A geological contact is a boundary which separates one rock body from another. A contact can be formed during deposition, by the intrusion of magma, or through faulting or other deformation of rock beds that brings distinct rock bodies into contact.

The geologic subdiscipline of stratigraphy is primarily concerned with depositional contacts, while faults and shear zones are of particular interest in structural geology. Faults and shear zones can be regarded as a form of secondary structure in the rock beds.

Conformable contacts represent no time gap in the geologic record. They are usually planar, though they may have slightly irregular topography. These contacts represent continual, uninterrupted deposition and accumulation of sedimentary rocks, or represent lava flows.

A conformable contact can be abrupt, where the contact separates beds of distinct lithology. Abrupt contacts coincide with bedding planes and represent a change in depositional environment, but with only a minor hiatus in deposition with no significant erosion. A brief hiatus without erosion is known as a diastem.

Gradational contacts occur where the change in depositional environment takes place over a longer period of time. They are further divided into progressive gradual contacts where the change in lithology is more or less continuous, or intercalated contacts where thin beds of a new lithology appear at the base of the contact zone, become thicker upwards, and completely replaced the old lithology at the top of the contact zone.

Closely related to intercalated contacts is interfingering (also known as intertonguing, interdigitating, or interlocking) of laterally adjacent sedimentary rock bodies. Here the contact breaks down into a series of wedges or tongues that penetrate the adjacent rock body and pinch out individually. These record fluctuations in the location of the boundary between different depositional environments where sediments were simultaneously being laid down.

Unconformities are gaps in the geologic record within a stratigraphic unit. These gaps can be caused by periods of non-deposition or by erosion. As a result, two adjacent rock units may have significantly different ages.

Intrusive contacts are the surfaces between host (or country) rock and an intrusive magmatic body. The older country rock is crosscut by a younger magmatic body. The nature of the intruding body depends on its composition and depth. Common examples are igneous dikes, sills, plutons, and batholiths. Depending on the composition of the magma, the intrusive body may have a complex internal structure which can provide insight into its emplacement.