A varve is a pattern in sediment which is made by annual processes (deposits which vary during the year). Varves are amongst the smallest-scale events in stratigraphy. They form only in fresh or brackish water. Varves are important in palynology, and can be fossilized as rhythmites.
During winter, when meltwater and its sediment is reduced and the lake surface freezes, fine clay-size sediment is deposited forming a dark-coloured layer.