The Yellowstone-Absaroka region of northwestern Wyoming is a distinctive subdivision of the Middle Rockies. A large magma chamber beneath the area has filled several times and caused the surface to bulge, only to then empty in a series of volcanic eruptions of basaltic and rhyolitic lava and ash.
Three such cycles have occurred in the past two million years, the most recent of which occurred about , years ago. The magma chamber is currently filling again, and the land surface in Yellowstone is rising or tilting a slight amount each year. The eastern and western ranges are separated by a series of high basins: from north to south they are North Park, the Arkansas River valley, and the San Luis Valley.
Only about 5, feet of sediment accumulated during middle Mesozoic times about to million years ago in the region now occupied by the Southern Rockies. Mountain building in these ranges resulted from compressional folding and high-angle faulting during the Laramide Orogeny, as the Mesozoic sedimentary rocks were arched upward over a massive batholith of crystalline rock.
Some 10, vertical feet of the sedimentary rocks were then eroded; otherwise the Front Range would be approximately twice its present height. The Southern Rockies experienced less of the low-angle thrust-faulting that characterizes the Canadian and Northern Rockies and the western portions of the Middle Rockies. The ranges of the Southern Rockies are higher than those of the Middle or Northern Rockies, with many peaks exceeding elevations of 14, feet. Colorado has 53 peaks over this elevation, the highest being Mount Elbert in the Sawatch Range , which at 14, feet 4, metres is the highest point in the Rockies.
These ranges were heavily eroded by several episodes of glaciation—the most recent ended about 7, years ago, and no active glaciers remain—resulting in spectacular alpine scenery. River valleys have been deepened in the past two million years, first from the direct action of glacier ice and subsequently by glacial meltwaters. Looping, knife-edged moraines occur in most valleys, marking the downslope extent of past glaciations.
The physiographic province called the Colorado Plateau in southeastern Utah , southwestern Colorado, northern Arizona, and northwestern New Mexico is another high-elevation region of the western United States , although it lacks the history of folding, faulting, and volcanic activity of adjacent regions.
The uplifts in the Colorado Plateau are not as great as those elsewhere in the Rockies, and therefore less erosion has occurred; Precambrian rocks have been exposed only in the deepest canyons, such as the Grand Canyon. The plateau is actually a series of plateaus at different elevations arranged in a stairstep sequence through faulting.
The horizontal sedimentary rocks have been dissected by the Green and Colorado rivers and their tributaries into a network of deep canyons. Some of these canyons are deeply entrenched meanders, such as the dramatic Goosenecks section of the San Juan River near Mexican Hat, Utah, where erosion through the canyon walls separating opposite sides of a meandering river loop has created a natural bridge. The Grand Canyon of the Colorado River cuts across the southern end of the Kaibab Upwarp in the southern plateau region.
The canyon is up to 6, feet 2, metres deep and exposes a remarkable sequence of sedimentary rocks. Weak rock types, such as shale and softer sandstone layers, form low-sloping benches, while more resistant rock types, such as limestone and harder sandstone layers, comprise cliff-forming units. Because of the alternating sequence of weak and resistant rocks in the canyon walls, a cliff-and-bench topography has formed that is typical of much of the Colorado Plateau region.
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The headward erosion of streams into the plateau surface eventually isolates sections of the plateau into mesas, buttes, monuments, and spires. Bedrock that has been fractured into series of parallel joints can weather into high rock walls known as fins. Subsequent weathering leads to the creation of natural arches. The same weathering processes on cliffs can create niches , which have been exploited by cliff-dwelling Native American cultures in the past. These domes are called laccoliths , and each of these mountain massifs is made up of a group of laccoliths.
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Thank you for your feedback. The lowest point of the state, located in Yuma County, has an elevation of 3, ft.
Phoenix is the capital of Arizona. The state borders five other US states as well as Mexico. Arizona is known for its varied landscape, which features deserts, canyons including the Grand Canyon , forests, and mountain ranges like the San Francisco Mountains. Arizona has a mean elevation of 4, ft, its highest point elevation is 12, ft tall Humphreys Peak, and its lowest point is 72 ft.
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The state of Utah is located in western part of the country and shares its borders with six other US states. Utah covers an area of , square km, has an estimated population of 3,,, and Salt Lake City serves as the state capital. The state has a mean elevation of 6, ft, which ranks third in the US. Utah is known for its varied terrain and rich biodiversity. The state of Nevada borders five other US states, encompasses an area of , square km, and has an estimated population of 2,, Nevada has a mean elevation of 5, ft.
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The 13, ft high Boundary Peak is the highest point in Nevada, while its lowest point is ft above sea level. New Mexico has an area of , square km, an estimated population of 2,,, and the state capital is Santa Fe. The state is fifth largest in the country by area, and borders five other US states and two Mexican states.
The landscape of the state varies widely, and includes deserts, mesas, as well as snow-capped peaks. New Mexico has the fourth highest mean elevation of the US states.
Idaho has an area of , square km, an estimated population of 1,,, and Boise is the state capital. The state borders six US states, as well as Canada.
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The state is relatively mountainous, and includes several ranges within the Rocky Mountains. Idaho has a mean elevation of 5, ft, its highest point is 12, ft Borah Peak , and its lowest point is ft above sea level. Montana occupies an area of , square km, has an estimated population of 1,,, and Helena is the state capital. As one of the eight Mountain States, Montana has numerous mountain ranges in the western part of the state, as well as several island range throughout. The state has a mean elevation of 3, ft, its highest point is 12, ft Granite Peak , and its lowest point is 1, ft above sea-level.
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