What is a varve?

A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera (varved clay) on the first map produced by the Geological Survey of Sweden in 1862. Initially, "varve" referred the separate components of annual layers in glacial lake sediments, but at the 1910 Geological Congress, the Swedish geologist Gerard De Geer (1858–1943) proposed a new formal definition, where varve means the whole of any annual sedimentary layer. More recently introduced terms such as 'annually laminated' are synonymous with varve.

Of the many rhythmites in the geological record, varves are one of the most important and illuminating in studies of past climate change. Varves are among the smallest-scale events recognized in stratigraphy.

An annual layer can be highly visible because the particles washed into the layer in the spring are much coarser than those deposited later in the year. This forms a pair of layers—one coarse and one fine—for each annual cycle. The classic varve is a light/dark coloured couplet deposited in a glacial lake. The light layer usually comprises a coarser lamina set of silt and fine sand deposited under higher energy conditions when meltwater introduces sediment load into the lake water. During winter suspended sediment input is reduced, finer clay sediments are deposited.