Grand Mesa

The Grand Mesa is a large mesa in western Colorado in the United States. It is the largest flat-topped mountain in the world.^[1] It has an area of about 500 square miles (1,300 km²) and stretches for about 40 miles (60 km) east of Grand Junction between the Colorado River and the Gunnison River, its tributary to the south. The north side of the mesa is drained largely by Plateau Creek, a smaller tributary of the Colorado. The west side is drained largely by Kannah Creek, which is received to the west by the lower Gunnison River. The mesa rises about 6,000 feet (1,800 m) above the surrounding river valleys, including the Grand Valley to the west, reaching an elevation of about 11,000 feet (3,400 m). Much of the mesa is within Grand Mesa National Forest. Over 300 lakes, including many reservoirs created and used for drinking and irrigation water, are scattered along the top of the formation. The Grand Mesa is flat in some areas, but quite rugged in others.

The mesa is topped by a hard layer of volcanic basalt. This layer formed between about 10.9 and 9.6 million years ago, by the eruption of at least 27 separate lava flows from a vent in the Crag Crest area and probably other vents. This built up a maximum total thickness of more than 93 meters (310 feet) of basalt.^[2] The resistant basalt layer suppressed erosion compared to the surrounding sedimentary rock layers, which suffered rapid downcutting from the action of the Colorado and the Gunnison rivers.^[3] The top layer rests on a thick sequence of Eocene shale and sandstone known as the Green River and Wasatch Formations. These layers in turn rest on a Cretaceous layer known as the Mesaverde Group that forms a cliff about halfway up the side of the mesa. The lowest layers are yellow and gray Mancos Shale of late Cretaceous age. The shale continues outward into the surrounding valleys in the vicinity of the mesa, providing a soil base that is fertile for various kinds of agriculture, especially in the Gunnison Valley to the south.

The flows of the Grand Mesa volcanic field may have originally covered an area of 1,300 square kilometers (500 square miles), but erosion has reduced the intact flows to just 166 square kilometers (64 square miles).^[2]

The presence of rounded river cobbles beneath Grand Mesa's basalt cap confirms that it was originally a valley when the volcanic eruption occurred, indicating that the mesa is an example of topographic inversion. This means that what was once the lowest point in the landscape is now the highest, with the Colorado River having incised approximately 1,500 meters since the basalt flow, resulting in an average downcutting rate of about 140 meters per million years.^[4]