Correlation (Geology)

Imagine that geologists a million years from now began studying Earth's surface within a 3 mi (5 kilometer) radius of your home. What would they find? They would probably discover considerable variation in the sediment deposits that are accumulating in your region today. They might find the remains of a river bed, a swamp, a lake, and other features. Geologists living today who study strata laid down millions of years ago make similar discoveries. As they follow an individual stratum for many kilometers, they find that its characteristics change. The stratum tends to have one set of characteristics in one region, which gradually changes into another set of characteristics farther along in the stratum. Those characteristics also change, at some distance farther along, into yet another set of characteristics.

Rocks with a particular set of characteristics are called a facies. Facies changes, changes in the characteristics of a stratum or series of strata, are important clues to Earth history. Suppose that a geologist finds that the facies in a particular stratum change from a limestone to a shale to a sandstone over a distance of a few miles. The geologist knows that limestone is laid down on a sea bottom, shale is formed from compacted mud, and sandstone is formed when sand is compressed. The limestone to shale to sandstone facies pattern may allow an astute geologist to reconstruct what Earth's surface looked like when this particular stratum was formed. For example, knowing these rocks were laid down in adjacent environments, the geologist might consider that the limestone was deposited on a coral reef, the shale in a quiet lagoon or coastal swamp, and the sandstone in a nearby beach. So facies changes indicate differences in the environments in which adjacent facies were deposited.