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    Principles of relative age dating

    Smith learned to recognize distinctive layers of sedimentary rock and to identify the fossil assemblage (the group of fossil species) that they contained.

    He also realized that a particular assemblage can be found only in a limited interval of strata, and not above or below this interval.

    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.

    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.

    If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the area that was later eroded by the river that formed the canyon.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.They then go further by interpreting the formation of each feature to be the consequence of a specific geologic event.Examples 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.

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