Riverbed on a hilltop

It looks like the Potomac river bed,
but minus the shells and fish.

I went poking around a rocky riverbed today that ran along the crest of a hill. It looked just like the bottom of the Potomac River, though without the fish and the shells. It also looked a little like a NASA photo of ancient riverbeds on Mars.

In all three cases, the giveaway was that the rocks are rounded. Some were as small as jelly beans, others as big as melons. I picked up a baseball-sized rock, and it felt good in my hand. Most were shades of cream, grey, or reddish brown. Some were startlingly white. They lay scattered about on the dark forest floor, like super-sized fairy dust.

They told a story about a restless river, a Potomac that shifted one way and then another, cutting down into the landscape and leaving evidence of its meanderings on what would become the hillsides we know today.

The rocks were mostly quartzite, a tough, hard material forged by heat and enormous pressure from sandstone lying deep under the earth’s surface millions of years ago. But there are no quartzite deposits here in the Piedmont province of the Potomac valley. The nearest is over 30 miles away in the Blue Ridge mountains, near Harper’s Ferry, West Virginia. According to geologists, chunks of the rock broke off from the mountains long ago and tumbled into the river. The river transported them downstream, bumping and scraping against themselves and the river bottom, like semiprecious stones in a rock tumbler.

Created by heat and pressure,
shaped by a raging river.

The river as sculptor. How long did did it take the river turn these jagged, angular pieces of rock into the rounded objects lying around me? Did it carry them downstream inch by inch, mile by mile, over hundreds or even thousands of years? Or did it whoosh them down in one or several full swoops?

Until recently, most geologists would have backed the slow-going scenario. According to the longstanding geological theory called uniformitarianism, geological events unfold in measured, incremental changes. But in recent decades, geologists have been taking a  livelier view of how geology works. The new theory, called catastrophism, recognizes the important role that sudden, violent events have played in shaping our world.

The catastrophist view is the apparent winner, according to the authors of a paper that describes the origin of similar deposits on the Virginia side of Great Falls a few miles away. They postulate that the rocks were transported and shaped by “one or a few” massive floods that were double the magnitude of the 1936 flood, which is the biggest on record.

The authors of the paper couldn’t pinpoint when these events took, though it could have been as recently as 70,000 years ago. Or it could have been much longer go. The rocks on the hillside have told part of their story, and we’ll just have to wait for the rest.

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