Paleodeserts

Data on ancient sand seas (vast regions of sand dunes), changing lake basins, archaeology, and vegetation analyses indicate that climatic conditions have changed considerably over vast areas of the Earth in the recent geologic past. During the last 12,500 years, for example, parts of the deserts were more arid than they are today. About 10 percent of the land between 30 degrees north and 30 degrees south is now covered by sand seas. Nearly 18,000 years ago, sand seas in two vast belts occupied almost 50 percent of this land area. As is the case today, tropical rainforests and savannas were between the two belts.

Fossil desert sediments that are as much as 500 million years old have been found in many parts of the world. Sand dune–like patterns have been recognized in presently nonarid environments. Many such relict dunes now receive from 3.15 to 5.9 inches (80 to 150 millimeters) of rain each year. Some ancient dunes are in areas now occupied by tropical rainforests. One of these is the Nebraska Sand Hills, which is an inactive 22,008-square-mile (57,000-square-kilometer) dune field in central Nebraska. The largest sand sea in the Western Hemisphere, it is now stabilized by vegetation and receives about 19.69 inches (500 millimeters) of rain each year. Dunes in the Sand Hills are up to 394 feet (120 meters) high.

Colorado River Basin

The Grand Canyon is a huge rift in the Colorado Plateau that exposes uplifted Proterozoic and Paleozoic strata, and is also one of the 19 distinct physiographic sections of the Colorado Plateau province. It is not the deepest canyon in the world (Kali Gandaki Gorge in Nepal is far deeper), nor the widest (Capertee Valley in Australia is about 0.6 mile, or 1 kilometer wider, and longer than Grand Canyon); however, the Grand Canyon is known for its visually overwhelming size and its intricate and colorful landscape. Geologically, it is significant because of the thick sequence of ancient rocks that are beautifully preserved and exposed in the walls of the canyon. These rock layers record much of the early geologic history of the North American continent.

Uplift associated with mountain formation later moved these sediments thousands of feet upward and created the Colorado Plateau. The higher elevation has also resulted in greater precipitation in the Colorado River drainage area, but not enough to change the Grand Canyon area from semi-arid. The uplift of the Colorado Plateau is uneven, and the Kaibab Plateau that the Grand Canyon bisects is over 1,000 feet (300 meters) higher at the North Rim than at the South Rim. Temperatures on the North Rim are generally lower than the South Rim because of the greater elevation (averaging 8,000 feet, or 2,438 meters, above sea level). Heavy rains are common on both rims during the summer months.

The Colorado River basin (of which the Grand Canyon is a part) has developed in the past 40 million years. A recent study places the origins of the canyon at some 17 million years ago. Previous estimates had placed the age of the canyon at 5 to 6 million years. The result of all this erosion taking place through the years is one of the most complete geologic columns on the planet. The major geologic exposures in the Grand Canyon range in age from the 2-billion-year-old Vishnu Schist at the bottom of the Inner Gorge to the 230-million-year-old Kaibab Limestone on the rim. There is a gap of about 1 billion years between the stratum that is about 500 million years old and the lower level, which is about 1.5 billion years old. This large unconformity indicates a period of erosion between two periods of deposition.

Many of the formations were deposited in warm, shallow seas, nearshore environments (such as beaches), and swamps as the seashore repeatedly advanced and retreated over the edge of a proto–North America. Major exceptions include the Permian Coconino Sandstone, which contains abundant geological evidence of aeolian sand dune deposition. Several parts of the Supai Group were also deposited in nonmarine environments. The great depth of the Grand Canyon, and especially the height of its strata (most of which formed below sea level) can be attributed to 5,000 to 10,000 feet (1,500 to 3,000 meters) of uplift of the Colorado Plateau, starting about 65 million years ago (during the Laramide Orogeny). This uplift has steepened the stream gradient of the Colorado River and its tributaries, which in turn has increased their speed, and thus their ability to cut through rock. Weather conditions during the ice ages also increased the amount of water in the Colorado River drainage system. The ancestral Colorado River responded by cutting its channel faster and deeper.

