-On October 26th, 2006 eight University of Wisconsin-Eau Claire Geomorphology Students explored Crystal Cave in Spring Valley Wisconsin in order to further our understanding of geomorphic processes in Karst Topography.
-Geology of Crystal Cave
The purpose of this page is a brief geologic explanation of what can be seen in the rocks at Crystal Cave. Crystal Cave is a unique geologic location amidst the dynamic karst topography of the surrounding area. As a series of erosional events took place over thousands of years Crystal Cave was carved out of the bedrock below. Because of this the geologic past of the region has been partially revealed to us today in the exposed rock within the cave walls. There is evidence of flooding, erosion, glaciation, and even a shallow sea which flowed atop present day Crystal Cave. Below is a short explanation of these events and the rocks that are the key to unlocking this dynamic past.
-The Ancient Past 450-500 million years ago
Crystal Cave consists of several types of rocks which can be seen in the walls of the cave. The first of note is Limestone (composed of calcium carbonate). Limestone is usually formed on ocean or sea floors and is from what Crystal Cave was originally carved out of. It is also our first clue in revealing the areas dynamic past. Around 450-500 million years ago, during the Ordovician Period of the Paleozoic Era, a shallow sea blanketed Wisconsin and began to precipitate the minerals to the sea floor which would later become this Limestone. At this time in the geologic past Wisconsin sat very near the equator and dealt with many trangressive and regressive sequences of sea water. Evidence of the sea can be seen in the numerous fossils including trilobites, crinoids, gastropods, cephalopods, and bryozoans found in the rock at Crystal Cave and other neighboring locations. As the sea water receded out of the area the minerals then began to harden into solid rock. This is when cave formation is thought to have begun. After thousands of years this Limestone rock was introduced, in some location to another groundwater source which exposed it to the element magnesium which replaced some of the calcium in the rock. This in turn formed Dolomite which is another of the exposed rock we see in the cave. The last rock seen in the cave is a type of Sandstone which is sandwiched between two layers of Dolomite/Limestone. This formation is known as the Prairie Du Chien Group.
-The Prairie Du Chien Group
The Prairie Du Chien Group consists of three main layers, as mentioned earlier. The first, oldest, and lowest layer of rock is known as the Oneota Dolomite. This layering of Limestone and Dolomite is thought to represent a deeper sea environment where precipitation of the required minerals could take place without disturbance. The next oldest rock, sandwiched in the middle, is the New Richmond Sandstone. This sandstone lies about 6-10 feet thick and is thought to have formed when the seas had regressed, or became shallow, and was deposited from runoff materials from the surrounding land. The youngest layer in the formation is the Shakopee Formation which was formed of another Limestone/Dolomite sequence.
(Top, Shakopee Formation. Left, New Richmond-Unconformity-Oneota from top to bottom. Bottom, New Richmond).
-The Ice Age Floods 20,000-12,000 years ago
Although intial cave creation began after the last of the sea water had receded out of what is now Wisconsin, the majority of the caves construction occured about 12,000 years ago. As the Wisconsinin Glacial Period began to come to an end some 20,000 years ago large quantities of meltwater from the vast ice sheets began to pour forth from glacial lakes and rivers. Much of Northern Wisconsin was covered in these vast ice sheets and as they began to melt enormous amounts of water poured across the Wisconsin landscape creating some of the most unique geologic features in North America. When these meltwaters reached Crystal Cave the torrent of water filled the cave and began eroding the rock with incredible force. Much of the sandstone and limestone was broke down and broke apart forming the large rooms and corridors of the cave. After numerous floods from the melting glaciers Crystal Cave was formed as we see it today.