and third levels of the cave, small nodules of chert can be found.  Called chert stringers, these gray to white pockets of rock are a form of silica dioxide (SiO2) or quartz which, when broken, forms very sharp, glass-like edges.  The chert in Crystal Cave was deposited at the same time as the limestone.  On a seafloor, small indentations or pockets will develop.  Each small depression is filled with sea water, but, because less circulation of the water occurs in these pockets, certain minerals will become more concentrated.  In those depressions where chert develops, sea water becomes saturated with silica.  Under a combination of the right physical conditions, such as temperature, depth of the water, water ph (acidity), oxygen concentration, etc., silica dioxide or chert will be deposited.
 
Chert has been used by man as a tool for hundreds of thousands of years.  Local Native Americans fashioned scrapers, arrowheads, and spear tips from chert.  It is still possible to find these tools in the local area.