Glen Canyon area rocks were laid down during the Mesozoic Era, a period of time that followed the interval when all Grand Canyon rock units were created. The Mesozoic Era lasted about 170 million years, from 251 to 65 million years ago. Some intriguing and fundamental facts about Glen Canyon’s Mesozoic rock sequence should be noted.
Earth’s fossil record reveals five mass extinction events. Because earth’s life was extremely primitive prior to 540 million years ago, all five extinctions occurred after the Cambrian Explosion of Life that marks the beginning of the Paleozoic (“early life”) Era.
The Mesozoic Period (Mesozoic means “middle life”) began with the most destructive of the five mass extinctions, an event so devastating that it was termed the Great Dying by early geologists.
Ninety percent of the planet’s plant and animal species perished. It was the third time in earth’s history that the extinction rate spiked elevated far above the norm.
Unfortunately, the exact cause is still unknown the most likely explanation suggests that several unusual circumstances, each one detrimental to life, occurred nearly simultaneously.
The combination came close to being a knockout punch. Those unfavorable circumstances included a dramatic drop in sea level, a change in atmospheric gas composition (coming from large and long-lasting volcanic eruptions in Siberia), and the loss of productive ocean habitat as the supercontinent Pangaea assembled from earlier continents.
The Great Dying, sometimes also known as the end-Permian or P-T Extinction, brought an end to the Paleozoic Period, the period of time responsible for the creation of the upper portions and most prominent Grand Canyon rock layers.
During the Paleozoic Era, the region that would eventually become the Colorado Plateau often subsided to below sea level positions to become the ocean floor.
Limestone, a rock type highly resistant to erosion in dry climates, accumulated to great depth with little interruption in this environment as is evident in the repeated layers of limestone found in the walls of Grand Canyon.
However, in the succeeding Mesozoic Period, after the Great Dying, the future Colorado Plateau region rested at or a little above sea level much of the time. Aeolian sandstones (wind- rather than water-deposited sandstones) piled on top of the older limestones.
Today we know the most important of the three aeolian beds as the Wingate, Navajo, and Entrada Sandstones. All of them are expressed as prominent cliffs in the Lake Powell/Glen Canyon area.
The change in the type of rock being laid down from Paleozoic to Mesozoic times is made manifest quite dramatically in the local landscape.
The Kaibab Formation, the rimrock of Grand Canyon, is the last of the Paleozoic rock units to have been emplaced. It can be spotted just down-river from the launch ramp at Lees Ferry. Upon it rests the Moenkopi Formation, the first of the Mesozoic rock units to be laid down.
While the Kaibab Formation is mostly a limestone extremely resistant to erosion in today’s dry climate, the overlying Moenkopi, mostly a shale layer, rapidly disintegrates. The result is this: a Grand Canyon whose rim is spectacularly well defined.
Along almost all of the edge of Grand Canyon (about 2,700 miles of it!), there is no doubt when you are standing on its rim. You can put your finger on it, or big toes on it. You can stand on it without having to guess if there’s another “rim” a little higher because the former higher rocks have been washed away. To take one more step is to risk a long, long fall INTO Grand Canyon.
The Mesozoic Era is the Age of Dinosaurs and the Glen Canyon area is loaded with evidence of their existence—bones, tracks, coprolites, even feathers, and more.
The Mesozoic Era ended when the second-largest mass extinction event occurred, the better known K-T or End-Cretaceous boundary event. About 70 % of Earth’s species perished then but this mass extinction is more famous than the Great Dying because it brought a sudden end to the reign of the dinosaurs.
The cause is, perhaps, better known, too because it was either induced by or exacerbated by the impact of a large asteroid or comet near the Yucatan Peninsula.
Almost all of the rock units of the Glen Canyon area can be seen between Lees Ferry (a part of the southwestern corner of Glen Canyon National Recreation Area even though Grand Canyon river trips famously begin there) and, by traveling NE, to the summit of the Kaiparowits Plateau.
To sum up, this stack of rock records much of the very interesting interval of time between the two largest mass extinctions in the history of the earth when Dinosaurs roamed the earth. Those layers are the ones found in Glen Canyon.
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Gary Ladd’s Publications
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