Pertaining to, produced by, or inhabiting a lake, the adjective lacustrine designates a lake-speciﬁc rather than marine quality or aspect. There is, for instance, a lacustrine age, a prehistoric period when lakeside dwellings were common. Lacustrine often refers to animals and plants inhabiting lakes, and also to the ecology of a lake area. Thus, lacustrine deposits are stratiﬁed geologic materials of the lake bottom. Sometimes the term indicates that these deposits have been exposed and made visible by the lowering of the water level or by the elevation of the land. Because, in contrast to seas and oceans, lakes are smaller, nearly closed systems with ﬁner-grained sediment (silt and clay)—and are often rich in organic shales, which, in turn, are important rock sources of petroleum—this distinction is not only geographically signiﬁcant (indicating a lake rather than a sea) but also geologically important (referring to certain deposits peculiar to lakes). Lacustrine deposits are found even in the Sahara, where the Cenozoic era left a sand sea of wide, shallow basins ﬁlled with alluvial and lacustrine drift.