4 relative dating principles
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.Canal digging provided fresh exposures of bedrock, which previously had been covered by vegetation.Law of Superposition: The principle of superposition states that in a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect.Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest.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.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.
Smith’s observation has been repeated at millions of locations around the world, and has been codiﬁed as the principle of fossil succession.Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.We develop a geologic history of the region, deﬁning the relative ages of events that took place there.Fossil Succession As 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.
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If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers.