Geology of the Grand Canyon
A professor of geology at Illinois Wesleyan, John Wesley Powell had more than just a passing interest in the layers upon layers of rocks exposed by the Colorado River. The expedition down the Green and Colorado Rivers, he would later claim, "was not made for adventure, but purely for scientific purposes, geographic and geologic." And indeed, he did devote a great deal of time to these purposes. In the afternoon, while his crew set up camp for the night, Powell would often climb into the canyon to investigate.
Much has been learned about the geology of the Grand Canyon and about the history of life on earth since Powell and his crew made their expedition in 1869. "Scroll Down through History" displays the layers of rock exposed within the Grand Canyon and shows when in earth's history they formed.
Scroll Down through History
mya: million years ago
Years before present
50-60 mya
The region where the Grand Canyon now lies begins to shift and rise, creating fault lines. Evidence of this activity is still visible today within the canyon and on the plateaus extending beyond the canyon.
230-280
A shallow sea advances and retreats several times during this period, leaving behind the layers named Kaibab Limestone, Toroweap Formation, and Hermit Shale. The Coconino Sandstone layer was deposited not by the sea, but by wind, which blew in sand across the region.
Fossils found in these layers include fern-like leaves, tracks left by reptiles, and numerous forms of marine life.
[Permian]
280-310
The Supai Group, consisting of deep-red siltstone, limestone, and sandstone, is 600 to 700 ft thick. Within these layers are marine fossils, including brachiopods, corals, crinoids, and gastropods. In some locations there are land plants and reptile tracks.
[Pennsylvanian]
310-345
This layer of rock, known as the Redwall Limestone, forms a sheer cliff 500 ft high or more in most areas. The limestone gets its color from the red siltstones above, which wash over the rock. Digging into the rock reveals that its color is actually bluish-gray. The limestone contains the fossilized remains of fish, mollusks, trilobites, crinoids, and corals.
[Mississippian period]
345-395
At some time during the Devonian period, a shallow sea deposited layers of white and gray dolomite.
[Devonian period]
500-570
These layers of sedimentary rock were deposited in Cambrian time, when another shallow sea covered this region. Among the fossilized remains found within these layers are brachiopods, trilobites, seaweed, and sponges.
[Cambrian period]
1,000 mya
Known as the Grand Canyon Supergroup, these layers of rock lie at an angle relative to the layers of rock above.
The layer labeled Bass Limestone contains the fossilized remains of algae.
[younger Precambrian era]
1,000 - 2,000
At the very bottom of the canyon lies the Vishnu Schist, a hard rock originally deposited mainly as sediments some 2 billion years ago. The layer was subsequently covered. Around 1.7 billion years ago, by then deep underground, the layer was transformed into schist (a hard metamorphic rock) through heat and pressure.
[older Precambrian era]
Sources:
"Geology of the Grand Canyon," edited by William J. Breed and Evelyn Roat
"Oasis in Space," by Preston Cloud
"Pages of Stone," by Halka Chronic
"Encyclopaedia Britannica"
"The Great Geographical Atlas," published by Rand McNally
