Stratigraphy

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Stratigraphy

Applications of stratigraphy in historical studies

Stratigraphy is the branch of geology concerned with the description and interpretation of sequences of rock layers or strata. In most cases the layers are of sedimentary origin, but can also include sequences of volcanic ash and lava, and even the study of different layers of human occupation at an archeological site. Sediment usually forms distinct strata with the most recent layers on top. Although the strata may be folded during episodes of mountain building, interrupted by inclusions and slippages, and even metamorphosized into other forms of rock, stratigraphic analyses can still be performed in many cases. The processes of sedimentation—including the presence of certain types of fossils—provide scientists with valuable clues about Earth history. These principles are thus valuable for many different types of scientist, ranging from prospecting geologists to city planners to archaeologists and paleontologists studying human and animal history and prehistory.

Stratigraphic fundamentals

The basic principle of sedimentation—that in any given set of layers of material the most recent levels are closest to the top—were established as long ago as the seventeenth century. By the nineteenth century such early geologists as Charles Lyell (1797–1875) recognized that sediment accumulation was not necessarily regular or obvious. Weathering can also influence the stratigraphic record by introducing trace elements into the various layers. Patterns in the very layering of sediments, such as ripple marks and flumes, can introduce discontinuities. Changes in climate, which bring about changes in sea level, also create discontinuities.

Equally as important as the composition of the layers themselves are the boundaries between them, which represent breaks in time or changes in sediment accumulation. Sediments do not deposit evenly—rates of sedimentation are influenced by extraordinary events as well as everyday processes. During periods of flood, for instance, rivers can drop tons of silt on what had been working farm land, and a single storm can carry away tons of beach sand into the ocean depths. The borders marked by the beginning and ends of such events can represent as little time as a single day. Because sediments generally accumulate over long periods of time, however, the borders between different layers usually represent a long-term change in local geography.

Geologists have adopted words to describe the different types of layers based on their thickness. Sediments are generally divided between laminae and beds, with the laminae represented by an accumulation of less than one centimeter, and the beds represented by accumulations ranging from 0.4 to 47 in (1-120 cm). The beds are subdivided into very thin, thin, thick, and very thick, respectively measuring from 0.4 to 2 in (1 to 5 cm), 2 to 24 in (5 to 60 cm), 24 to 47 in (60 to 120 cm), and more than 47 in (120 cm) across. Beds are also graded on the size and type of the individual sand grains. Beds and laminae together form primary sedimentary structures, which indicate the way in which strata are laid down.

Geologists have also introduced various sub-disciplines of stratigraphy. Lithostratigraphers study changes in layers of rock. This is the type of stratigraphy most commonly seen on geological survey maps. Biostratigraphy uses microscopic fossils to determine the relative ages of rocks and helps paleontologists trace local variations in climate. Tephrostratigraphy is the study of deposits of volcanic ash, while magneto-stratigraphers trace fluctuations in Earth’s magnetic field—specifically, reversals in its polarity—over millions of years. Other useful applications of strati-graphic analysis include seismic stratigraphy, which applies the principles of acoustics (sending shock waves through the earth) to determine the positions of pockets of petroleum and other substances.

Applications of stratigraphy in historical studies

Strata can be used to study history of both Earth and, on a shorter time scale, of humankind. Anthropologists and archaeologists use stratigraphic principles to understand how and under what circumstances an archeological site was occupied, how long the people that lived there stayed, and how they lived while they were in residence. Archaeologists regularly apply microstratigraphic principles such as observation of the process of soil formation and landscape development based on weathering and sediment accumulation to their sites in order to classify and date artifacts. For example, excavators working at the Paleolithic site of Mezhirich in what is now Ukraine uncovered the bones of hundreds of mammoths, some of which had been burnt, others of which had been arranged to form houses. The scientists suggested, based on stratigraphic principles—the thinness of accumulated layers of occupation debris—that the site was not occupied year-round, but instead was a seasonal dwelling-place. Paleontologists studying the border regions between the Eocene and Oligocene periods in ancient history have studied eastern Oregon’s stratigraphy to draw conclusions about global climactic conditions. They suggest that the regularity of sediments in the area reflect, not a gradual change in sea level, but a cooling trend in which the area changed from a subtropical to a temperate climate.

Archaeologists have applied stratigraphic principles to understanding the history of the famous Roman city of Pompeii, which was buried during an eruption of the volcano known as Vesuvius in AD 79. Although the historical record of the explosion itself is quite clear, scientists use stratigraphy to help understand life in the city before the eruption. Excavators had assumed, based on the testimony of ancient written sources, that parts of Pompeii had been built as long ago as the fifth century BC and had been occupied ever since. Nineteenth and early twentieth-century archaeologists had accepted this reasoning, based on analyses of building styles and construction. Beginning in the 1930s, however, scientists began to revise their thinking using observations of the micro-stratigraphy of the site. Modern excavations suggest that most of the buildings standing at the time the volcano erupted were built in the period of Roman occupation—in other words, no earlier than the second century BC. Some finds of debris unrelated to the AD 79 eruption can be dated back to the fifth century BC, but these are not directly connected with standing houses. Stratigraphy promises to change the history of a site historians believed they knew very well.