his is the first of a series of three articles about the age of rocks exposed within Grand Canyon. This article presents a compilation of “best” numeric ages for Grand Canyon’s major rock units.The second article will provide more details on the age and overall geology of rocks exposed in Grand Canyon. And finally, a third article will explore the science of geologic dating. The subsequent articles will be published in future editions of bqr.

Grand Canyon is one of the best places in the world to gain a sense of geologic, or “deep,” time because the canyon exposes a great swath of geologic history. Rocks exposed in Grand Canyon are truly ancient, ranging from 1840 million years old (or 1.84 billion years old) to 270 million years old. The Grand Canyon landscape is geologically young, carved within just the last six million years. There are younger geologic deposits in Grand Canyon too, like the Ice Age fossils found in caves, a 1000-year-old lava flow in the western canyon, and even the debris flow deposits that form each year.
Yet, it is the canyon’s rock walls that allow people to develop their greatest perspective on geologic time because of these rocks’ immense age, their fossil record, and because these rocks formed in environments so different than those found in northern Arizona today. With a rock record spanning more than 1500 million years, Grand Canyon is truly a panoramic view into the geologic past.
Nonetheless, how geologists determine the age of rocks is a mystery to many members of the public, and even to some river guides and others who share the canyon’s geologic story. It is natural to wonder “How do you know that?” when a geologist says a Grand Canyon rock formed 270 million years ago. Further confusion arises when one publication or geologist says, for example, that the Kaibab Formation is 270 million years old and another says 255 million years old. The same questions arise for the other rocks at Grand Canyon. Which ages are more correct, and why?
We hope this series of articles will answer these questions. We’ve developed a list of the “best” numeric ages for rocks exposed in Grand Canyon based on the scientific literature, and short primer on geologic dating methods and how they were applied to Grand Canyon rocks. We also describe three “sets” or “packages” of Grand Canyon rocks, each with unique geologic histories.

Dating Rocks
Peeking ahead to the third article, geologists use two major approaches to determine the age of rocks: relative dating and absolute age determinations. Relative dating determines the order in which a sequence of geologic events occurred, but does not determine exactly when they happened. The simplest method of relative dating is the Law of Superposition: rocks on top must be younger than rock layers below because sedimentary rocks are stacked up subsequently.
Absolute age determinations are numeric and identify the time in years when specific events happened, such as the formation of a rock. Radiometric dating is the most commonly used type of absolute age determination in geology. It takes advantage of the decay of radioisotopes naturally present in rocks.
Both types of dating methods are important in different geologic situations and have been used together to discern the ages of individual rock units exposed in Grand Canyon (Table 1). Articles in future editions of bqr will provide more details on geologic dating techniques and how these techniques were applied to Grand Canyon.

The Numeric Ages of Rocks Exposed in
Grand Canyon
Most of the rocks exposed in Grand Canyon, including nearly all the sedimentary rocks, cannot be radiometrically dated, and so absolute age determinations are not available for these rocks. However, through fossil correlation, relative age relationships, and other information, geologists can determine their geologic age, such as a period (like “Permian” or “Cambrian”) from the Geologic Time Scale. For example, the geologic literature reports that the Kaibab Formation formed during the Permian Period, more specifically, during the Late Early Permian in the Roadian or Leonardian age.

These terms are very accurate and meaningful to a geologist, but they do not say how old the Kaibab Formation is in numerical terms (such as 270 million years), and mean little to folks who don’t have the Geologic Time Scale memorized. Hence, using numerical ages are essential in dealing with general audiences. Even though they may not find a description of the Kaibab Formation as “Late Early Permian” or “Roadian” meaningful, they could comprehend a numeric value of 270 million years old (at least to the degree that geologic time is understandable to humans). They may not know that Cambrian rocks are older than Permian rocks, but will intuitively know that 525 million years old rocks are older than 270 million years old rocks.
We compiled a set of numeric ages for Grand Canyon rocks for interpretive purposes (Table 1) as a shorthand or translation for the public of the geologic age of the rocks. To develop this chart, we relied on the geologic ages reported in the professional literature and used the most up-to-date geologic time scales (specifically, the International Stratigraphic Chart [2004], and Geologic Time Scale [2004]), and used absolute age determinations whenever possible. For example, we used the reported Leonardian and/or Roadian (subdivisions of the Permian Period) age of the Kaibab Formation and the Geologic Time Scale (2004) to determine that the Kaibab Formation is approximately 270 million years old. We used a similar process for all of the rock units in Table 1 that lacked absolute (radiometric) ages. We also consulted with geologists who have worked on Grand Canyon rocks (notably with Dr. Ronald Blakey at Northern Arizona University, who works with Mesozoic and Paleozoic rocks of the Colorado Plateau, and Dr. Michael Timmons, at the New Mexico Bureau of Geology and Mineral Resources, who researches the Precambrian Grand Canyon Supergroup).
Our goal in making this compilation was to be as accurate as possible in assigning numeric ages, even though it was impossible to be entirely precise. Additionally, where the science allowed, we wanted to have rounded numbers (they’re just easier to learn and remember). Of course, assigning a single numeric age can be misleading, since it may take millions of years to deposit some formations. Numeric ages are an important translation for the public to understand rocks at Grand Canyon. It is our hope that people who interpret the age of Grand Canyon rocks, including river guides, other tour guides, and rangers, will use these numeric ages. We believe that they are as accurate and as precise as possible, at least, until the next refinement of the Geologic Time Scale, additional stratigraphic fieldwork, or new age determinations of rocks exposed within Grand Canyon.

Summary
Table 1 contains what we believe are the most accurate numeric values for the age of rocks exposed in Grand Canyon. Of course, without further information on the geology of Grand Canyon rocks and geologic dating techniques, Table 1 is just a list of names and numbers. In the next two articles in this series, we elaborate on the geology of Grand Canyon, and the science of geologic dating techniques. We hope all of this information will increase your understanding of the canyon’s geology, and even let you answer the question “How do you know that?” when someone wonders how geologists can say a rock is millions, or even billions, of years old.

Allyson Mathis and Carl Bowman
Allyson_mathis@nps.gov

References:
Beus, Stanley. S., and Morales, Michael, eds., 2003, Grand Canyon Geology, Second Edition, Oxford University Press, 432 p.
Blakey, Ronald, 2004, personal communication.
International Commission on Stratigraphy, 2004, Geologic Time Scale 2004: Cambridge University Press and www.stratigraphy.org/scale04.pdf.
International Commission on Stratigraphy, 2004, International Stratigraphic Chart: www.stratigraphy.org/chus.pdf.
Karlstrom, Karl, 2004, personal communication.
Timmons, Michael, 2005–2003, personal communication.

Acknowledgments: Mike Timmons, Ron Blakey, and Karl Karlstrom provided valuable insight into the ages of rocks exposed in Grand Canyon and assisted us with our compilation of best numeric ages.

About the Authors: Allyson Mathis is a geologist by training and an interpretive park ranger at Grand Canyon National Park. Carl Bowman is the Air Quality Specialist for Grand Canyon National Park’s Science Center.