Relative dating of geological features
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.These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages.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.We can use these principles to determine relative ages of the features.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.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.
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.
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.
Smith’s observation has been repeated at millions of locations around the world, and has been codiﬁed as the principle of fossil succession.
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.
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.