USGS: Science for
a Changing World


We study Earth's history by studying the record of past events that is preserved in the rocks. The layers of the rocks are the pages in our history book.

Most of the rocks exposed at the surface of Earth are sedimentary--formed from particles of older rocks that have been broken apart by water or wind. The gravel, sand, and mud settle to the bottom in rivers, lakes, and oceans. These sedimentary particles may bury living and dead animals and plants on the lake or sea bottom. With the passage of time and the accumulation of more particles, and often with chemical changes, the sediments at the bottom of the pile become rock. Gravel becomes a rock called conglomerate, sand becomes sandstone, mud becomes mudstone or shale, and the animal skeletons and plant pieces can become fossils.

Diagram showing modern landscape 
and potential fossils
An idealized view of a modern landscape and some of the plants and animals that could be preserved as fossils.
Originations of major life forms
Originations of major life forms.
As early as the mid-1600's, the Danish scientist Nicholas Steno studied the relative positions of sedimentary rocks. He found that solid particles settle from a fluid according to their relative weight or size. The largest, or heaviest, settle first, and the smallest, or lightest, settle last. Slight changes in particle size or composition result in the formation of layers, also called beds, in the rock. Layering, or bedding, is the most obvious feature of sedimentary rocks.

Sedimentary rocks are formed particle by particle and bed by bed, and the layers are piled one on top of the other. Thus, in any sequence of layered rocks, a given bed must be older than any bed on top of it. This Law of Superposition is fundamental to the interpretation of Earth history, because at any one location it indicates the relative ages of rock layers and the fossils in them.

Layered rocks form when particles settle from water or air. Steno's Law of Original Horizontality states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting during episodes of mountain building.

Rock layers are also called strata (the plural form of the Latin word stratum), and stratigraphy is the science of strata. Stratigraphy deals with all the characteristics of layered rocks; it includes the study of how these rocks relate to time.

Photograph of outcrop of Ordovician 
Lexington Limestone rich in fossil shells near Lexington, Kentucky
Outcrop of the Ordovician Lexington Limestone, which is rich in fossil shells, near Lexington, Kentucky. These horizontally layered beds were deposited about 450 million years ago. The dark stains on the rocks are formed by water seeping from springs. The vertical marks on the rocks are drill holes in which dynamite charges were exploded to remove the rock so that an interstate highway could be built. Photograph courtesy of O.L. Karlkins.
Photograph of nearly vertical limestone beds 
near Ardmore, Oklahoma, that were disturbed from their original horizontal position by mountain 
building and contain Silurian fossil shells
Nearly vertical limestone beds that were disturbed from their original horizontal position by mountain building. The men are collecting Silurian fossil shells. These rocks are in the Arbuckle Mountains, near Ardmore, Oklahoma. Photograph courtesy of J.E Repetski.

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Last updated 14 August 1997 (krw)
Maintained by John Watson