Just downstream from Glen Canyon lies Grand Canyon, a somewhat better known segment of Colorado River canyon. Grand Canyon is a profound cleft in Earth’s crust that almost always prompts questions about its creation; how long ago did it form and why did it form?
In contrast, Glen Canyon’s landscape typically prompts inquiries about its fantastic details—the arches, bridges, slot canyons, draperies of desert varnish, balanced rocks, alcoves and rounded sandstone domes.
The questions are different because most visitors to Grand Canyon, standing on the rim, can not possibly notice Grand Canyon’s details given the canyon’s titanic long-distance vistas while most Glen Canyon visitors view the canyon from its interior while boating on Lake Powell where the canyon’s details are preeminent and its large scale structure is lost to view.
Because they are adjacent to one another, Grand Canyon and Glen Canyon are closely related—both were carved by the Colorado River, both are geologically young (a few millions to a few tens of millions of years old) and both were carved from sections the same thick sequence of rock layers.
And yet the two canyons are visually very different because Glen Canyon lies within the upper (younger) portion of the Colorado Plateau’s rock sequence while today’s Grand Canyon is carved from the lower (older) portion of the sequence. Grand Canyon is profoundly deep; Glen Canyon is not. The Colorado River in Grand Canyon is loaded with large rapids; the Colorado River in Glen Canyon was placid. All of these topographic expressions are derived from the kinds of rock layers that bound the canyons and the different degrees of uplift the canyons have experienced.
I use the phrase “today’s Grand Canyon” because there is convincing evidence and theoretical reasons to believe that a “Proto-Grand Canyon” existed where today’s Grand Canyon sits that was carved from the same younger group of rock layers that cradle today’s Glen Canyon. That is, before the Colorado River carved as deep into the land as it has today, it was carving into younger rock layers that have since been subsequently entirely erased by erosion in the Grand Canyon area. Those rock layers exist now only where they were not elevated to such heights as was the Grand Canyon region; they survive to the north of Grand Canyon at Glen Canyon and Zion and other nearby locations.