TheGrand Mesa is a largemesa in westernColorado in theUnited 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 ofGrand Junction between theColorado River and theGunnison River, its tributary to the south. The north side of the mesa is drained largely byPlateau Creek, a smaller tributary of the Colorado. The west side is drained largely byKannah 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 theGrand Valley to the west, reaching an elevation of about 11,000 feet (3,400 m). Much of the mesa is withinGrand 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 ofvolcanicbasalt. This layer formed between about 10.9 and 9.6 million years ago, by the eruption of at least 27 separatelava 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 surroundingsedimentary rock layers, which suffered rapiddowncutting from the action of the Colorado and the Gunnison rivers.^[3] The top layer rests on a thick sequence ofEoceneshale andsandstone known as theGreen River andWasatch Formations. These layers in turn rest on aCretaceous layer known as theMesaverde Group that forms a cliff about halfway up the side of the mesa. The lowest layers are yellow and grayMancos Shale of late Cretaceous age. The shale continues outward into the surrounding valleys in the vicinity of the mesa, providing asoil base that is fertile for various kinds ofagriculture, 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]

Climate on Grand Mesa varies by elevation. Higher elevations tend to be cooler and receive more precipitation. The top of the mesa is more than 2 miles (3,220 m) above sea level, and experiences analpine climate with substantial amounts of precipitation throughout the year. Because of the high altitude, summer days are mild, and temperaturesdrop sharply after sunset. Winter temperatures rarely rise above freezing, and can be extremely cold, often falling far below zero, especially at night. The top of the mesa is usually snow-free from late June through early October; however, the mesa receives heavy snowfall in the winter, rendering the forest service roads impassable. Accumulations of more than 300 inches (760 cm) of snow in the winter are typical. Surrounding areas with lower elevation are warmer and more arid throughout the year.

The following climate data is for Mesa Lakes Resort, located on the north slopes of the mesa at 9,800 feet (3,000 m) above sea level, which experiences asubalpine climate (KöppenDfc).

The mesa is traversed by theGrand Mesa Scenic and Historic Byway, which includesSH 65, between the town ofMesa on the north and the town ofCedaredge 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 terrain near thePowderhorn Resort ski area. The forested top of the mesa remains snowbound much later in the spring than the surrounding valleys, and is a popular location forcross-country skiing and snowmobiling.

A panoramic view of the Grand Mesa, with Mt. Garfield and theBook Cliffs visible on the left

• 
  Cliff edge on western face of the mesa.
• 
  Typical landscape on top of the mesa.
• 
  In the summer, cattle graze atop the mesa.
• 
  View from Grand Mesa looking south towards Island Lake and theSan Juan Mountains.
• 
  Numerous ponds and small lakes dot the top of the Mesa.
• 
  Fall colors atop the mesa.
• 
  Western edge of the mesa.
• 
  Typical Forest Service road atop the mesa.
• 
  The view towards Grand Junction.
• 
  An aerial photograph of Grand Mesa.

• Mountain ranges of Colorado
• Table (landform)
• 2014 West Salt Creek landslide

• Halka Chronic (1980).Roadside Geology of Colorado. Missoula Montana: Mountain Press Publishing Co.ISBN 0-87842-105-X.

Wikimedia Commons has media related toGrand Mesa.

• "Grand Mesa".Peakbagger.com.

39°1′36″N108°4′53″W / 39.02667°N 108.08139°W /39.02667; -108.08139

• Colorado Plateau
• Geology of the Rocky Mountains
• Mesas of Colorado
• Rocky Mountains
• Landforms of Delta County, Colorado
• Landforms of Mesa County, Colorado
• Grand Mesa National Forest

• Pages using gadget WikiMiniAtlas
• Articles with short description
• Short description is different from Wikidata
• Commons category link is on Wikidata
• Coordinates on Wikidata