Deriving from the Swedish word for layer, a varve is a pair of two thin, adjoining sheets of sedimentary clay and silt. The word was originally used to refer exclusively to the layered, annual sediments that were deposited (as long as eighteen thousand years ago) in ancient glacial lakes and other still water pools formed from glacial meltwater. However, varves continue to form today, and have been found to form in lakes (even tropical lakes) as well as artificial reservoirs. They can be expected in any part of the country where seasonal sedimentation in lake basins occurs. The thicker and coarser band of sediment deposited in summer (by rapid streams fed by melting ice) is lighter in color and often includes iron oxides. The thinner band of fine clay and other organic material that settles out in winter (from grains suspended in still pools) is darker in color. Because successive layers of glacial varves were deposited annually, they can be counted and compared in thickness and used to calculate the age of the deposits, to track the retreats and advances of glaciers, and to establish data about climate change—much the way tree rings can be use to identify age and varying growth conditions. Such use of varves, developed in 1878 by the Swedish geologist Gerard de Geer, is called varve chronology (or geochronology) and is one of the main techniques that confirmed the Ice Age as not merely a hypothesis, but a fact of prehistory. Contemplating the abundance of varved clays around the Baltic Sea, de Geer wrote that “nature must have conserved all the years that have passed in these sediments.” Recently, as scientists have learned that varves occur in marine habitats as well as in lakes, an alternative term, rhythmite (from the seasonal nature of the deposits), has come into favor.

Emily Hiestand