The colluvium and alluvium problem: Historical review and current state of definitions

In the history of alluvium and colluvium, the definitions have been shifted and rearranged several times, and this evolution is ongoing. Although field books, textbooks, and dictionaries provide standardized references, the authors of those definitions must wrestle with a wide variety of previous definitions, especially for colluvium. Contributing to this challenge is the need for Earth scientists to match available terminology to fit specific cases, which has resulting in the stretching of definitions to the point of overlap and sometimes contradiction. Alluvium was first used to describe gradual and imperceptible deposition of new earth material along rivers by floods. In the 19th century, as coarse, alluvium-like sediment in higher landscape positions was unlikely deposited by annual flooding, geologists explained those older alluvium deposits by the biblical deluge and named such materials diluvium. Although not the original definition, cases where alluvium and diluvium could not be distinguished, were named colluvium. Later, colluvial soils replaced 'overplacement soils' to describe regolith that had been transported down slope by mass-gravity processes. Concurrently other scientists used colluvium to describe deposits from slope wash. Modern definitions tend to connect to one, or more frequently both, of those concepts. The resulting spectrum of definitions presents the potential for confusion and miscommunication. This pattern was observed in a survey of 376 Earth scientists who generally agreed with defining colluvium as produced by mass-gravity-driven transport on steep slopes and alluvium as produced by water-driven transport on floodplains. However, diverse perspectives were evident for the spectrum between those two end members. This paper synthesizes the history, the logic, current perspectives, and potential strategies going forward.