Varve

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.
The classic varve archetype is a light/dark-coloured couplet deposited in a glacial lake. The light layer is usually silt and fine sand deposited when spring meltwater brings sediment into the lake. 
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.
Varve formation needs the absence of bioturbation (lake bottom disturbed by animals). Therefore, varves usually form under anoxic conditions.