A Geologic Wonder
The Colorado Plateau is not just one plateau, but rather a huge area filled with stacked plateaus, surrounded by highlands to the north and lowlands to the south and west. Relatively stable, the roughly circular Colorado Plateau nevertheless experienced uplift, folding, faulting, and fracturing as a result of the violent Tertiary-period geologic events that formed the surrounding mountains. The oldest rocks on the Plateau are more than 570 million years old. During the North American continent’s turbulent detachment from Africa, Asia, and South America 300 to 400 million years ago and subsequent slow drift to its present location, the physiographic province that would later be known as the Colorado Plateau remained relatively undisturbed.
Over those hundreds of million years, warm shallow seas alternately inundated and receded from the Colorado Plateau, and large quantities of sediment accumulated. The layers gradually sank under their own weight until heat and pressure hardened them into layers of sandstone, siltstone, shale, and limestone, in places several kilometers thick.
Then, between 80 and 40 million years ago, a violent uplift formed the Rocky Mountains, rainfall levels rose, and the Colorado River was born. The extraordinary mountain-building forces are reflected in the Colorado Plateau in its domes, shallow basins, fractures, faults, and long folds called “reefs”. Finally, around 10 million years ago, the entire western United States began to rise again as a result of tectonic compressional stresses; some portions eventually rose to altitudes of 5 km above sea level. The Colorado Plateau rose also — by more than 1.5 km— but remained structurally stable, perhaps “riding” a protective cushion of magma. Following this uplift, rivers began to carve deep, narrow canyons into the Plateau’s multi-layered rock. Today, even as the Plateau continues slowly and gently rising, erosion is simultaneously wearing it down.
There is plentiful evidence of Cenozoic volcanic and magmatic activity within the Plateau, including intrusions that have caused upwarps, tilting, and doming of strata, and extrusions in the form of volcanic landforms and lava-capped mountains. There are five major mountain ranges within the Plateau, all formed by the action of magma that never breached the surface.
Within the Plateau, rivers have incised thousands of kilometers of canyons, whose walls bear additional erosional features, such as alcoves, grottoes, windows, towers, and honeycombs. Many of these canyons and gorges are remarkably deep and narrow, with ever-narrower side canyons branching off from main passages. Labyrinths of canyons pattern the Plateau. For example, the Escalante River Canyon and its side canyons constitute a network of nearly 1,600 km. Yet the Escalante is itself only a side canyon — one of 50 major side canyon systems that are tributaries of the Colorado and Green Rivers.
Shelly Fischman and Jim Rolfes
Though wind and frost erosion contribute to the creation of some Plateau landforms, direct erosion by water (in rain, groundwater, runoff and rivers) is the primary sculpting force for most of the unusual shapes. Generally, the odd forms result from differential erosion, which occurs when there are significant differences in relative hardness or cementation especially where fracturing has occurred. For example, in the Plateau, fractured sandstone is more susceptible to erosion by water than is massive limestone, which often comprises erosion-resistant caprock.
In addition to the thousands of mesas, buttes, domes, towers, monuments, temples, and spires carved by water, there is also a multitude of natural rock arches and bridges in the Plateau, where wind (aeolian erosion) and ice have been contributory sculptors. At least five bridges have spans of more than 60 m.
Surface features also reveal the Plateau’s evolution. There are beautifully colored, lithified sand dunes, ripple marks frozen in rock, and whole forests of petrified trees. The area boasts hundreds of classic examples of geologic phenomena, often decorated in the brilliant reds, purples, and oranges of iron and other minerals.