Colorado’s landforms fall into three broad belts: the Eastern Plains, part of the Great Plains, rising from about 1,100 to 2,100 m (3,500 to 7,000 ft); the Rocky Mountains through the centre of the state, with the Front Range, Sawatch, San Juan and Sangre de Cristo ranges, high basins called “parks”, and more than 50 peaks of 14,000 ft or more; and the mesas and canyons of the Colorado Plateau on the Western Slope. The highest point is Mount Elbert in the Sawatch Range at 4,401 m (14,440 ft), also the highest summit in the entire Rocky Mountains. The lowest point, where the Arikaree River crosses into Kansas at 1,011 m (3,317 ft), is the highest low point of any US state. Across its 269,837 km² (104,185 sq mi), almost every one of these features records a specific event in the state’s geologic past, and read in order, those events explain the map.
1.7 billion years ago: the gneiss of the Black Canyon and the Pikes Peak granite
The foundation of Colorado was assembled between about 1.78 and 1.65 billion years ago, when a series of volcanic island arcs collided with the southern edge of the ancient Wyoming Craton. The collisions squeezed and heated the rock into gneiss and schist, and granite and pegmatite later forced their way into it. That basement is still exposed today wherever uplift and erosion have stripped the younger layers off. The most dramatic place to see it is the Black Canyon of the Gunnison, where the canyon walls are 1.7-billion-year-old gneiss and schist crossed by pale pegmatite dikes.
A second pulse of intrusion followed. About 1.08 billion years ago, magma rose beneath what is now the central Front Range and cooled into the Pikes Peak granite, a coarse, pink to brick-red rock that makes up almost all of Pikes Peak, 4,302 m (14,115 ft). Then came an enormous gap. For hundreds of millions of years the surface was simply worn down, until the granite mountains were planed almost flat by about 510 million years ago. The missing time is part of the Great Unconformity, and in Colorado roughly 10 km of basement rock disappeared during it.
300 million years ago: Frontrangia, Uncompahgria and the red Fountain Formation
Through most of the Paleozoic, Colorado lay under a shallow tropical sea that laid sandstone, shale and limestone on the planed-off basement. Then, around 300 million years ago, as the continent collided with Gondwana during the assembly of Pangaea, two mountain ranges rose through the middle of the state. Geologists call them the Ancestral Rocky Mountains, or by the names of their two main blocks, Frontrangia and Uncompahgria. They may have stood around 3,000 m (10,000 ft) high.
They did not last. Rivers stripped them almost as fast as they rose and spread the debris across alluvial fans at their feet. That coarse, red, feldspar-rich gravel and sand hardened into the Fountain Formation, which takes its colour from the granites and gneisses it was eroded from. Deposited in the Late Pennsylvanian, around 300 million years ago, it now forms many of the best-known red rocks of the Front Range: the Flatirons above Boulder, Red Rocks Park at Morrison, Roxborough State Park and Garden of the Gods in Colorado Springs.
The Cretaceous Western Interior Seaway and the Dakota Hogback
Through the Mesozoic, new layers piled up over the worn-down stumps of the Ancestral Rockies. Red Triassic shales, Jurassic dune sands and the Morrison Formation covered them. Then the sea came back. From about 100 to 66 million years ago the Western Interior Seaway split North America in two, stretching from the Gulf of Mexico to the Arctic; at its largest it was about 970 km (600 mi) wide and up to 760 m (2,500 ft) deep.
The seaway left layer after layer across Colorado. When the sea level briefly fell, sand and pebbles became the Dakota Formation sandstone. Deeper water then deposited thousands of feet of mud that became the Mancos and Pierre shales, with the Niobrara limestone between them, which holds fossils of marine reptiles such as mosasaurs and plesiosaurs. Coastal swamps along the retreating shore left the coal-bearing Mesaverde Group. The hard Dakota sandstone matters most to the modern landscape: when the mountains were later pushed up, it was bent upward along their eastern edge and now caps the Dakota Hogback, the long ridge that forms the first line of foothills along the plains from southern Wyoming through Colorado into northern New Mexico. Interstate 25 runs parallel to it.
The Laramide orogeny, from about 80 million years ago: today’s Front Range and Sawatch
The present Rocky Mountains began to rise in the Late Cretaceous. Far to the west, the Farallon Plate was sliding beneath North America at an unusually shallow angle, and the drag it put on the underside of the continent pushed up blocks of Precambrian basement hundreds of kilometres inland. This mountain-building episode, the Laramide orogeny, began 80 to 70 million years ago and ended 55 to 35 million years ago, though the exact dates are disputed. The Front Range began rising about 68 million years ago.
