America’s national parks are showcases of extraordinary geological phenomena. While some of them are famous, others are unsung, unexplained, or just plain strange. But, to be sure, all of them spur the imagination, helping us to appreciate the forces that have shaped the nation and its parks.

Here are five of the most fascinating geological oddities you can find in America’s national parks:

> Badlands Wall, Badlands National Park (South Dakota)

The condensed story: Once there were volcanoes and volcanic ash. There were forests and river flood plains. There was a sea, then uplift, then an island sea, and each episode left a layer on the site of the Badlands in South Dakota. Half a million years ago, rivers began to carve it all into the strange and stratified badlands seen today.

The most representative formation in the park, found in the North unit, is the Badlands Wall–a 60-mile spine of buttes left behind as wind, rain, freezing, and thawing have had their way with the badlands sediments.

Badlands National Park is a geological wonderland featuring striking buttes, pinnacles, and spires. (Photograph by acmace, Flickr)

Badlands National Park is a geological wonderland featuring striking buttes, pinnacles, and spires. (Photograph by acmace, Flickr)

A scenic drive hugs the wall, providing access to the Door and Window trails, which lead to openings in the 150- to 450-foot wall that reveal views of the upper grasslands to the north and the lower grasslands to the south. Both the drive and the trails are excellent ways to marvel at the park’s strange geology, particularly evident in the colorful horizontal layers clearly revealed in the wall’s facades.

> North America’s Tallest Dunes, Great Sand Dunes National Park & Preserve (Colorado)

The magnificent dunes that rise from the San Luis Valley against the soaring backdrop of Colorado’s Sangre de Cristo Mountains are composed of sand left behind by ancient lakes and blown up against the massive mountain windbreak by prevailing westerly winds that funnel the sand toward three passes in the range. Because the winds occasionally change direction and blow from the mountains back toward the valley, the Great Sand Dunes get sculpted and grow higher than most. They are, in fact, the tallest dunes in North America, up to 750 feet high. Only about 10 percent of the park’s sand is actually in the dunes. The rest is in what’s known as the sand sheet, grasslands that surround the dunes on three sides.

But it’s the soaring dunes of the 30-square-mile dune field that capture attention in the park. There dunes are wind sculpted into star dunes, whose crests intersect in star-shaped patterns. The tallest dune in the park is the 750-foot-high star dune called, imaginatively, the Star Dune.

Reversing winds also form “Chinese Walls,” long, low ridges that arc across the ridges of high dunes. Getting around in the dunes isn’t easy–rangers recommend walking in a zigzag fashion up the ridges-but gaining a vantage on sand summits like High Dune, which looms 650 feet above the valley, or Star Dune yield extraordinary perspectives on the whimsical forms and shapes of their neighbors.

  • Tip: Sandboarding down those dunes is possible on a well-waxed old snowboard or plastic sled.

> The Racetrack, Death Valley National Park (California)

In this California desert park full of geological oddities, from its salt-pan floor to fluted canyons to cinder cone crater, none is stranger or more baffling than Death Valley’s mysterious Racetrack.

The Racetrack is a playa, a dry lake bed, in the northern part of the park where rocks tumble from a bordering bluff. Then, as evidenced by tracks in the playa, the rocks–which range from golf-ball size to several hundred pounds–inexplicably proceed to move on their own across the playa. They zigzag, loop, even cross each other’s tracks.

The “races” seem to take place overnight; no one has actually witnessed the rocks’ movement. The long-standing theoretical explanation: Rain causes the playa surface to become slick, and strong winds blow and push the rocks across its surface. One problem with that theory is the fact that the rocks sometimes seem to move in concert with one another, but other times split up and go their own way. No one really knows for sure how they move.

The road to Racetrack Valley starts near Ubehebe Crater. Four-wheel drive is necessary to reach the dry lake.

> Great Kobuk Sand Dunes, Kobuk Valley National Park (Alaska)

Hanging out on the sand with temperatures in the 80s would not be unusual in, say, southern California, but in the Alaska Arctic? Strange but possible.

Half a million caribou migrate through Alaska's Kobuk Valley. (Photograph by alaskanps, Flickr)

Half a million caribou migrate through Alaska’s Kobuk Valley. (Photograph by alaskanps, Flickr)

The Great Kobuk Sand Dunes stretch for miles just south of the Kobuk River between the villages of Ambler and Kiana, 35 miles north of the Arctic Circle. The inland dunes are the result of two periods of glaciation, during which retreating glaciers pulverized rock into sand, which was borne by strong east winds to form the high dunes of the valley floor. The dunes crest at about 100 feet high near the river and rise to 500 feet closer to the Waring Mountains.

The youngest, most active dunes are crescent-shaped mounds called barchans, a sand formation that’s the result of strong winds prevailing from a single direction. They’re reachable by a hike from the river if you’re on a float trip, or by air taxi from Bettles or Kotzebue. The park also has two smaller active dune fields that can be visited, the Little Kobuk Sand Dunes and the Hunt River Sand Dunes.

> Waterpocket Fold, Capitol Reef National Park (Utah)

Though Utah’s Capitol Reef was named for a dome formation flanked by cliffs and ridges that resemble an ocean reef, it’s the remarkable Waterpocket Fold that is its main attraction. “Fold” is apt, because it’s like a wrinkle in the Earth’s crust, of which the dome is one small part. People often come into the park visitor center asking where to see the fold, only to learn that they’re standing on it. The 100-mile-long fold is what geologist’s call a monocline, a long line of cliffs created by a faulting action. Erosion over the last 15 to 20 million years has laced the fold with domes and twisting slot canyons.

One way to see it is to “loop the fold” on a 147-mile scenic drive. East of the visitor center, Utah 24 runs west right through the fold. Then follow Notom-Bullfrog Road south to Burr Trail Road, which makes some spectacular switchbacks up and east over the fold, with fantastic views of its cliffs. Eventually Burr Trail joins Utah 12, which leads back north toward park headquarters.

Hikers can plunge into the fold on Upper Muley Twist Trail and Lower Muley Twist Trail. Upper Muley is a 9-miler that starts down in a canyon near the Strike Valley Overlook and climbs to the ridgeline that is the top of the fold. Lower Muley’s trailhead is at mile 44.6 on Burr Trail, and the trail zigzags so tightly it can “twist a mule.” It runs for 23.4 miles, but even a short hike gives you the sense of venturing deep into one of the planet’s most interesting formations.

This article was excerpted from the National Geographic book The 10 Best of Everything: National Parks.

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