The Earth Story

These are some big fossils. 

Petosky Stone

Michigan's stage stone is a lovely fossil coral dating from the late Devonian to early Carboniferous that was spread around parts of the state much more recently by grinding ice sheets during the recent ice ages, often as eroded rounded pebbles. Hexagonaria percarinata (named for the hexagonal patterns) belongs to an extinct group of corals called rugose that characterise reef building organisms of the Paleozoic. In those days what is now Michigan was the floor of a shallow equatorial sea filled with coral reefs.

In some areas then entire head of fossilised colonies transformed into limestone can be discovered in the Traverse group strata, though most are recovered on the beaches of Lake Michigan, where annual frosts heave and turn stones each winter revealing new specimens. A polish on the lapidary's bench is necessary to reveal the patterns in their full glory. Each hexagon was the home of a polyp that lived within the colony, who then fed via the tentacles that emerged from the central mouth. A small number of specimens have a rosy hue due traces of iron that infiltrated during the process of the reef turning into rock (called lithification or diagenesis).

The common name comes from that of an Ottawa chief called Pet O Saga (meaning rising sun), reputedly half French, who established himself as a fur trader in the last quarter of the 19th century. The stone has been a popular souvenir of the area since the Victorian era.

Loz

Image credit: Cobalt 123

http://www.petoskeyarea.com/petoskey-stone-73/ http://www.michigan.gov/documents/deq/ogs-gimdl-GGPS_263213_7.pdf http://web2.geo.msu.edu/geogmich/petoskystone.html

Drone flyover of the Trilobite Corn Maze in Wisconsin