The different stratigraphic units (litho-, bio-, chrono- and sequence stratigraphic) are organised and named in hierarchic systems, which are practical tools for descriptions and communications; e.g. lithostratigraphic units are define as beds, members, formations, groups and super groups.
The description of the relation and contact of the geological layers is essential for an analysis of the rock forming geological processes. The term ”inconformity” is used to describe the contact between two successive layers in a rock succession, which are not formed during continuous depositional processes in contrast to an “conformity” used for contacts between layers formed during continuous processes. If the layers are parallel, the contact is named a ”disconformity”; if the contact is characterised by a distinct angle (difference in dip angle) between the layers, the contact is named a ”angular discordance”. The first mentioned contact reflects an interruption in the sedimentation processes, while the latter contact illustrates an interruption where tectonic influence (tilting) and erosion occurred prior to the deposition of the overlying layer.
As a result of an unconformity, evidences in the form of sedimentary layers are missing from a time period in the stratigraphic column; this missing part (“hole”) in the stratigraphic record is called a hiatus (pluralis: hiati), which may be of local or regional distribution.