The base level and course of the Colorado River (or its ancestral equivalent) changed 5.3 million years ago when the Gulf of California opened and lowered the river's base level (its lowest point). This increased the rate of erosion and cut nearly all of the Grand Canyon's current depth by 1.2 million years ago. The terraced walls of the canyon were created by differential erosion. Between 3 million and 100,000 years ago, volcanic activity deposited ash and lava over the area, which at times completely obstructed the river. These volcanic rocks are the youngest in the canyon. The ancient Pueblo people were a Native American culture centered on the present-day Four Corners area of the United States. The ancient Pueblos were the first people to live in the Grand Canyon area. There are approximately 1,737 known species of vascular plants, 167 species of fungi, 64 species of moss, and 195 species of lichen found in.

The Grand Canyon boasts a dozen endemic plants (known only within the park's boundaries), whereas only 10 percent of the park's flora is exotic. A total of 63 plants found here have been given special status by the U.S. Fish and Wildlife Service. Of the 34 mammal species found along the Colorado River corridor, 15 are rodents and eight are bats.

Sahara Desert

Some 12,000 years ago, the only place to live along the eastern Sahara Desert was the Nile Valley. It was so crowded that prime real estate in the Nile Valley was difficult to find. Disputes over land were often settled with the fist, as evidenced by the cemetery of Jebel Sahaba, where many of the buried individuals had died a violent death. But around 10,500 years ago, a sudden burst of monsoon rains over the vast desert transformed the region into habitable land. This opened the door for humans to move into the area, as evidenced by 500 radiocarbon dates of human and animal remains from more than 150 excavation sites. “The climate change at [10,500 years ago] which turned most of the [3.8-million-square-mile] large Sahara into a savanna-type environment happened within a few hundred years only, certainly within less than 500 years,” said study team member Stefan Kroepelin of the University of Cologne in Germany.

In the Egyptian Sahara, semi-arid conditions allowed for grasses and shrubs to grow, with some trees sprouting in valleys and near groundwater sources. The vegetation and small, episodic rain pools also enticed animals well adapted to dry conditions, such as giraffes, to enter the area. Humans also frolicked in the rain pools, as depicted in rock art from southwest Egypt. In the more southern Sudanese Sahara, lush vegetation, hearty trees, and permanent freshwater lakes persisted over millennia. There were even large rivers, such as the Wadi Howar, once the largest tributary to the Nile from the Sahara. Wildlife included species such as elephants, rhinos, hippos, crocodiles, and more than 30 species of fish up to 6.5 feet (2 meters) long. A brief history of the Sahara Desert follows:

• • 22,000 to 10,500 years ago: The Sahara was devoid of any human occupation outside the Nile Valley, and extended 250 miles further south than it does today.
• • 10,500 to 9,000 years ago: Monsoon rains begin sweeping into the Sahara, transforming the region into a habitable area that was swiftly settled by Nile Valley dwellers.
• • 9,000 to 7,300 years ago: Continued rains, vegetation growth, and animal migrations lead to well-established human settlements, including the introduction of domesticated livestock such as sheep and goats.
• • 7,300 to 5,500 years ago: Retreating monsoonal rains initiate desiccation in the Egyptian Sahara, prompting humans to move to remaining habitable niches in Sudanese Sahara. The end of the rains and return of desert conditions throughout the Sahara after 5,500 years ago coincides with population return to the Nile Valley and the beginning of pharaonic society.

At the end of the last Ice Age, the Sahara Desert was just as dry and uninviting as it is today. However, sandwiched between two periods of extreme dryness were a few millennia of plentiful rainfall and lush vegetation. During these few thousand years, prehistoric humans left the congested Nile Valley and established settlements around rain pools, green valleys, and rivers.