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.
Geologists apply geologic principles to determine the relative ages of rocks, structures, and other geologic features at a given location.
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.
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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.
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.
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.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.Cross cutting relations: The principle of cross-cutting relations states that if one geologic feature cuts across another, the feature that has been cut is older.For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike.If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified.