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.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.Principle of baked contacts: The principle of baked contacts states that an igneous intrusion “bakes” (metamorphoses) surrounding rocks, so the rock that has been baked must be older than the intrusion.Principle of inclusions: The principle of inclusions states that a rock containing an inclusion (fragment of another rock) must be younger than the inclusion.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.
Different ways to correlate: -Physical continuity- can physically trace a rock unit from one area to another.
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.
As we will see, painstaking work over many years eventually allowed geologists to assign numerical age ranges to fossil species.
Of note, some fossil species are widespread, but survived only for a relatively short interval of geologic time.
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.