A varve is an annual layer of Sediment or Sedimentary rock.
The word Varve is derived 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 was used to describe 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 described the whole of any annual sedimentary layer. More recently introduced terms such as annually laminated are synonymous with varve.
Of the many rhythmites found in the geological record, varves are one of the most important and illuminating to studies of past climate change. amongst the smallest-scale events recognised in Stratigraphy varves fall within the calendar band of orbitally forced cyclicity.
The first varve Chronology was constructed by De Geer in Stockholm in the late 19th century. Further work soon followed, and a network of sites along the east coast of Sweden was eventually established. The varved sediments exposed in these sites had formed in glaciolacustrine and glacimarine conditions in the Baltic basin as the last ice sheet retreated northwards. By 1914, De Geer had discovered that it was possible to compare varve sequences across long distances by matching variations in varve thickness, and distinct marker laminae. However, this discovery led De Geer and many of his co-workers, to making incorrect correlations, or teleconnections, between continents, a process criticised by other varve pioneers like Ernst Antevs.
In 1924 a special laboratory dedicated to varve research - the Geochronological Institute - was established. De Geer and his co-workers and students made trips to other countries and continents to investigate varved sediments. Ernst Antevs studied sites from Long Island, U.S.A. to Lake Timiskaming and Hudson Bay, Canada, and created a North American varve chronology. Carl Caldenius visited Patagonia and Tierra del Fuego, and Erik Norin visited central Asia. By this stage, research outside the activities of the Geochronological Institute team had also developed, and Matti Sauramo had constructed a varve chronology of the last deglaciation in Finland.
1940 saw the publication of a now classic scientific paper by De Geer, the Geochronologia Suecica, in which he presented the Swedish Time Scale, a floating varve chronology for ice recession from Skåne to Indalsšlven. Lidén made the first attempts to link this time scale with the present day. Since then, there has been an ongoing process of revision as new sites are discovered, and old ones reassessed. At present, the Swedish varve chronology is based on thousands of sites, and covers 13,200 varve years.