The Principle of Fossil Succession is valid and does not depend on any pre-existing ideas about evolution. Geologists, however, interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time. There are two basic types of contacts between rock units, conformable and unconformable . Conformable contacts between beds of sedimentary rocks may be either abrupt or gradational . Most abrupt contacts are bedding planes resulting from sudden minor changes in depositional conditions.
Gradational contacts represent more gradual changes in depositional conditions.
There are several criteria for recognizing unconformities. Structural criteria for recognizing unconformities include an irregular contact which cuts across bedding planes in the underlying unit, a difference in the angle of dip of the beds above and below the contact, and truncation of dikes or faults along a sedimentary contact. Overlying strata are deposited basically parallel with the erosion surface. An examination of the fossils shows that there is a considerable time gap represented by the surface. Igneous contacts may also be helpful in relative dating, particularly when considered along with the Principle of Cross-cutting relationships.