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Your question concerns the geologic column, the stratigraphic reconstruction of the geologic history of the earth. In stratigraphy, the fundamental unit is the Formation, defined as a laterally mappable unit with a minimal extent and thickness and with distinct upper and lower bounds. Formations can be subdivided into Members and Beds, and may themselves belong to a Group of formations. The formation itself is without time implications. It is a physically defined entity, apart from interpretations with respect to its genesis.

When geologists adopted the "Law of Faunal Succession", and applied this to rock units, they superimposed an interpretive time on these units and generated what are called "Time-Stratigraphic Units". These units may or may not coincide with rock units, such as Formations, since they are dependent on the presence of unique clusters of fossils being contained in the units in question. The classification scheme used terminology as follows:

Erathem encompasses the major units of the Phanerozoic geologic column: Paleozoic, Mesozoic, Cenozoic.
System includes the familiar divisions of the erathems: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian for the Paleozoic, Triassic, Jurassic, Cretaceous for the Mesozoic, and Tertiary and Quaternary for the Cenozoic. The Systems are further subdivided into
, for example Lower Cambrian, Middle Cambrian, Upper Cambrian, etc., and the divisions of the Tertiary (Paleocene, Eocene, Oligocene, Miocene, Pliocene).

The term you have specifically asked about: epoch, belongs to another concept in stratigraphy, the Time Units. Time units might most simply be defined as the time during which the time-stratigraphic units were laid down. Thus the Paleozoic Era was the time during which the Paleozoic time-stratigraphic units were accumulated, and the time during which the Paleozoic formations were accumulated. The Cambrian Period was the time during which the Cambrian time stratigraphic units were laid down, or the time during which the Cambrian Formations were accumulated. The Eocene EPOCH (here I capitalize only because this is the term you asked me to consider) was the time during which the Eocene time-stratigraphic units were laid down, or the time during which the Eocene Formations accumulated.

There are subdivisions of the Time Unit EPOCH (called Age), and of the time-stratigraphic Series (called Stage), as well.

Now to the question you pose. An EPOCH can be a very different amount of time depending upon your assumptions. The Pleistocene is assumed to be the shortest EPOCH by those who accept the uniformitarian principles. Yet for those who believe in a worldwide flood and catastrophic geological processes, the Pleistocene may well be the longest geological EPOCH. Even in uniformitarian thought there is no specific time period for an EPOCH. It is best described as a major subdivision of a Period. I hope this helps.

______________________________________________________ Ó 2010 Arthur V. Chadwick, Ph.D.