The remarkable crisscrossing pattern on this sandstone was formed by an extinct animal. It crawled across moist, ripple-marked sand at the edge of a shallow tropical ocean, which covered Wisconsin about 520 million years ago during the Cambrian geological period. Ripple marks near the center and also on the bottom of the rock were formed by waves during high tide, but most were destroyed by the animal as it moved across the sand during low tide. The fossil track is named Climactichnites (meaning "rope ladder shape"), but the animal that made it is known only from its tire-track-like trail.
During the Cambrian geological period, North America lay along the equator with Wisconsin in the southern tropics. A shallow sea covered most of the continent and tropical storms churned that sea as they do today in the Gulf of Mexico. The Cambrian seashores of Wisconsin were pounded by such storms countless times. Subsequently, the tropical landmass was rotated and moved north by continental drift to become present mid-latitude North America.
[graphic] W represents the location of tropical Wisconsin 520 Million Years Ago. [below Equator]
Paleontologists believe that the track maker was a large, slug-like animal without a skeleton, which fed by sucking microbes from damp sand as it crawled at low tide. The cross bars were made as the animal crept forward and the ridges along the sides probably helped preserve the track from later destruction by waves or currents. Besides Wisconsin, such trails are 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.
Boulder discovered and donated by Milestone Materials Div. of Mathy Construction Co., Onalaska WI.
Text provided by Robert H. Dott Jr., Geologist, UW-Madison .
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