Nature Notes; Grandfather Rocks: Red Rock Ranch & the Precambrian Eon

One billion years ago. The planet, with its rocks and seas, was bare of all but the simplest forms of single-celled life. In the waters, algae were the sum of plant life.

The Precambrian Eon preceded the emergence of complex animal life. It makes up 85 percent of the Earth’s history – but precious little is known about this period.

But in mounds of deep red rock west of Van Horn, this ancient age speaks.

As early as the 1840s, naturalists found fossil evidence suggesting that animal life appeared on the planet in a burst of diversity.

This was the Cambrian explosion. It began about 542 million years ago. Over the course of 25 million years, life diversified exponentially from its single-celled form. The Cambrian explosion produced most of the animal phyla that we see today.

But when you look further back – at the Precambrian Eon that lasted 4 billion years – the evidence is limited.

Darice McVay provides guided tours of her family’s ranch near Van Horn. The family’s Red Rock Ranch contains some the most extensive examples of Precambrian geology in Texas.

“Here we have all this beautiful rock that has been eroded by thousands of years of erosion, wind and rain, that has been exposed,” McVay said. “And it’s absolutely gorgeous.”

McVay drives visitors through the desert scrub west from Van Horn. Passing Turtleback Mountain, the first Precambrian formations come into view. Mounds of sandstone emerge from the surrounding hills, and McVay invited visitors to walk on “some of the oldest dirt in the world.”

The Trans-Pecos contains tremendous geological diversity. Its national parks feature rocks shaped by two ancient seas, by volcanism and by the continent’s continued faulting. But a West Texas rockhound sees immediately that this is something different.

These rocks seem to exude a primal force and simplicity.

The Van Horn Sandstone appears in stacks, in rounded outcroppings and intricate hoodoos. But whatever life existed in the Precambrian left no fossil evidence.

From a high ridge, visitors survey the scene above Hackberry Creek, looking east at a red-rock valley. McVay described the scene – Van Horn and the Wylie Mountains in the distance to the southeast, the Carrizos to the south and the Beach Mountains to the north.

“Now the Precambrian is underneath all of these mountains,” McVay said, “and there are place you can see here where the wind and water have eroded it away, so you can see the Precambrian.”

At the ranch, the Van Horn Sandstone is perhaps the most striking. But there are several Precambrian formations in the area, composed of both volcanic and sedimentary materials.

The oldest is known as the Allamoore Formation. It was created in an island arc, more than 1 billion years ago. Lava flowed from the sea floor and at the margins of an ancient continent.

This rock was then transformed in a mountain-building event called the Grenville Orogeny. As land masses collided to form a supercontinent called Rodinia, mountains rose, roughly along the line of the present-day Appalachians. Evidence is found from Canada to Mexico and in the Llano Uplift of Central Texas. And as the mountains eroded, the Hazel Formation was deposited here at the Red Rock Ranch.

The Allamoore and Hazel formations eroded further. The eroded material formed the massive beds of the Van Horn Sandstone.

In West Texas, the Precambrian rocks are exposed at Red Rock Ranch and in the Franklin Mountains near El Paso. But they lie beneath the entire region – as the basement of West Texas. Geologists believe these Precambrian rocks form the “craton” – the stable continental core – for much of Texas.

When we see the continent’s grandfather rocks in these red outcroppings, we see back into the planet’s elemental past.

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.