Mushroom Rock State Park is noted for its mushroom rock formations. It is located in the Smoky Hills region of north-central Kansas in Ellsworth County, Kansas, United States. These rocks are the remains of beach sands and sediments of the Cretaceous Period, the interval of geologic time from about 144 to 66 million years ago. Sandstone and sedimentary rock is held together by natural cement. The concretions that make up Mushroom Rocks are cemented calcium carbonate.