Varve

Varve, (Swedish, varv, a layer), annual deposit of sediment forming a distinctive layer. It is most frequently used to refer to deposits formed in lakes on the margins of glaciers. In the summer, meltwater from the ice carries a mixture of sand, silt, and clay into the lake and the larger, coarser particles are deposited. Over the winter, freezing cuts off supply and only the finer particles, hitherto suspended in the lake, sink slowly to the floor forming a fine-grained layer over the sand or silt below. The lifetime of the lake is recorded in the succession of layers made up of fine-coarse couplets.

The Swedish geologist, de Geer, counted and measured the varves in Scandinavian lakes and concluded that ice lay in the south of Sweden 13,500 years ago and, after retreating, finally separated into two smaller ice caps some 8,700 years ago. The latter event is taken as the end of the Pleistocene Ice Age. Radiocarbon dating methods give substantially the same results.

A different type of varve in the Green River rocks formed some 50 million years ago in western United States in an extensive lake under a mild seasonal climate. This varve consists of a couplet of fine-grained, muddy sediment, one part organic-poor, the other, organic-rich produced by an annual bloom of algae.