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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.
Rock type (or lithology) is usually not a good basis for correlation.
Some rock types look much the same regardless of their age (the 275 m.y.
Coconino Sandstone in Grand Canyon looks similar to the 160 m.y Navajo Sandstone in southern Utah) [see Photos 6a&b].