varve

A layer or series of layers of sediment deposited in a body of still water in one year. Varves are typically associated with glacial lake deposits and consist of two layers—a lower, light-colored layer that consists primarily of sand and silt, and a darker upper layer that consists primarily of clay and organic matter. The lower layer is typically deposited in the summer by the rapid melt of glacial ice, and the upper layer is usually deposited in the winter by the slower settling of sediment through calm water. The thickness of the layers in a varve varies depending on the proximity to the margin of the glacier, with thicker layers forming closer to the glacial margin and thinner layers forming farther away from it. Varves have been used, like tree rings, to measure the ages of glacial deposits from the Pleistocene.