In the Triassic era, North America was part of Pangea, a supercontinent whose massive scale sparked extreme monsoons that oxidized ancient sediments into striking red rocks.
These “red beds,” rich in iron minerals, are now visible in places like the Bighorn Basin, where folded rock formations tell the story of tectonic collisions and mountain-building. One vivid streak of these formations cuts through Bighorn Canyon, a deep chasm formed over millions of years and now a stunning recreation area teeming with wildlife, fossils, and echoes of Native American history.
Ancient Megamonsoons on Supercontinent Pangea
About 220 million years ago, during the Triassic Period, what is now North America was part of the massive supercontinent Pangea. Because of its enormous size, Pangea had equally large-scale weather systems. Scientists believe that its tropical regions were hit by intense seasonal “megamonsoons,” which brought dramatic swings between very wet and very dry conditions.
Red Beds and Rusted Rock Across Continents
These extreme climate cycles likely caused widespread oxidation – essentially, rusting – of surface sediments. Over time, this process produced reddish rock layers known as “red beds,” colored by the iron-rich mineral hematite. Today, these ancient red rocks appear on several continents. In North America, they are found in a few regions, including the Bighorn Basin in northern Wyoming and southern Montana. This oval-shaped basin, about 150 miles (240 kilometers) long and 80 miles (130 kilometers) wide, is surrounded by six mountain ranges and is known for its rich fossil record and oil and gas deposits.
Chugwater Formation and the Rise of the Rockies
Sharp folds of the distinct reddish Chugwater Formation cut across the image above, acquired with the OLI-2 (Operational Land Imager-2) on Landsat 9 in June 2024. In the tens of millions of years since the Chugwater’s Triassic conception, the Pangean supercontinent fragmented, and landmasses recombined. The sedimentary rock layers seen here were folded and tilted in the continental collisions that created the Rocky Mountains. These folds are located near the northern end of Bighorn Basin; other outcrops of the Chugwater appear in the basin, often along its perimeter.
This jagged streak of red rock intersects with Bighorn Canyon. The canyon makes a gouge in the landscape as it extends north into Montana, with depths reaching up to 2,500 feet (760 meters). Bighorn Canyon National Recreation Area, outlined in the image above, was established in 1966 after the Bureau of Reclamation constructed Yellowtail Dam on the Bighorn River and created a lake more than 70 miles (110 kilometers) long.
The recreation area encompasses a wide range of natural and cultural wonders. Researchers have discovered fossils dating back to the Jurassic Period within its boundaries. Today, the shrubland, woodland, and steppe habitats are home to wildlife such as bighorn sheep, wild horses, and coyotes. Artifacts along parts of the Bad Pass Trail, traveled by Native Americans for thousands of years, are preserved within the park. Now, more than 200,000 people visit the area each year for hiking, boating, fishing, and other activities.
NASA Earth Observatory images by Wanmei Liang, using Landsat data from the U.S. Geological Survey.
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1 Comment
It takes more than just water to create red beds. They are typically a product of chemical weathering, such as the laterites found in the Amazon today, and notably abundant during the Eocene epoch. That means that temperature plays an important role in increasing the rate of chemical weathering. Probably the pH of rainwater, presently about 5.5, played a role and was probably lower than today.