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Colorado Plateau: The region of plateaus, mesas, and canyons encompassing much of Colorado, Utah, Arizona, and New Mexico, and eight national parks—Arches, Bryce Canyon, Canyonlands, Capitol Reef, Grand Canyon, Mesa Verde, Petrified Forest, and Zion)—is known as the Colorado Plateau.

The Continental Divide: The Continental Divide of North America is a ridge that crosses the continent from north to south, separating water flowing east and water flowing west. The divide follows the crest of the Rockies through Glacier, Yellowstone, and Rocky Mountain national parks.

Desert Pavement: High winds can scour away sand, silt, and other small particles of soil, leaving behind a densely packed layer of coarse pebbles and gravel known as desert pavement. You can see it in arid areas such as the plains of Big Bend National Park.

Desert Varnish: This reddish-brown or black coating seems to drip down canyon walls as if from a spilled can of paint. Windblown dust or rain containing iron and manganese, along with microorganisms living on the rock’s surface, create the color. Ancestral Puebloans and other ancient American Indians scratched drawings called petroglyphs into the desert varnish.

Fault Zones: Shifting underground land masses such as those of California’s famous San Andreas Fault have helped create many of Joshua Tree National Park’s upper elevations, including the Pinto Mountains on the northern border and the Little San Bernardino Mountains on the southwest side. This same geologic upheaval has shifted and cracked underground rocks, damming up the flow of groundwater and forcing it to the surface, making precious moisture available to wildlife and plants.

Fossil Reef: Much of the desert region of Texas and New Mexico shares a common geologic past, when a warm, shallow inland sea covered a large area. As the sea evaporated, a nearly 400-mi-long limestone reef was deposited, made up of sediment, dead plants, and the skeletons of tiny sea creatures. Movement in the Earth’s surface later thrust the reef upward, helping to shape the Guadalupe Mountains. One of the largest and most visible examples of fossil reef is Capitan Reef in Guadalupe Mountains National Park, which attracts geologists from around the world.

Geothermal Features: Hissing geysers, burbling mud pots, and boiling cauldrons have fascinated travelers ever since mountain man John Coulter described them to an unbelieving public in 1810. Today, these "freaks of a fiery nature," as Rudyard Kipling described them, are still clearly visible in Lassen Volcanic and Yellowstone national parks. Geysers, hot springs, and fumaroles are created when superheated water rises to the earth’s surface from a magma chamber below. In the case of geysers, the water is trapped under the surface until the pressure is so great that it bursts through. Mud pots, also known as paint pots, are a combination of hot water, hydrogen sulfide gas, and dissolved volcanic rock. The mixture looks like a hot, smelly, burping pudding.

Glaciers: Heavy snow compacted by centuries of accumulation forms the distinctive blue, dense ice of a glacier. Incremental movement, usually inches a year, is what distinguishes a glacier from a snowfield; most of the glaciers in the contiguous United States are in Washington and Oregon, and most are receding due to global warming.

Hot Springs: Past the eastern escarpment of the Sierra, evidence of the region’s volcanic history remains in the giant calderas of the high-mountain desert. In the area west of Death Valley National Park, countless hot springs issue from the earth and form small to medium-size pools in the ground, some of which are suitable for bathing. If you should stumble upon one, never jump in without first testing the water’s temperature. The locals keep many of the springs secret, but in some places people have piped the hot waters into developed pools. While driving on U.S. 395, look for road signs pointing the way to numerous hot springs.

Laccolithic Mountains: Throughout the Colorado