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 far different than those found in northern Arizona today.

With a rock record that spans more than 1500 m.y., Grand Canyon is truly a panoramic view into the geologic past.

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.

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

Geologists usually correlate sedimentary rocks based on their fossil assemblages, particularly on the presence of index fossils.

Both techniques are important in different geologic situations.

And both techniques are used together to develop the geologic time scale and to discern the ages of rocks exposed in Grand Canyon. In flat-lying sedimentary rocks it is simply the Principle of Superposition [see Photo 3] (or the “rule of pancakes”): the rocks at the bottom are oldest, and the rocks on top are the youngest. When rocks are folded, faulted, overturned, or intruded by igneous rocks, it is still possible to determine the relative age of the rocks.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.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.It is also how geologists relate rocks from one region to another.A good collection of index fossils in a rock layer allows a very precise assignment of relative age.