Missing Time at Grand Canyon National Park


Grand Canyon’s three sets of rocks are separated by sharp and interesting rock contacts known as unconformities that represent gaps in the rock record (Figure 12). There has been considerable work trying to assign numeric ages to the time missing along the unconformities to complement knowledge of the time recorded by rocks.

two drawings of rock layers in the walls of grand canyon.

Figure 12. John Wesley Powell (1875) recognized the three main packages of rocks exposed in Grand Canyon (left). The image on the right shows newer names and an important correction; note how the contact between the Vishnu Basement Rocks and the Grand Canyon Supergroup was tilted the same amount as the overlying Grand Canyon Supergroup prior to the deposition of the Layered Paleozoic Rocks.

As noted by Grand Canyon’s preeminent stratigrapher and former park naturalist Eddie McKee (1969):

“These unconformities were discussed by Powell (1875, p. 212), who pointed out that each represents a sequence of events of tremendous importance in Earth history, including the formation of mountains by tectonic forces, the erosion of these mountains to a condition of base level, and finally, the burial of the erosion surface by sediments of advancing seas.”

For clarity, different names for the major unconformities between Grand Canyon’s three sets of rock have been introduced:

The Great Unconformity

Photo of rocky cliffs.
Figure 18. The Great Unconformity above the Granite Gorge has Vishnu Basement Rocks below the white line and the Layered Paleozoic Rocks above it. At this location, there are 1.2 billion years (about 25 percent of Earth history) missing (not recorded) across this contact.

Photo by Laurie Crossey.

Powell’s original term, the Great Unconformity, is used for the contact below the Tonto Group where the Layered Paleozoic Rocks overlie the Vishnu Basement Rocks (Figures 17 and 18). There is approximately 1.2 billion years (or about 30 percent) of Earth history missing (e.g., not recorded) at this contact.

Photo of a rock outcrop.
Figure 17. The Great Unconformity is the erosional contact that separates the vertical layering of the Vishnu Schist (below) from horizontal bedding of the Paleozoic Rocks (above).

Photo by Laurie Crossey.

The Great Nonconformity

Photo of cliff with a line showing a geologic contact.

The Great Nonconformity refers to the contact below the Grand Canyon Supergroup where it overlies the Vishnu Basement Rocks (Figure 18). This sub-Supergroup contact can represent up to 500 million years of missing record when the Vishnu Basement Rocks were exhumed by erosion from 12 mile (20 km) depths.

The Great Angular Unconformity

Photo of banded rocky cliffs.

Photo is by James St. John (Flickr).

The Great Angular Unconformity is the contact between the Layered Paleozoic Rocks and the Grand Canyon Supergroup; this contact can span up to 750 million years of missing rock record.

All Three Unconformities in one View

All three unconformities can be seen from several places from the rim.

Lipan Point

Photo of rocky cliffs and buttes with a line overlay highlighting a geologic contact.

Figure 19. The Grand Canyon Supergroup is not exposed everywhere, but this view, looking north from Horseshoe Mesa in eastern Grand Canyon, shows the Great Nonconformity (white line) with the 1.25 Ga basal Grand Canyon Supergroup resting on the 1.75 Ga Vishnu Basement Rocks. Up to 500 million years of Earth history is missing (not recorded) at this contact. Also shown is the Great Angular Unconformity (red line) where the 510 Ma Layered Paleozoic Rocks rest on the tilted 1,100 to 1,250 Ma Unkar Group of the Grand Canyon Supergroup, with 590 to 740 million years of history missing (not recorded) along this contact. These two profound erosion surfaces come together to form the Great Unconformity (black line) where the Layered Paleozoic Rocks overlie Vishnu Basement Rocks (black line).
NPS photo by Carl Bowman.

Yavapai Geology Museum and Vicinity of the Grand Canyon Village

Photo of view looking down into the Grand Canyon.

Figure 20. View from the center window of Yavapai Geology Museum. Vishnu Basement Rocks are at the bottom of Grand Canyon in the Granite Gorge. Look within the layers to see the ancient island (monadnock) of tilted Grand Canyon Supergroup rocks that was fringed by 505 million year old beach sands and eventually got covered up by muds of the Tonto Group as the lowest layers of the Layered Paleozoic Rocks accumulated.
Photo by Laurie Crossey.

Disconformities within the Layered Paleozoic Rocks

Another kind of unconformity is a disconformity, a gap in time across a contact between sedimentary strata above and below it (Figure 21). Disconformities are present between some formations in the Layered Paleozoic Rocks (Figure 22).

Photo showing details of a rock layer within the cliffs of the Grand Canyon.

Figure 21. Some disconformities, like this minor one in the Supai Group between the Esplanade Sandstone and Watahomigi Formation, are subtly revealed by a pebble conglomerate layer left at the end of a period of erosion in the Supai Group.
Photo by Carl Bowman.

Photo of rocky cliffs and buttes with a line overlay highlighting a geologic contact.

Figure 22. A major disconformity occurs at the base of the Redwall Limestone (along line) (left). Without the fossil evidence, it would be hard to recognize that about 150 million years of rock record is missing along this flat lying contact.
Photo by Laurie Crossey.

Photo of a section of cliff face with rock formation names and dates shown in text overlay.
Figure 22 b. In detail, in eastern Grand Canyon, one can see channels that reveal some of the missing layers.

Photo by Laurie Crossey.

Important Concepts Related to Grand Canyon’s Unconformities

An evaluation of the unconformities present between Grand Canyon’s sets of rocks and within them reveals three important concepts (Figure 23):

  1. Although the vertical mile of rock revealed in Grand Canyon looks like a spectacularly complete rock record, more time is missing in it than is preserved. While it is tempting to equate missing time with unknown events, this is not entirely true. By looking at the rock layers on either side of an unconformity, a geologist may deduce significant events that occurred in the gap. For example, during the Great Nonconformity, erosion reduced a rugged mountain landscape to a nearly smooth plain with a prodigious amount of erosion over a prodigious amount of time.

  2. Numeric ages can be assigned to the time missing along the unconformities based on the best numeric age of rocks directly above versus directly below these erosion surfaces.

  3. It is often possible to understand part of what went on across a given unconformity by looking elsewhere, sometimes nearby as in the case where the Grand Canyon Supergroup is present between the basement and the Layered Paleozoic Rocks, or more globally.

Illustration of rock layers and stratigraphic colunm.

Figure 23. Rock and Time. Grand Canyon has one of the world’s most complete geologic records, yet more time is missing (black = time not recorded in the column on the right) than preserved. We assign approximate numeric ages to the time missing along the unconformities based on the age range of rocks directly above versus below these erosion surfaces. Note that the modern erosion surface on top of the Kaibab Formation has 270 million years missing. Diagram does not show all formations and unconformities in the Layered Paleozoic Rocks because of space considerations.

Learn More

Tiny image of the cover of a report titled Telling Time at Grand Canyon National Park.

To learn more about the age of Grand Canyon’s rocks, please see:

Karlstrom, K., L. Crossey, A. Mathis, and C. Bowman. 2021. Telling time at Grand Canyon National Park: 2020 update. Natural Resource Report NPS/GRCA/NRR—2021/2246. National Park Service, Fort Collins, Colorado. [IRMA Portal]

Photos and Illustrations

Part of a series of articles titled Telling Time at Grand Canyon National Park.

Grand Canyon National Park

Last updated: January 30, 2024