By Jim Brace-Thompson
North Dakota sports just a single state rockhound symbol even while the neighboring state, with the same surname (South Dakota), sports a state mineral, gemstone, fossil, and jewelry. The sole North Dakota state rockhound symbol is the state fossil Teredo petrified wood. This peculiar petrified wood has been compared in appearance to Swiss cheese.
While it may have just one rockhound symbol, North Dakota can take pride in knowing it was one of the early states to designate such a symbol. Rockhound author June Culp Zeitner, encouraged the practice starting in the early 1960s, with articles in bulletins of the American and Midwest Federations of Mineralogical Societies. Fanned by June’s efforts, five states designated a state mineral, rock, or fossil in 1965.
Two more states followed suit in 1966, then eight more — including North Dakota—jumped on the bandwagon in 1967. Now, nearly every state in the union has at least one rockhound symbol.
Teredo petrified wood gives fossil collectors a twofer. It is both a fossil and a trace fossil. The fossil is petrified wood of varied species (ginkgo, metasequoia, bald cypress, etc.).
During the Paleocene Epoch, 60 million years ago, logs of such trees became driftwood, floating in an inland ocean called the Cannonball Sea. Such logs served as home base for small, worm-like “Teredo” clams that bored into them with rasping shells that left long tunnels, or trace fossils. These subsequently filled with sediments of what has been named the Cannonball formation.
This formation represents the last inland sea to cover North Dakota. Its sediments may be found in the areas immediately around and southwest of the cities of Bismarck and Mandan.
Revealing Sea Life
When petrified Teredo wood is cut and slabbed, you often can see outlines of the clamshells as tiny white crescents within the elongated in-filled tubes they carved. Similar petrified wood from Australia is commonly referred to as peanut wood.
Throughout history, sailors have cursed such so-called shipworms for the damage they did to wooden sailing vessels.
But today, fossil collectors appreciate the intricate designs they left in petrified wood that has been designated the official state fossil of North Dakota.