There are several other kinds of absolute dating methods. In some lakes, ice covers the lake in the winter which limits the amount of sediment deposited at the bottom of the lake to a thin dark layer. In the summer when the ice melts, a thicker lighter colored layer is deposited on the top of the darker colored layers. Thus, each pair of dark/light layers represents one year of sediment deposition. Such layers are called varves and can be used to count the number of years the lake has been receiving sediment. [Photo courtesy of National Park Service. Photo by P. Carrera.]