Correlation (Geology)

Sedimentary rocks provide information about Earth history that is generally not available from igneous or metamorphic rocks. To understand why this is so, imagine a region in which sediments have been laid down for millions of years. For example, suppose that for many millions of years a river has emptied into an ocean, laying down, or depositing, sediments eroded from the land. During that period of time, layers of sediments would have collected one on top of the other at the mouth of the river.

These layers of sediments are likely to be very different from each other, depending on a number of factors, such as the course followed by the river, the climate of the area, the rock types exposed along the river course, and many other geological factors in the region. One of the most obvious differences in layers is thickness. Layers of sedimentary rock may range in thickness from less than an inch to many feet.

Sedimentary layers that are identifiably different from each other are called beds or strata. In many places on Earth's surface, dozens of strata are stacked one on top of each other. Strata are often separated from each other by relatively well-defined surfaces known as bedding planes.

In 1669, the Danish physician and theologian Nicolaus Steno made a seemingly obvious assertion about the nature of sedimentary strata. Steno stated that in any sequence of sedimentary rocks, any one layer (stratum) is older than the layer below it and younger than the layer above it. Steno's discovery is now known as the law of superposition.

The law of superposition applies only to sedimentary rocks that have not been overturned by geologic forces. Igneous rocks, by comparison, may form in any horizontal sequence whatsoever. A flow of magma may force itself, for example, underneath, in the middle or, or on top of an existing rock stratum. It is very difficult to look back millions of years later, then, and determine the age of the igneous rock compared to rock layers around it.