The Alpena CVB’s page on Rockport State Recreation Area says:
Rockport State Park, Michigan’s 100th State Park and an official Dark Sky Preserve, has over 4,237 acres of land located on the shores of Lake Huron north of Alpena. The property includes a deep-water protected harbor, an old limestone quarry of approximately 300 acres, a unique series of sinkholes, Devonian Era fossils, the Besser Natural Area, and a broad range of land types, vegetative cover, cultural resources and recreation opportunities. At the harbor the DNR has a boat launch facility, and there is a small park with picnic areas.
If you click through, they have a nifty guide that includes more information on the offerings including the fossils and sinkholes! You can get a map and more info from the State of Michigan’s page on Rockport Recreation Area.
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