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The Making of a Mountain: the Glass Range

glass-mountains-capitan-reef-map
North of Marathon, the Glass Mountains mark the threshold between the Great Plains and the mountain West. These “gateway” mountains are the exposed remains of a reef, which formed around an inland sea when West Texas lay near the equator, as part of the supercontinent of Pangaea.

The writer Edward Abbey described his first sight of the Rocky Mountains, as a 17-year-old hitchhiker from Pennsylvania:

“On the Western horizon... was a magical vision, a legend come true. The image of those mountains struck a fundamental chord in my imagination that has sounded ever since.”

It's an essential American experience – the westering traveler's first intoxicating glimpse of mountains above the plains. In Texas, the Glass Mountains, north of Marathon, play that role. Rising to 6,500 feet, they may not have the scale of Colorado's Front Range. But they mark the same threshold, and their meaning is unquestionable: One has entered the West. 

What is the story of these gateway Texas mountains?

Blaine Hall was a native West Texan whose geological career took him around the world. 

“The story of the Glass Mountains is the story of the whole reef system,” he said. 

The limestone strata that give these mountains their glassy look, their lunar pallor, have their origins in the deep Paleozoic past, Hall said.

Some 270 million years ago, the planet's land masses were fused as the supercontinent Pangaea. What's now West Texas was submerged by an inland sea, connected to the globe-encircling ocean by a narrow channel.

Mammals wouldn't make their appearance for tens of millions of years. But the sea teemed with life – algae and sponges, clams and sea urchins, the extinct arthropods known as trilobites. Creatures called bryozoans – “moss animals” – formed colonies, that resembled lacy fans.

And all these lifeforms created an immense reef.

“The nature of the reef system is you have a basin,” Hall said, “and fringing that basin you have these carbonate buildups. The Glass Mountains are an integral part of that system.”

The reef ringed the basin for some 400 miles. But about 260 million years ago, the outlet connecting the inland sea to the ocean began to close – and the reef died. Its remains were buried under new rocks during the ensuing millennia.

But more than 200 million years later, powerful tectonic forces would exhume the reef here.

The mountainous landscapes of the American West are tied to a collision – between North America and the oceanic Farallon Plate. Across millions of years, the Farallon Plate drove itself beneath our continent, in a process called “subduction.” It made it as far as the location of the Glass Mountains. As the subduction ceased, the land here was faulted and broken – raising up the buried reef in the form of the Glass Range. Elsewhere in West Texas, the reef was exhumed as the Apache and Guadalupe mountains.

The Glass Mountains are the threshold of the Big Bend, and the mountain West. Their shining cliffs are also a gateway into deep time.

Drew Stuart is the producer for the Marfa Public Radio series Nature Notes.