Long Before The Potomac River, There Were Rocks
If you think we’re living in exciting times, you should have been around here 400 million years ago.
I tried to imagine the scene. Far offshore, an arc of volcanic islands appeared, advanced and collided with our region, thrusting up the land to form great mountains.
Things calmed down, but not for long. Next, a whole microcontinent loomed in the distance, and it too made its calamitous landfall, complete with volcanic eruptions.
Then it happened a third time. The megacontinent of Godwanna took aim at our region, locking present-day Montgomery County in the middle of a supercontinent called Pangea for many millions of years.
Evidence of all of this turmoil lay in the slab of rock under my feet by the shore of the Potomac River, downstream from Carderock. The rock was faintly patterned with brown and grey, but otherwise unremarkable.
But to geologist Joe Marx, this rock tells about events that took place an unimaginably long time ago.
Lean and lanky, Marx could have passed for a local shaman. But instead of divining bones, he was showing his band of followers how patterns in the rocks tell us about the past. His stories are grounded in new knowledge produced by hundreds of researchers, much of it just during the past 50 years.
Marx nimbly scouted out a rocky peninsula jutting into the river for a place to assemble the members of his geology walk, and began to describe the makings of our area. The field trip was organized by Montgomery County’s Audubon Naturalist Society.
Sediments flowed down the mountains created by these ancient collisions, and washing into the sea, they fanned out, layer piling upon layer.
As the sediment was buried, it hardened into rock. Much deeper, heat and pressure turned the rock into the consistency of taffy. As the layers built up and twisted they developed the swirls and folds that we saw in the rock before us.
The rock had now changed, or as geologists say, metamorphosed. This kind of metamorphic rock is called schist.
Marx ran his hand down the end of a rock slab corrugated with layers, like pages in a giant book. This was called “foliation,” and it was evidence of the continental collision. Today the layers lie at right angles to the coastline.
One group member asked about the globs of white rock stuck into the dark schist. How did they get there?
The white rock is quartz, Marx explained, and it has a low melting point. Originally, the surrounding rock contained tiny particles of quartz. When the rock heated up, the quartz melted and migrated through the rock’s pores and collected between the rock’s layers.
Birth of the Potomac
I found all of this about rocks fascinating. But what did it have to do with the river?
First, the river is much younger than the rocks, Marx said. As recently as a few million years ago this area was a coastal plain with small streams meandering around. There was no Potomac.
Then the earth’s crust began to lift up in our region and much of the east coast. This forced the flowing water to dig down and form more permanent channels, marking the beginning of our river and the beginnings of many others.
When the Potomac finally arrived on the scene, it used the relative weakness of the earth’s fault lines to etch its pathway across the landscape.
You can get the feel for these lines and the rock’s jointing by looking at a map of the river. At Great Falls, the river makes a sharp turn, and then another one as it straightens out into Mather Gorge. At Sandy Bottom it turns the opposite way, and then, a short distance later, it turns again.
Very interesting all this about the past, I said. But how about the future? When will the river end?
Marx gave me an exasperated look. I was asking a scientist to put on a shaman headdress and enter that frightening world of conjecture, where there is no evidence.
“Who knows?” he said. The land could rise. It could sink. Geology keeps going on, and the only thing certain is that the future will not be like the present.
But take it from me, the Potomac will be around for a while yet. So get out there and enjoy it.