HOW OLD IS IT?
Nothing is more apparent at Chaco Canyon than the passage of time. The crumbling walls of the
stand as a patent reminder that this desolate canyon once bustled with human activity.
Geologists know the age of these and other rocks and fossils thanks to an arsenal of dating techniques. Radiometric dating uses the predictable decay of naturally occurring radioactive elements to establish the age of rocks. To establish the relative age of rocks, geologists compare layers in the rock (a technique called stratigraphy) and also look for index fossils, fossils of organisms that are known to have lived within a narrow time range.
The rock that comprises Chaco Canyon is sedimentary, built from layers of compacted ocean sediment. Sedimentary rock is difficult to date absolutely using radiometric dating, because it is composed of tiny bits of many types of preexisting rocks, all of various ages. To date sedimentary rock, geologists correlate fossil-bearing rock samples with other samples having both the same fossils as well as nearby layers of igneous rock—such as those formed by lava flows—which can be dated radiometrically.