The Laramide uplift explains the basic shape of Colorado. It raised the basement blocks that became the Front Range, the Sawatch Range and their neighbours; it tilted the sedimentary layers along their flanks into the hogbacks and upturned fins seen at Garden of the Gods and Red Rocks; and between the ranges it left high basins, the “parks”. North Park drains north to Wyoming via the North Platte, Middle Park feeds the Colorado River, and South Park is the headwater of the South Platte. The Continental Divide now follows the crest of these ranges, sending water west to the Pacific by way of the Colorado River and east to the Gulf of Mexico.
The San Juan volcanoes and the La Garita supereruption
After the Laramide squeeze ended, volcanoes took over in the southwest. Between about 35 and 40 million years ago, composite volcanoes built up across what are now the San Juan Mountains, part of a vast volcanic field that covered most of the southern Rockies during the middle Tertiary. From around 35 to 30 million years ago the eruptions turned violently explosive, and 15 to 18 volcanoes collapsed into calderas. Ash flows eventually covered about a third of the region.
The best known is La Garita Caldera, near today’s town of Creede. About 28 million years ago it produced the Fish Canyon Tuff, roughly 5,000 km³ (1,200 cu mi) of ash-flow deposits covering at least 28,000 km² (11,000 sq mi) to an average thickness of about 100 m (330 ft). It ranks among the largest known explosive eruptions in Earth’s history. The San Juans that hikers see today, some of the most rugged mountains in the state, are largely the eroded remnants of this volcanic pile. Farther east, beyond the Sangre de Cristo Range, the Spanish Peaks, West Spanish Peak at 4,155 m (13,631 ft) and East Spanish Peak at 3,867 m (12,688 ft), formed when magma intruded near the surface about 24.6 and 23.4 million years ago; the walls of igneous rock, called dikes, that radiate from them are among the best-known examples in the country.
Rifting and Miocene uplift: the San Luis Valley and Grand Mesa
Over the past 35 million years the crust of the western United States has also been stretching, and Colorado’s share of that extension is the Rio Grande Rift. Its northernmost basin, the San Luis Valley, dropped between the San Juan Mountains and the Sangre de Cristo Range. About 196 km (122 mi) long and 119 km (74 mi) wide, with an average elevation of 2,336 m (7,664 ft), it is often called the largest alpine valley in the world. The Rio Grande rises in the San Juans west of the valley and flows south across it into New Mexico.
At the same time, broad regional uplift during the Miocene lifted Colorado, Utah, New Mexico and Arizona to about 1,500 m (5,000 ft) above sea level, and rivers began cutting down hard. Grand Mesa, east of Grand Junction, shows how rock type controls the result. Between about 10.9 and 9.6 million years ago at least 27 lava flows spread a basalt cap more than 93 m (310 ft) thick over soft shale and sandstone. The Colorado and Gunnison rivers stripped away the unprotected rock around it, leaving a flat-topped mountain about 1,300 km² (500 sq mi) in area that stands roughly 1,800 m (6,000 ft) above the surrounding valleys at about 3,400 m (11,000 ft), often called the largest flat-topped mountain in the world.
The last 3 million years: the Gunnison cuts the Black Canyon
The Gunnison River set its course about 15 million years ago across soft volcanic debris from the West Elk Mountains and nearby ranges. When the land rose again 2 to 3 million years ago, the river cut straight through that cover into the 1.7-billion-year-old gneiss below. Trapped in its channel, it kept cutting at about 25 mm (1 in) every 100 years. The result is the Black Canyon, where the river drops an average of 6.4 m per km (34 ft per mile), steepening to 45 m per km (240 ft per mile) at Chasm View, and the gorge narrows to 12 m (40 ft) at the bottom.
Pleistocene ice and wind: cirques in Rocky Mountain National Park and the Great Sand Dunes
The last touches came in the ice ages of the Pleistocene. Glaciers formed high in the ranges, carving out valleys and cirques. In Rocky Mountain National Park, where elevations run from 2,396 m to 4,346 m (7,860 to 14,259 ft) at the top of Longs Peak, ice and streams removed up to 1,500 m (5,000 ft) of sedimentary rock from the old seabed and exposed the basement beneath. Small glaciers still survive there, including Andrews, Sprague, Tyndall and Taylor, and the east side of the Continental Divide, which is drier, is marked by heavily glaciated peaks and cirques.
Meanwhile, in the San Luis Valley, sand washed down from the surrounding mountains was left exposed as lakes on the valley floor shrank. Prevailing southwest winds blew it toward the Sangre de Cristo Range, where it piled up over an estimated tens of thousands of years into the tallest dunes in North America, up to 230 m (750 ft) high, covering about 78 km² (30 sq mi).
First protected as a national monument in 1932, the dunes became Great Sand Dunes National Park and Preserve in 2004. They hold an estimated 5 billion m³ or more of sand. For a peak-by-peak look at the ranges this history produced, read our guide to the mountains in Colorado, and for the red-rock country that continues west into Utah and Arizona, see the Colorado Plateau.