A varve is a layer of sediment that represents one year of deposition. It is typically composed of two layers, a light layer and a dark layer. The light layer is deposited during the summer months when there is more sunlight and the water is warmer. The dark layer is deposited during the winter months when there is less sunlight and the water is colder. The thickness of a varve can vary depending on the amount of sediment that is deposited each year.