Relative dating of geological features

If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers.But if a layer of sediment buries a fault, the sediment must be younger than the fault.Lateral continuity: The principle of lateral continuity states that sediments generally accumulate in continuous sheets within a given region.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.For example, a conglomerate containing pebbles of basalt is younger than the basalt, and a sill containing fragments of sandstone must be younger than the sandstone.

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.It provides the geologic underpinning for the theory of evolution.Example: 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.