As we will see, painstaking work over many years eventually allowed geologists to assign numerical age ranges to fossil species. For example, if a bed contains Fossil F (from the succession specified above), geologists can say the bed is older than a bed containing Fossil A, even if the two beds do not crop out in the same area. Once the relative ages of a number of fossils have been determined, the fossils can be used to determine the relative age of the beds containing them. The sequence contains a definable succession of fossils (A, B, C, D, E, F), that the range in which a particular species occurs may overlap with the range of other species, and that once a species vanishes, it does not reappear higher in the sequence. From these data, we can define the range of specific fossils in the sequence, meaning the interval in the sequence in which the fossils occur. It provides the geologic underpinning for the theory of evolution.Įxample: Bed 1 at the base contains fossil species A, Bed 2 contains fossil species A and B, Bed 3 contains B and C, Bed 4 contains C, and so on. Smith’s observation has been repeated at millions of locations around the world, and has been codified as the principle of fossil succession. Thus, once a fossil species disappears at a horizon in a sequence of strata, it never reappears higher in the sequence or, put another way, extinction is forever. He also realized that a particular assemblage can be found only in a limited interval of strata, and not above or below this interval. Smith learned to recognize distinctive layers of sedimentary rock and to identify the fossil assemblage (the group of fossil species) that they contained. Canal digging provided fresh exposures of bedrock, which previously had been covered by vegetation. Investors decided to construct a network of canals to transport coal and iron, and hired an engineer named William Smith (1769–1839) to survey some of the excavations. We develop a geologic history of the region, defining the relative ages of events that took place there.Īs Britain entered the industrial revolution in the late 18th and early 19th centuries, new factories demanded coal to fire their steam engines and needed an inexpensive means to transport goods. We can use these principles to determine relative ages of the features. The succession of events in order of relative age that have produced the rock, structure, and landscape of a region is called the geologic history of the region. They then go further by interpreting the formation of each feature to be the consequence of a specific geologic event.Įxamples of geologic events include: Deposition of sedimentary beds erosion of the land surface intrusion or extrusion of igneous rocks deformation (folding and/or faulting) and episodes of metamorphism. Geologists apply geologic principles to determine the relative ages of rocks, structures, and other geologic features at a given location. With this principle in mind, geologists conclude that examples of folds and tilted beds represent the consequences of deformation after deposition. If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified. Original horizontality: The principle of original horizontality states that layers of sediment, when first deposited, are fairly horizontal because sediments accumulate on surfaces of low relief (such as floodplains or the sea floor) in a gravitational field. Uniformitarianism: The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates, so the present is the key to the past. These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages. Building from the work of Steno, Hutton, and others, the British geologist Charles Lyell (1797–1875) laid out a set of formal, usable geologic principles.
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