A crisscrossing pattern on this sandstone is the fossil trail of an extinct animal that crawled across moist, ripple-marked sand at the edge of a shallow tropical ocean that covered Wisconsin about 520 million years ago during the Cambrian geological period. Ripple marks near the center and on the bottom of the rock were formed by waves during high tide, but most were destroyed as the animal moved across the sand during low tide. The fossil track is named Climactichnites, meaning “rope ladder shape,” though the animal itself is known only from its tire-track-like trail. Paleontologists believe the track maker was a large, slug-like animal without a skeleton that fed by sucking microbes from damp sand as it crawled at low tide, leaving cross bars as it crept forward while ridges along the sides probably helped preserve the track from later destruction by waves or currents. During the Cambrian geological period, North America lay along the equator with Wisconsin in the southern tropics, and a shallow sea covered most of the continent while tropical storms churned the water and repeatedly pounded Wisconsin’s seashores. Later, continental drift rotated and moved that tropical landmass north to become present mid-latitude North America. Similar trails are also known from eastern North America and Missouri in sandstone of the same age. This boulder was carried by Wisconsin’s last glacier from 20 or 30 miles northeast of here and was dropped about 20,000 years ago near the ice front, which lay about 6 miles to the west.