The discovery of the geologic time scale adds a new dimension to hiking in the national parks of the south-western United States. By correlating strata from the Grand Canyon region with strata from the Painted Desert, Petrified Forest, Bryce Canyon, and Zion Canyon National Parks, geologists have constructed a stratigraphic column that represents a partial record of Earth history beginning in the Proterozoic era and continuing into the Eocene epoch. These rocks have been exposed by erosion that began about 10 million years ago when a large block of land, encompassing parts of Arizona, New Mexico, Colorado, and Utah uplifted to form a broad highland called Colorado Plateau.

When you walk down the trail from the Grand Canyon’s rim to the river on the canyon floor, you walk through a 1.5-km-thick column of strata. Geologists have subdivided the strata into several geological formations, and have interpreted the environment in which each formation developed by looking at their rock types, fossils, and sedimentary structures. By the principle of superposition, we know that the youngest rocks in the canyon occur at the rim, while the oldest rocks are found at the base, so walking down the trail from the rim to the river is like walking back in time.

At the rim of the canyon, you stand on the Kaibab Limestone, remnants of ancient reefs deposited 250 million years ago when the region was not the high plateau it is today by a balmy, shallow sea, much like the one surrounding the modern Bahamas. Along the trail, you cross the cliff-forming CocoNiño Sandstone, which contains gigantic cross beds that formed in desert sand dunes. Large reptiles plodded across these dunes 270 million years ago,leaving footprints in the loose sand that we can see today, preserved as fossils in solid rock. Beneath the dunes, the rocks record a wide floodplain on which rivers deposited mud and silt, littered with fragments of ferns. These deposits compromise the 280-million-year-old Hermit Shale and the 300-million-year-old Supai Formation. Beneath the Supai, you enter the 350-million-year-old Redwall Limestone, deposited when the sea flooded the region. Farther down the trail, beds of 500-million-year-old Temple Butte Limestone (containing fossils of strange armored fish) and Bright Angel Shale (containing fossils of trilobites and other shelled invertebrates) crop out. On the shelf overlooking the inner gorge, you stand on the Tapeats Sandstone, which includes the deposits of 505-million-year-old beaches.

So far, all the beds we have crossed roughly horizontal. Along the lip of the inner gorge, we find more sedimentary rocks, called the Unkar Group (a group consists of several formations lumped together). These beds tilt at about 20-30, and they contain no fossils of shelled organisms, only the vague imprints of soft-bodied invertebrates. Based on correlation with similar units elsewhere, these rock layers probably represent deposition in an 800-million-year-old rift. Underneath lie black schists (the Vishnu Schist) and pink granite (the Zoroaster Granite), metamorphic and igneous rocks that contain no fossils at all, but have yielded radiometric dates of about 1.6 billion years. The existence of these rocks means that a volcanic mountain range once existed at the site of the Grand Canyon; rocks were buried deeply and metamorphosed, then were intruded by large volumes of magma beneath this range. Later, many kilometers of rock were removed, exposing the Visnu Schist at the surface.

Is the record of Earth history visible on the walls of the Grand Canyon complete? No. The nonconformity between the Vishnu Schist and the Unkar Group represents about 800 million years of missing time, while the angular unconformity between the Unkar Group and the Tapeats Sandstone represents almost 300 million years. In fact, the column of flat-lying rocks above the Unkar Group records less than 25% of time between the end of the Precambrian time and the beginning of the Mesozoic era. Further major disconformities occur at the base of the Redwall and the base of the Supai.

The strata that once lay directly above the Kaibab Limestone have been eroded from the Grand Canyon but still crop out in the Painted Desert and the Petrified Forest, 150 km to the east. These strata appear as vivid stripes of red, cream, gray, and green shale across the desert. They contain the petrified remains of 60-m-high conifer trees and the footprints of early (220-million-year-old) dinosaurs. Cross-bedded sandstones, remnants of giant dune fields that, 200 million years ago, made the region look like a huge Sahara Desert, comprise the Navajo Sandstone, which now form the spectacular white and orange cliffs of Zion Canyon. Above the sandstone, a several-kilometer-thick layer of gray shale records the time between 130 million and 70 million years ago when a shallow sea stretched across the interior of North America from the Arctic to the Gulf of Mexico. This shale contains abundant fossil shells, including the cone-shaped shell of the nautilus, a squid-like creature. Still younger sandstones and shale form the muticolored walls of Bryce Canyon and Cedar Breaks National Parks. These strata were deposited in a huge lake between 80 million and 40 million years ago.

What did we learn on our journey through time? Conditions we find at a particular location today were not necessarily the same throughout Earth history. The high and dry region of the Grand Canyon today was sometimes a shallow sea, sometimes a river floodplain, sometimes a sandy desert, and sometimes a rugged mountain range. Such changes reflect the activity of plate tectonics, the global rise and fall of the sea level and climate changes. And while all these changes take place on the Earth's surface, life forms evolve.

Other Feature Articles in this chapter:  1  :  2

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