Crystal Cave was formed in 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 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.
The Dissolving Agents
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 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.