Geology was my first love. As a young girl, I discovered a treasure trove of seashells on top of a mountain and ever since, I’ve been in love with the rise and fall of the lay of the land. Now 30 years old (almost 31!), with a degree in geology and a fair bit of geo-minded traveling under my belt, I can usually stand at an overlook, summon my experiences with orogeny and erosion and tell a fairly accurate story about how the landscape came to be.
From my essay Into the Ojito Wilderness: After wandering around the badlands, we climbed up a steep trail to the top of the mesa and headed northwest, towards a massive eroded anticline with views of Cabezon Peak, a magnificent volcanic neck that I climbed last spring.
Sitting on the edge, my textbook knowledge of anticlines – a convex dome of geologic layers – gradually aligned with the giant scoop-shaped valley in front of me. Here there was once a dome, but over time, the raised center had eroded away, leaving a long, deep, dark red trench capped by a thick angled layer of white crystalline gypsum. A geology lesson, in the flesh!
My geology prowess is not invincible, however. Occasionally, I stumble. A few weeks ago, on my first trip to the Devil’s Marbleyard in Virginia, I totally wiped out.
Located near the geology-gifted town of Natural Bridge, Virginia, the Devil’s Marbleyard is a jumbled morass of refrigerator, car and bus-sized boulders, strewn over eight acres on the side of a mountain.
When I see fields of boulders, I think glaciers. When ice moves over a landscape, it tears up the underlying rocks and carries them along, often depositing boulders far from their source. I’ve seen a lot of glacial moraines out west and most recently, in Rhode Island. With ice on my mind, the tracks in the rocks, all running parallel, looked like glacial striations: scratches gouged out when two rocks are rubbed together by moving ice.
As it turns out, I was wayyy off. For starters: glaciers never got as far south as Virginia during the last Ice Age. Duh. The Laurentide ice sheet advanced from the Arctic as far south as New York City and Chicago, but no farther. The Appalachian Mountains may have held some ice, but the peaks were roughly the same height 10,000 years ago as they are now, so we’re not talking Rocky Mountain-esque glacier fields (which are fast disappearing).
So what happened here? As it turns out, the bizarre and beautiful geology of the Devil’s Marbleyard tells a story of ancient beaches, extinct worms, supercontinents and ice crystals. According to the U.S. Geologic Survey, the white rocks of the Marbleyard began as a white sand beach, much like the Outer Banks of North Carolina, around 500 million years ago. The sand was riddled with worms, which left behind copious worm-track tunnels as they moved through the sediment.
Over a period of several hundred million years, the beach was buried and the quartz-rich sediments were compressed and baked into quartzite, one of the hardest metamorphic rocks on the planet. A few of the worm tracks were preserved, all in one parallel plane; tracks in other orientations were obliterated.
Then around 325 million years ago, the continental plate under what is now North America (then known as Euramerica) began colliding with the African plate (known as Gondwana), eventually forming the supercontinent Pangaea and pushing up the Appalachian Mountain chain to Himalayan heights.
For more than 200 million years, the hard quartzite of the Marbleyard lay intact, within the bowels of the mountains, until it was eroded to the surface, largely still in one piece. Then, ice went to work on the rocks, but not in the spectacular glacier-driven methods I imagined. Instead, tiny ice crystals found their way into cracks and fissures in the rocks, perhaps along the ancient worm-trails, and when the ice expanded, it cracked the rocks. Eventually, the entire quartzite ridge tumbled downslope, creating the Devil’s Marbleyard.
The moral of this story: given enough geologic time, worms can bring down mountains!
The Devil’s Marbleyard is a great day hike, around 3 miles round trip on the Belfast Trail. You can also connect with the Appalachian Trail from the Marbleyard. Dogs are permitted off leash, but they’ll need to have some serious rock-hopping skills to follow you into the Marbleyard. The rocks are fairly stable for scrambling, but a few move underfoot and I did hear a couple of minor rockfalls. Click here for directions to the trailhead.