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.
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.Return to Cave Geology