We often get queries that ask about local geology that we do not have easy access to. However, it’s fairly easy to sleuth things in the broad brush by locating state geologic maps. I can’t say much about a rock found in someone’s backyard, because glaciers and rivers could have moved that rock hundreds of kilometers from its original source. The following is a local-geology question that I CAN reasonably respond to.
Q: Can you tell me when was the last time North Carolina was under water? I'm finding fossil seashells yet I live nowhere near any ocean. I live in Jacksonville, NC (Onslow County)
- Brandon F
A: You live on the Outer Coastal Plain of North Carolina; Onslow County runs all the way to the ocean. The Outer Coastal Plain, or Tidewater is extremely flat, averaging less than 20 feet above sea level. It contains large swamps and lakes indicative of poor drainage conditions, which have hosted both freshwater and marine mollusks at different times. The coastal margin north of Cape Lookout is a “drowned coast,” in which sea level rise associated with the end of the last Ice Age, and continual melting of the ice caps, has caused the ocean to invade the lower reaches of river valleys including where you live. This drowning has produced large embayments such as Albemarle and Pamlico Sounds. New River (where you live) lies between this region and the Cape Fear uplift.
You might wish to look at the North Carolina geologic map for more detail:
To your east you have the Belgrade Formation, with oyster shells embedded in sand. To your north and west you have the River Bend Formation, also fossiliferous with limestone among other rocks. Both formations are listed as Tertiary in age (66 million to 2.6 million years ago). However, the shells you are seeing could conceivably be from the last several tens of millennia if I read your elevations and location correctly.
I hope this helps. You have some excellent geologists in your state, both at the state and university levels. It should be fairly easy to contact one - perhaps visit the closest university and ask to talk with a geologist there.