Ichnofossils in the Making

by Sarah Doyle

For three summers, I worked weekends on a tour boat on the Connecticut River. As the Quinnetukut II glided downstream, I talked about the geology and history of a 6-mile section of river from Northfield, MA, down to Barton Cove in the Turners Falls area. It’s a lovely stretch. As it shoots through the French King Gorge, the river hugs right up against the Eastern Border Fault, a rift created by the break-up of Pangaea. When that supercontinent split apart about 100 miles east of us and the Atlantic Ocean began to form, the breakage created stress fractures (“faults”) in the bedrock further inland, in this case forming the Connecticut River Valley. A spot just north of the French King Bridge always amazes me, because the rocks on the east and west sides of the river are entirely different in a way that illustrates this process: ancient metamorphics on the east and younger sandstones and conglomerates on the west, built up after Pangaea split.

But about the potential ichnofossils: We were very, very fortunate that our area was not directly hit by Hurricane Irene on August 27, 2011, but it did plenty of damage to towns just 15 miles to the west of us and to neighbors nearby in Vermont. It took a few days for the watershed runoff to make its way down to the Connecticut River and a little longer to reach the Northfield/Turners Falls/Greenfield area, but when it did, the water rose pretty high. For weeks afterward, it looked like a river of chocolate milk racing toward the sea. The first time I was back at the dock at Riverview, where we pick up passengers, I saw that the largish rocks along the river bank were thickly covered in silt.

People had been walking there and left their footprints behind, and rising and lowering water levels had left their marks on the silt as well. It reminded me immediately of one of the topics I talk about on the boat: the formation of dinosaur footprints. This spot is just five or six miles from one of the richest fossil footmark beds of the 19th century, one of the places where Dexter Marsh, James Deane, and Roswell Field quarried tracks to sell to Edward Hitchcock at Amherst College, James Hall in Albany, John Collins Warren in Boston, and colleges and museums all over the American east, Britain, and Europe. We can safely assume that these human tracks won’t last long enough to fossilize, but it’s fun to see them looking so fresh, especially being so close to where many dinosaur tracks, raindrop impressions, mud cracks, and water ripple marks were created 200 million years ago.

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