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sea level. The strata were also compressed, leading to the formation of the East Kaibab Monocline, the arching of the rock layers into an elongate dome in the central Grand Canyon region. This episode of uplift was completed by 40 million years ago.
The Colorado Plateau, although raised far above sea level, remained lower than the surrounding mountain ranges. During this period, rivers in the Grand Canyon region flowed to the northeast, as sediment and water were transported out of the Mogollon Highlands, into giant lakes in northern Arizona and Utah. This drainage pattern persisted until at least 30 million years ago and possibly as recently as 18 million years ago.
Sometime between 30 and 18 million years ago drainage patterns may have begun to change, but very few details are known. The Colorado River system did not exist. Some water may have drained southward, but most stream systems still headed north. Much water may have drained into internal basins that did not connect to the ocean.
WHY ARE THE RIMS DIFFERENT ELEVATIONS—AND DOES IT MATTER?
Both the South Rim and North Rim sit atop the Kaibab Formation. Although the layers appear flat from either vantage point, they are slightly slanted, and the North Rim is approximately 1000 feet higher than the South Rim, for these layers are bent into a broad arch, the East Kaibab Monocline. The apex of the arch is 12 miles north of the North Rim. The southern side of the arch slants gently downward from that point and drops 1000 feet between the North and South rims.
This difference in elevation has other implications: The angle of the rock layers means that water falling on the rims flows away from the South Rim, but is funneled toward the North Rim. Therefore, relatively little water flows down creeks from the South Rim, but considerable amounts flow down drainages beginning on the North Rim. As a result, less side-stream erosion has occurred on the south side of the canyon and the descent from the canyon rim to the Colorado River is much shorter and steeper than from the North Rim. The landscape is accordingly less complex; there are far more buttes and intricate side canyons on the north side of the river.
Extension to the west, 18 million to 6 million years ago: During this time period, stretching and thinning of western North America caused the Basin and Range Province in Nevada to form and the land surrounding the Colorado Plateau to decrease in elevation. Once the regions to the south and west were lower than the plateau, runoff began to etch south-flowing drainages, the direction of today’s Colorado River.
However, sediment deposits suggest that the Colorado River itself did not exist until approximately 6 million years ago. Instead, there were still large inland lakes and shorter waterways that were not connected. Water likely flowed along some of the Colorado River’s present path and through some of its major tributaries, although possibly opposite from the direction it does today. For instance, water may have flowed down the Little Colorado and then up Marble Canyon, but not through the main Inner Gorge of the Grand Canyon to the west of the confluence of these two drainages.
What caused these many rivers to coalesce into a single large drainage system is not definitively known, but two theories rise to the forefront. One is that the inland lakes suddenly joined together, possibly because of a catastrophic overflow of one of them, providing the force to cut through topographic barriers and integrate previously disconnected drainage systems. A second theory is headward erosion, whereby erosion causes waterways to move progressively upstream, cutting into slopes. Eventually this process destroys a ridge that had previously divided two drainages and connects the two stream systems. A combination of these two processes likely integrated the Colorado River system.
The final big event required for the creation of the modern Colorado River course is the creation of the Gulf of California, which occurred about 6 million years ago, and provided the Colorado River’s outlet to the ocean.
Glaciations and fault movements, 6 million years ago to present: By 6 million years ago, the Colorado River followed its current course to the Gulf of California. Events during the past 2 million years contributed to significant downward erosion, allowing the canyon to achieve its depth. First, the past 2 million years have been a time of recurring glaciations and at the end of each glacial cycle, greatly increased runoff from the Rocky Mountains would have resulted in giant floods with enormous erosive power.
Second, the Colorado River is crosscut by multiple faults, where the side of the fault that descends coincides with the downhill side of the river. Each time the fault moved, the downhill side of streambed would suddenly have been lowered, creating a step termed a knickpoint. A river’s erosive power slowly moves knickpoints upstream, in the process deepening the riverbed.
A second geologic event also created large and probably temporary knickpoints: There were large lava flows that dammed the Colorado River approximately 640,000 years ago. Although probably short-lived, these dams would have allowed enormous reservoirs to form along the Colorado River’s course. When the dams broke or were eroded upstream, the water stored behind the dam would have been instantaneously released, providing massive erosive force.
AN EVOLUTIONARY STORY
The physical conditions that create the rock strata you traverse are repeated again and again, creating multiple layers of limestone, shale, or sandstone. However, the Grand Canyon’s sedimentary strata were deposited over 500 million years, and the fossils in successive strata record much of the evolutionary history of life.
At the time the Bass Limestone was deposited, the only lifeform was single-celled colonial bacteria called stromatolites, visible in the rock as wavy bands. Multicellular, shell-bearing, aquatic animals evolved by the start of the Cambrian era, 542 million years ago; the Bright Angel Shale contains abundant trilobites, an early shell-bearing creature. Worm burrows, termed trace fossils, are also abundant. By the time the Redwall Limestone was deposited, different invertebrates dominated the seas, and crinoids (a stalklike relative of starfish) and brachiopods are abundant in this layer. Unlike at the time of the Bright Angel Shale’s deposition, plants and animals now colonized the land. Along the South Kaibab Trail, plant fossils are visible in the Hermit Formation; some specimens of ferns are on display on the west side of Cedar Ridge for all to observe. The Coconino Sandstone preserves reptile footprints. Stop and consider that these critters didn’t exist when you are just a couple of miles farther down the trail.
VEGETATION
Most hikers will consider their walk from the Grand Canyon’s rim to the Colorado River—and back up again—to be simply “desert.” Spring, summer, and fall temperatures are hot, the humidity is low, and the vegetation is sparse and prickly. However, you will in fact pass through four vegetation zones, each existing due to a specific combination of elevation, moisture availability, and temperature and each dominated by specific species. They are:
Pinyon-juniper woodland: Pinyon pine and juniper are the dominant species in this widespread southwestern plant community. At the Grand Canyon, this community exists on the South Rim and inside the canyon down to the Redwall Limestone. Ecologically, it demarcates the elevation that receives significant snow during the winter months. (For the first 1000 feet below the rim on the Bright Angel Trail, this community is interspersed with the mountain scrub and chaparral community, composed of species including Gambel oak, fernbush, serviceberry, and snowberry.)
Blackbrush scrub: This community is defined by the dominance of blackbrush and covers the Tonto Platform. Unless you head toward Plateau Point, the characteristic monostands of blackbrush are not observed along the Bright Angel Trail, since the trail descends along the Garden Creek drainage, first down the dry wash below Jacobs Ladder and then through riparian (streamside) vegetation below Indian Garden. On the South Kaibab Trail, you are in blackbrush scrub as you cross the Tonto Platform and approach the Tipoff.
Mojave Desert scrub: This community exists below the Tonto Platform down to the Colorado River. The species growing here can withstand extremely high summer temperatures and many require milder winters. Although it is informally termed Mojave Desert scrub, the plant community contains many species not present in the Mojave Desert of southeastern California. For instance, species such as brittlebush and catclaw acacia that are typical of the warmer Sonoran