Nonetheless, how geologists determine the age of rocks is a mystery to many members of the public, and even to many park rangers [see Photo 2], guides, and others who share the canyon’s geologic story with others. ” when geologists say 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 m.y. The same questions arise for the other rock units at Grand Canyon. This article will answer these questions by providing a short primer on geologic dating methods and how they were applied to Grand Canyon rocks.Correlation determines if rocks are the same age, and is a key tool that geologists use to identify the relative ages of rocks where a rock layer is not continuously exposed.

We then describe Grand Canyon rocks as belonging to three “sets,” or packages of rocks, each with unique geologic histories, and present a compilation of “best” numeric ages for Grand Canyon rocks to use when explaining Grand Canyon geology to the public.

Photo 1: Many people consider Grand Canyon National Park the world’s premiere geologic landscape and a “geologic wonder”.

The Grand Canyon landscape is geologically young, being carved within just the last 6 m.y.

There are younger geologic deposits in Grand Canyon too, such as the Ice Age fossils found in caves, a 1000-year-old lava flow in the western canyon, and even the debris flow deposits that continue form each year.

Coconino Sandstone in Grand Canyon looks similar to the 160 m.y Navajo Sandstone in southern Utah) [see Photos 6a&b].

Lithology is also a poor basis for correlation because different rock types can be deposited at the same time in adjacent areas because of different environments.

Grand Canyon contains many important geologic resources, including a vast fossil record ranging from Precambrian stromatolites to Ice Age mammal bones and dung found in caves; a potentially active volcanic field in the western Grand Canyon; a geologic history ranging more than 1.7 billion years; and the canyon landscape itself as the greatest example of arid land erosion.

There are two major categories of geologic dating techniques: relative dating and absolute age determinations .

A good collection of index fossils in a rock layer allows a very precise assignment of relative age.

An “index fossil” in the realm of clothing fashion could be polyester leisure suits – they were widespread, and thankfully, only around for a brief time.

For example, in Grand Canyon, there are many examples of sandstone deposition on a beach while limestone was forming off shore.