Guided tours at national parks are a rocking good time
By Tom Nichols
Last year, Judy and I became very big fans of tours led by rangers and volunteers at America’s national parks and monuments. So before departing Big Bend National Park in west Texas, our first national park of 2016, we couldn’t leave without one more tour, this one on geology.
We hiked at Santa Elena and Bouquillas canyons, Mule Ear Peaks, the Chisos mountains and Chihuahuan Desert lowlands. All of it was majestic. Now all I wanted was a clear, brief recap about how Big Bend came together. Turns out that’s impossible. The convoluted rock forms here can be confounding, even for geologists.
Our volunteer geology guides sketched Big Bend from the perspective of Sotol Vista, west of Panther Junction. They asked us to imagine a cinematic opening scene for the Big Bend region 300 million years ago.
It was a deep inland sea with shallower waters near Persimmon Gap to the north, where the oldest rocks in the park are 500 million years old.
Gravels and sands transported from mountains to the north filled the region with sediments as deep as 8,000 feet. They formed shale and sandstone masses, which were compressed, eroded and buried.
Later, a shallower sea deposited masses of mud, which left limestone layers that contain the fossil remains of giant clams, crocodiles and a flying reptile with a wingspan of 35 feet.
The central and youngest feature in the park are the Chisos Mountains, a fractured birthday cake formed by episodes of volcanic activity, peaking inside the park about 35 million years ago, followed by erosion, magma intrusion and faulting.
There’s a lot of rhyolite, an igneous rock with bits of pyrite, or Fool’s Gold, to admire in the Chisos. Oxidation on the surface of the igneous rock helps to create the signature brown-reddish hues. Emory Peak, which rises to 7,825 feet, is the tallest point in the park.
Looking toward the U.S.-Mexico border, we gazed at dusty gray mountain ridges on both sides and learned about the history of Santa Elena Canyon. Water flow from the Rio Grande and Rio Conchos rivers etched the limestone canyon and carved a path to the Gulf of Mexico during the last 2 million years, making this the youngest major river system in the United States.
Movement along the west coast of the North America plate about 25 million years ago stretched the Big Bend region, creating fracture zones near Santa Elena Canyon. Rock masses near the canyon slid downward during episodes of faulting and earthquakes. The activity left Santa Elena Canyon about 1,500 feet above the Rio Grande River while a layer of similar rock lies about 1,500 hundred feet under a parking lot where the trail begins.
West of Santa Elena Canyon is Mesa de Anguila, a remnant of the Appalachian Mountains. The younger Rocky Mountains are also represented at Big Bend, with Mariscal Mountain, the southern tip of the Rockies in the United States.
Later in the tour, from a lookout near Goat Mountain, we saw how intrusive magma from under the earth’s surface rose and punched up enormous domes, how magma moved laterally and broke through rock formations, how it filled in gaps in inside the mountain, and how long, straight dikes of igneous rocks remain. They are more resistant to erosion from wind and water than layers of sandstone and clay once covering them.
We ended our tour at Tuff Canyon, a misnomer because the smooth, light gray volcanic particles plastered on the eroded river bed are too large to be considered textbook tuff. Yes, a tour guide can get too geeky at times, even for me.
Anyway, the most important thing to remember about the Big Bend geology hike is that you don’t have to walk much, just a quarter of a mile or so during the two-hour vehicle caravan tour.
In 2015, we took guided or self-guided tours at White Sands and Carlsbad Cavern in New Mexico; Guadalupe Mountains and Alibates Flint Quarries in Texas; Badlands in South Dakota; Devils Tower, Yellowstone and Grand Teton in Wyoming; Glacier in Montana; Theodore Roosevelt in North Dakota; Effigy Mounds in Iowa; Craters of the Moon in Idaho, and Olympic and Lewis and Clark in Washington. Only Alibates Flint in the Texas Panhandle and Effigy Mounds on the Mississippi River bluffs involved strenuous walking. Most routes are a mile or less, relatively flat, and often paved or on boardwalks.
We say the best way to celebrate the 100th anniversary of the National Park Service is with a ranger or volunteer. Show some love to our dedicated public servants.