Classification of Quaternary Time and Deposits
Over the years, Quaternary geologists have developed terms and systems to classify layers of sediment (strata). These terms and classification systems help people communicate when discussing geologic history or referring to specific layers of sediment. Geologists describe the lithologic (physical) characteristics of strata that allow them to be mapped and traced regionally. A unit is named after a place where it is well exposed and the lithologic characteristics are typical ("type locality"). For example, the Wedron Group was named for the village of Wedron and the Wedron Quarry, where strata of Quaternary sediments (tills and proglacial river and lake sediments) are well exposed above bedrock in pits in which silica sand is being mined. Such units are known as lithostratigraphic units. The basic unit in lithostratigraphic classification is the formation, which must be identifiable on the basis of easily recognizable physical properties and must be widespread or thick enough to be mappable on a regional scale. A formation may be subdivided into smaller units called members, and several formations can be combined into larger units, called groups.
Similarly, geologists have subdivided geologic time into various named intervals that are useful in discussing geologic history. For example, the Quaternary Period is subdivided into smaller intervals, such as the pre-Illinois, Illinois, Sangamon, Wisconsin, and Hudson episodes, which are represented by deposits and soils of major glacial and interglacial intervals. Some episodes are subdivided into smaller units of time called subepisodes.