The Bedrock of Crystal Cave

 The bedrock that makes up Crystal Cave is sedimentary rock, or limestone and sandstone to be specific. It dates back to the Ordovician period and is approximately 550 million years old. There are three layers that make up the bedrock and they are the Shakopee, New Richmond Sandstone, and the Oneota. These three layers can be categorized into a group called the Prairie du Chien and it crops out over a broad portion of southeastern Minnesota and southwestern Wisconsin. It was originally named for the set of excellent exposures near the city of Prairie du Chien.

 The Prairie du Chien group was formed during periods of transgression and regressions of the ocean during the Ordovician. The Shakopee Dolomite layer is the youngest and was formed during a period of transgression or deep water. It is comprised of layers of limestone and dolostone. Also layers of amorphous churt or flint can be seen deposited within the dolostone. Native Americans used this flint to make arrowheads for hunting.

 The next layer down is the New Richmond Sandstone and is the bottom portion of the Shakopee. It was formed during a regressional period. Oceanic waters receded and created a shallow water or beach environment. It is oriented in many one to two inch layers which would indicate the constant deposition from waves washing sediment ashore. Its layers are oriented at a slight angle. You can also see worm burrows which would also be indicative of a shallow water or beach environment. If you look closely you can also see little calcite crystals.

 The last and oldest layer is the Oneota Dolomite. This layer was formed during a transgressional period and its layers are thicker. The layers in the Oneota as well as the Shakopee are very flat which means that the waters were calm during these periods. You can also see fossilized gastropods in the Oneota.