Category: Rocks

Stalking the wild amphibolite

Hikers clamber over remnants of oceanic crust created half a billion years ago.

The park ranger set the map on the countertop and traced his finger along a dotted line, first down the edge of Bear Island. and then upstream along the Potomac River’s Mather’s Gorge. Just past a spot marked TM-3 his finger stopped.

“Here it is,” he said quietly, almost to himself. “Amphibolite isn’t something you see much around here.”

I didn’t know much about amphibolite except that it was a dark rock with a violent and mysterious past. Even its name is beguiling. “Amphibole,” one of its  constituent minerals, means “ambiguous” in the ancient Greek.

Getting to know a new rock is always exciting—at least to me. It was getting late, but the trail looked short. If I hurried I could reach the spot well before dark.

Race to the rocks. Turning off the C&O Canal towpath I followed the blue marks on tree trunks that traced the trail through the stunted forest. Past a stream I came to a wall of rock. Up I scrambled, finding footholds for my boots and  feeling my leg muscles stretch as I stretched my arm to grab a handhold.

Every so often I looked around at the jumble of rocks, some grey, others black, many covered with blotches of light green lichen and tufts of moss, or even small trees growing out of cracks. Many of the rocks were cleaved into sharp angles, as if by a master stonecutter. I walked on bedrock smoothed into curves and hollows by the river’s abrasive floodwaters.

The rockscape was created over hundreds of millions of years by collisions of sections of the earth’s crust as they migrated over the earth’s mantle. Each collision threw up mighty mountain ranges and compressed and deformed sediments and debris and the very magma that welled up from deep under the earth’s surface. The Potomac region sits in the epicenter of these continental collisions.

I crossed a little beach and climbed another cliffside, then dropped down again, taking care with each footstep. Up and down, again and again—this was starting to be more work than pleasure. It was also getting late. I kept glancing at the sun as it sank over the hills across the river.

Maybe this wasn’t such a good idea? The rock I had wanted to meet as a new friend was now luring me along a path of slippery ledges, sharp crevasses, and plunging holes, each with the potential to twist my ankle, or worse. I checked my cell phone; the screen said, “No Signal.” I hadn’t left a note to say where I was going. I didn’t have a flashlight, or even my plastic orange whistle that was top rated by survival-mastery.com.

Then I came to a post—the same trail marker the ranger had showed me on his map.

Just as the amphibolite had intruded into the overlying sedimentary rocks, a tree drives apart the amphibolite.

Ancient fortress. A few steps further and I found myself in a realm of rock slabs like the remains of an ancient fortress. I examined a rock face and its coarse-grained pepper-and-salt pattern, heavy on the “pepper.” The dark mineral is hornblende, the common name for group of minerals found in many types of igneous and metamorphic rocks called amphiboles. The “salt” was  feldspar.

I ran my hand over the rock and felt its pebbly texture. These were amphibole crystals that had been exposed by weathering. The rock surface looked like the skin of the marine iguanas I once saw on the Galapagos Islands.

Now this was starting to get fun again. No matter that amphibolite is actually a common rock that is used for very mundane purposes. Polished to a shiny black, it’s a favorite for building facades and kitchen counter tops. Its toughness makes it a good aggregate for road construction and ballast for laying train track. The local Indians shaped amphibolite into tools for grinding corn and other foodstuffs.

Heat and pressure.The amphibilite was created here some 540 million years ago. This would put it in the period between two great mountain building events, the first when continents collided 1.1 billion years ago to form a super continent called Rodinia, and the second when an arc of volcanic islands slammed into North America’s east coast around 460 million years ago. In between, vast amounts of sediments swept into the ocean and cascaded down the slope on the edge of the continental shelf. These submarine landslides created sediment layers sometimes miles thick, producing heat and extreme pressure that forged the mud sand sand into shale and sandstone.

Framed by lichen, the weathered surface displays pebble-like crystals of amphibole.

Next, tabular masses of oceanic crust, called gabbro, punched up into these sedimentary rocks, where the same heat and pressure transformed them into amphibolite. I was standing on such a mass, one of a series of parallel deposits shaped like fingers that began on the opposite shore and passed underneath the river.

OK, time to go. Hurry, but take care, I kept telling myself. I continued on, up the rocks and down. I found the cutoff trail leading back to the C&O Canal towpath just as the moon appeared as a thin crescent over the hills. Back down by the river the usual owl was making its usual comments.

 

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.