More than two billion years ago, this area lay beneath an ancient Precambrian sea where sand was deposited on the sea floor, and wave action formed ripple marks in that sand. Over time, the sand grains cemented into sandstone and later, under greater heat and pressure, changed into hard quartzite, preserving the ripple marks. During the next several hundred years, extensive weathering and erosion reduced the surrounding region to a flat peneplain, but Rib Mountain, made of resistant quartzite, remained high above it. About 600 million years ago, expanding oceans covered Wisconsin and deposited thousands of feet of sediment. As those layers slowly wore away, Rib Mountain and its ancient ripple marks were revealed again. Algal stromatolites of blue-green algae, bacteria, and silt formed on the bottom of the Precambrian sea and are believed to have been the first life-forms to colonize the earth.