Sedimentary rock: Dave pointed out Cambrian sandstone which lays along the edge of Lake Mendota. It is approximated to be between 500 and 600 million years old and also probably has some calcium carbonate from organisms that became trapped between the layers of this rock. Another sedimentary rock found at the tops of the hills in Raymer’s Cove is a light-colored calcium-magnesium-carbonate rock known as dolomite. Interestingly, the dolomite can sometimes have gaps filled with a substance called “chert”. Chert is another sedentary, silicone-dioxide rock which is produced from microorganisms like algae. Dave mentioned a mechanism to explain why we sometimes find chert sandwiched between dolomite layers: imagine a decaying trilobite that will become integrated into the dolomite… trilobite decaying is an acidic process which causes the silicon based dolomite to crystalize and aggregate in a particular way that allows spaces in the dolomite to form—where chert can eventually fill in!
Igneous rock: In the Preserve, igneous rocks are what geologists would call “erratic rocks” or a rock that is assumed to have been brought over via glaciers. A common igneous rock found in the Preserve is the basalt rock—a very dark-colored rock with high levels of iron and magnesium.
Metamorphic rock: Like igneous rocks, metamorphic rocks are also considered erratic rocks in the Preserve. These rocks have a “gneissic texture” which refers to how the lighter and darker colored rocks appear to separate in a pattern— an arrangement which occurs when the rocks’ arrangement changes due to exposure to intense heating.
Finally, Dave also distributed the free booklet, Landscapes of Dane County, Wisconsin, authored by him, and produced by the WI Geological and Natural History Survey, 2007. Report and Photos by Olympia Mathiaparanam.