After mapping the rock layers in a particular area, the geologic history can be reconstructed by using a set of principles developed mostly before the beginning of the 19th century. According to the principle of uniformitarianism, the same processes that formed the oldest geologic features are still at work today. Stratigraphers assume that in any sequence of undisturbed strata, the bottom layer is the oldest and the layers above it are successively younger. This is called the principle of superposition. However, stratified rocks are rarely undisturbed: strata can be found in a variety of positions, ranging from horizontal to vertical. No matter what the current position of the strata is, stratigraphers assume that, according to the principle of original horizontality, the sediments were originally deposited in nearly horizontal layers. The principle of original lateral continuity extends this view: strata, as originally laid down, either extended in every direction culminating in a feathered edge, or terminated abruptly against the edges of a basin or some other preexisting geologic feature.

Rock strata are often found to have fossils of plants or animals preserved in them. The principle of faunal succession uses this fossil record to correlate and date various layers. This principle states that there are unique life-forms from each period of the Earth's history and that the fossilized remains of these creatures can be used to match contemporaneous deposits from around the world.

STRATIGRAPHIC UNITS

To map the rock layers in a given region, the strata must be classified on the basis of their physical characteristics, or lithology, on the basis of their ages, or on the basis of the fossils found in them. The fundamental stratigraphic rock unit is the formation, which is defined as a mappable body of rock with a distinct lithology. Formation names often have two parts: the first is related to the geographic area where the formation is well-exposed at the surface of the Earth, or where it was first described, while the other describes the rock's primary lithologic type. For example, the St. Louis Limestone, primarily composed of CARBONATE MINERALS, is named after exposures located near St. Louis, Mo. When a formation contains more than one lithologic type, the geographic name is simply followed by "formation."

Because the age of a particular rock unit might vary from place to place, stratigraphers have developed the time-rock, or chronostratigraphic, unit to group rock strata by age. The fundamental time-rock unit is called a system. Strata are also organized by the fossils they contain, called biostratigraphic units. The fundamental biostratigraphic unit is the zone.

STRATIGRAPHIC CORRELATION

Once the strata are mapped, the stratigrapher must then correlate them according to temporal, lithographic, or paleontological similarities. Often the temporal correlation of units that are separated by relatively short distances can be accomplished by noting lithologic similarities. Particularly useful are thin layers of unusual composition that were deposited over a large area, such as a bed of volcanic ash. Correlation of units separated by large distances usually depends on identification of similar fossils. This technique must be employed with caution because the fossil occurrence of most organisms depends on environment at the time of deposition. In addition, the migration of a particular species may take a substantial amount of time. For these reasons, rocks of different ages can have similar fossil contents. Thus, two rocks of different age may have similar fossil content. However, as the segment of time involved becomes greater, the reliability of the correlation also tends to become greater.

The use of fossils permits the establishment of only a relative order of events. Absolute ages of deposition for sedimentary units are determined by radiometric age-dating. This technique measures the age of crystallization of a mineral in which small amounts of a radioactive element are incorporated. As the element decays, the decay products are trapped in the mineral. The ratio of radioactive element to decay product indicates the time since crystallization.

Radiometric age-dating is ideally suited to dating igneous and metamorphic rocks, in which minerals have crystallized under near-equilibrium conditions of high temperatures. Sedimentary depositional ages are much more difficult to measure, because little recrystallization takes place as these rocks are formed. Fossils have been dated by the isotopic content of their skeletons. The most reliable stratigraphic dates are those established by measuring the ages of adjacent igneous or metamorphic rocks. For example, a sedimentary unit that nonconformably (the granite was eroded before the sedimentary unit was deposited over it) overlies granite and is crosscut by an igneous dike must be younger than the granite and older than the dike.

HISTORY

Although the fundamental concepts of stratigraphy--like the law of superposition--seem obvious, the development of stratigraphy as a science is a relatively recent event. Nicolaus Steno (1669) is credited with the discovery of principles of superposition, original horizontality, and lateral continuity. Several European geologists, notably Johann Gottlob Lehmann (1756), Giovanni Arduino (1760), and Georg Christian Fuchsel (1762), mapped thick stratigraphic sequences in England and Germany. Although these early stratigraphers also noticed that layers in a particular region would often occur in the same order, their interpretations of these strata were limited by their belief that most sediments had been laid down during the great biblical flood.

The theory of a "Universal Ocean," which was thought to have covered the entire Earth, was consistent with biblical accounts of the flood, and neatly explained the presence of seashells and other marine fossils on mountaintops. This theory was championed by Abraham Werner, who dominated geological thinking in the late 18th century.

The theories of these "Neptunists" were contested by James Hutton, a Scottish geologist responsible for introducing (1788) the concepts of Uniformitarianism and the vastness of geologic time. Although Hutton's writings originally received little critical attention, they were subsequently popularized by Charles Lyell, particularly in his widely used text, Principles of Geology (1830-33). As Hutton's ideas attained prominence, mapping and subdivision of stratigraphic units proceeded quickly. The surveyor William Smith constructed (1815) an excellent geologic map of southeastern England. He successfully delineated successive strata on the basis of distinctive rock type and fossil content. Other geologists took note of Smith's techniques, and, using the principles of rock-type division and fossil correlation, quickly recognized major stratigraphic divisions throughout Europe and the eastern United States. By the middle of the 19th century all of the geological eras and periods had been recognized and defined.

During the 20th century, stratigraphy has been tied to the development of the petroleum industry. Because understanding the origin and distribution of sedimentary rocks is a prerequisite for locating oil and gas concentrations, various petroleum companies have sponsored detailed mapping programs throughout the world. They also have organized large research programs directed toward understanding the origin of particular rock types, such as reef limestones.

Although the practice of stratigraphy depends heavily on fieldwork, laboratory measurements are often used to refine field observations. Lithological determinations involve microscopic analysis of thin rock sections for trace elements and microfossils. Field observations were once restricted to surface outcrops, but now geophysical techniques are used to map the rocks beneath the surface.

Linda Carroll