Walking in the Ocean(s)

When I was a kid on the Piedmont in Montgomery County, Maryland, I was crazy for dinosaurs. I got hooked (like most kids) around third grade, and by fifth grade had read the books by Roy Chapmann Andrews, a paleontologist from the American Museum who roamed the Gobi looking for fossils in the 1930's. Andrews was a larger than life character. He fought off bandits, nearly died of thirst, and discovered the first dinosaur eggs. Steven Spielberg read the same books and morphed Roy into Indiana Jones. I must have turned over every rock within a five-mile radius of my house, always with the same result -nothing. They were just rocks. As luck and GI-Bill housing economics would have it, all the rocks I picked up were wrong for at least two reasons: they were almost all metamorphic, and they were all too old.

Turns out, I was too far north by a county; some of the first dinosaur fossils in North America were found on the Coastal Plain in Prince Georges County and they are still being dug out of the clay there.

It wasn't until I took a geology course, complete with field trips, in the Master Naturalist program sponsored by the Northern Virginia Audubon Society, (insert shameless plug here: http://www.asnv.org/) that I began to get a clue about what had happened here on the Piedmont. The field trips were great; I got to travel to strange and different worlds. The worlds in question is the one we are sitting on; geology is the science of the way-back machine. I learned that the mid-Atlantic, especially Northern Virginia, has some of most complex geology on the planet, with titanic continental collisions, volcanos spewing lava a thousand feet thick, and general mayhem going back nearly a billion years.

If you plan it right, you can walk on the floor of two oceans (two and a half if you make the drive to the beach). The rocks at Great Falls on the Potomac are old ocean floor. Over 500 million years ago the African Continent pushed up against the North American Continent in a slow motion collision. This closed the Iapetus Ocean (one of the Titans, Iapetus was the father of Atlantis). The ocean floor that was between them had no place to go but up and folded into tall mountain peaks. Asia and India are doing the bump right now and the Himalayas grow microscopically taller day by day. Climbers bring back fossil snails and clams as souvenirs from Everest and K-2. The mountains that formed when Africa slammed into North America (one would need the time scale of the gods to really see the crunch) were as high or higher than the Himalayas. Time passed. The mountains wore down bit by bit, grain by grain, until now just the roots are left. The oceanic crust at the heart of the mountain range transformed into the metamorphic rocks that are now found throughout the Potomac Gorge. The original muds and silts of ocean floor were changed by enormous pressure and heat into mica schists and related rocks. Here and there, gnarly crystalline chunks stand out in relief, harder than the rock surrounding them and more resistant to wear.

Jump in your time machine, or trusty Toyota extended cab pickup, and fast forward about 400 million years to the Triassic, or drive upstream about 15 miles to Seneca, Maryland and Riley's Lock on the C&O Canal. Walk upstream from the parking lot, cross the partially rebuilt aqueduct holding the canal bed, and come to a wide place on the water. The rocks here are all red sandstone. The wide place was used as a turning basin for canal barges back when the C&O was a going concern. It was also a dock and loading point for the quarries. Walk back in the woods and you can find the remnants of the quarries and the ruins of the buildings that housed the giant saws used to cut the sandstone into building blocks. The Smithsonian Castle is made from red Seneca sandstone, as is the interior of Washington Monument. You are standing in the Culpepper Basin; a dip in the Earth's surface formed when Africa and North America ended their embrace and cracked back apart. As they broke, the widening rift filled and became the Atlantic Ocean. Africa and North America are still pulling apart at about the same rate as your fingernails grow. The Culpepper Basin is one of several basins formed as subordinate cracks to the Atlantic. Over time, streams deposited sand in the crack, filling it up. The brownstones of New York are made of the same rock from a different but related basin. You can carefully ease back a layer of stone from a boulder in the quarry, and looking at the fresh surface at an oblique angle, you can see ripples made when the stone was still sand and water flowed over it. You can find the tracks of early dinosaurs in some basins. The best I have done so far is find fossil tides.