With one of the clearest exposures of the rock record and a long, diverse geologic history, Grand Canyon is an ideal place to gain a sense of geologic or “deep” time. The oldest rocks exposed in the canyon are ancient, 1,840 million years old. Conversely, the canyon itself is geologically young, having been carved in the last 6 million years. Even younger deposits, including ice age fossils in caves, 1,000 year-old lava flows in the western canyon, and recently deposited debris flows, bring Grand Canyon’s geologic record to the present.

Understanding the park’s natural resources is undeniably intertwined with its geologic history; therefore, telling geologic time is an important part of the interpretive efforts at Grand Canyon National Park (Arizona). An appreciation of geologic time places geologic topics such as geomorphology, the origin and evolution of the Colorado River, stratigraphy, historical geology, and paleontology in context. Unfortunately, telling geologic time is a mystery to visitors and park staff without backgrounds in Earth science. Naturally a non-geoscientist may wonder, “How do you know that?” when a geologist or interpreter says “That rock formed 270 million years ago.” To add to the confusion, both technical and popular literature report a wide variety of numeric ages for Grand Canyon rocks. For example, one publication may say that the Kaibab Formation is 270 million years old, while another says 255 million years old. The same inconsistencies arise for the other rock units in the park. At best, readers are left wondering which are the correct (or “best”) ages and why. At worst, they may discount the scientific processes used to measure deep time.

When one’s objective is simply to learn how old a rock layer is, sorting through the subdivisions of geologic periods, the scientific names of microscopic index fossils (diagnostic assemblages of past life), and the nuances of radiometric dating techniques is very confusing. Moreover, most non-geoscientists will not find a description of the Kaibab Formation as Leonardian or Roadian (stages) meaningful. However, they will be able to comprehend the numeric value of 270 million years (at least to the degree that geologic time is understandable). Therefore, numeric ages are essential when interpreters and resource managers communicate geology to the public and to one another. However, finding such numbers in the scientific literature is not easy. Unless researchers used absolute-dating techniques in a study, only the relative geologic age (i.e., period, epoch, or stage) of a rock unit is usually reported. Moreover, the scientific papers that do publish absolute age determinations are not always clear about the geologic significance of these dates.

Given the inconsistencies in reported numeric ages for Grand Canyon rocks and the difficulty in determining their ages, we reviewed the technical literature and consulted with researchers to compile the “best” ages of Grand Canyon rocks. By “best” we mean the most accurate and precise ages, given the parameters of geologic dating techniques and available information from the rock record. The primary audiences for this work were interpreters (including NPS rangers, commercial guides, authors, and publishers) and resource managers. The goal was to develop a single list of numeric ages that users could apply consistently, thereby facilitating comprehension of the geologic history and features of the Grand Canyon.