Grand Mesa

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The Grand Mesa is a large geologic formation in western Colorado in the United States. The largest mesa in the world, it has an area of about 50 square miles (130 km²) and stretches for about 40 miles (64 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. It rises about 5,000 feet above the surrounding river valleys, including the Grand Valley to the west, reaching an elevation of about 10,000 feet (3,000 m), with a maximum elevation of about 11,600 feet. Much of the mesa is within Grand Mesa National Forest. Over 200 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.

Geologically the mesa is the result of a hard volcanic basalt layer on its top. This volcanic layer, created during the birth of the modern Rocky Mountains approximately 30 million years ago, suppressed erosion compared to the surrounding sedimentary rock layers, which suffered rapid downcutting from the Colorado and the Gunnison. The top layer rests on a thick sequence of Tertiary 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 from the early Cretaceous. 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 mesa is traversed by Colorado State Highway 65 between the town of Mesa on the north and the town of Cedaredge on the south. The route over the mesa provides a dramatic contrast in landscape, climate and vegetation. On the north side, the road climbs the steep cliffs near the Powderhorn ski area. The forested top of the mesa remains snowbound much later in the spring that the surrounding valley, and is a popular location for cross-country skiing.