A piece of Caballos novaculite – an ancient rock found in the hills and mountains of the Marathon Basin.
Africa collided with what’s now the eastern United States, while South and North America fused near the equator. Three hundred million years ago, the continents had joined to form a single land mass – the supercontinent known as Pangaea.
The Appalachian Mountains were produced in that collision. But Western landscapes were shaped by more recent tectonic events.
Yet, near Marathon, Texas, lines of low mountains speak of an ancient chapter in the earth’s history.
For even a casual student of Texas mountains, the landscape between Marathon and Big Bend National Park is anomalous – it’s unexpected. Across the region, most mountains trend to the northwest. But here, low ranges run east to west: the Woods Hollow, Peña Blanca and Tinaja mountains, Simpson Springs and Horse Mountain. These hills are capped and lined by distinctive bands of white rock.
On Hwy. 385, a historical marker announces, here, the “old,” Appalachian-era mountains meet the “young” Rockies. It’s “a fusion,” the marker says, “unmatched at any other site in North America.”
What produced this landscape? And what’s the striking white rock that forms these ridges and flatirons?
That white rock is harder than steel. And it formed some 400 million years ago. It’s a silica-rich, crystalline chert called the Caballos novaculite.
There’s some dispute over its source. Most geologists believe it formed in a deep-sea basin. They trace it back to the skeletal material of sponges and single-celled creatures called radiolarians.
For hundreds of millions of years, sediments accumulated in this ocean basin, off the southern coast of North America. In addition to the chert, the sediments formed limestones, sandstones and shales.
Then, about 300 million years ago, the sea floor began to feel a pinch.
David Rohr is a geology professor at Sul Ross State University in Alpine.
“This would have been deposited – if we go for the deep origin – in the big marine basin adjacent to the North American continent,” Rohr said. “Toward the end of the Paleozoic, the continents began to come together. And the southern continent, basically South America, a portion of Gondwana, collided with North America – and this material got caught in between, and it was deformed, folded, faulted and then actually thrust-faulted quite a considerable distance to the north, to where it is now. So that’s why you can see the big folds and the deformational features in the Marathon area.”
What happens when continents collide? Well, today, the Indian subcontinent is smashing into Asia. It’s raising the highest mountains on earth – the Himalayas.
The collision of South and North America produced what’s called the “Ouachita Orogeny.” An orogeny is a mountain-building event. The rocks of the ocean basin were lifted and moved 10s of miles to the north and west.
The mountains stretched from present-day Alabama to Sonora, Mexico. But in the subsequent ages, they were worn away and buried in new rock.
Today, the Ouachita Mountains of Oklahoma and Arkansas are representative of this ancient range. Their novaculite rocks are known as “Arkansas stones.” Prized in prehistoric times for spearpoints, they’re still used today for sharpening knives and other steel tools.
In West Texas, tectonic forces have lifted and exposed the ancient range. It’s visible in the Marathon Basin, and in the Solitario, in Big Bend Ranch State Park.
The Caballos novaculite caps many of these ridges and hills – because it’s so resistant to erosion.
The novaculite remains buried in a band across much of Texas. In recent years, petroleum companies have produced natural gas from novaculite in Pecos County.
Questions about the Marathon Basin persist. Some of the ancient mountains are surrounded by unusual “boulder beds.” There are boulders hundreds of feet wide. And they contain rocks that are out of place in the Trans-Pecos – ones found nowhere else in the region.
Sul Ross geologist Liz Measures says these rocks may have “spilled over” from South America in the ancient collision.
“So these exotic rocks would have had to be eroded off of whatever was in South America and Africa that ended up smacking into us,” Measures said. “So it’s like leaving scrapes of your paint on a car you hit and you ran off.”
The Trans-Pecos is a portrait of geological diversity. On your next visit to Big Bend or the Post Park south of Marathon, look for the white bands of Caballos novaculite – and remember its 400-million-year-old story.
Nature Notes is underwritten by the Dixon Water Foundation and is produced by Marfa Public Radio in cooperation with the Sibley Nature Center in Midland, Texas. This episode was written by Andrew Stuart.