In many lakes, especially those near ice-sheets, formations called varves -- banded layers of silt and sand -- are deposited annually. Course and pale material is left in summer, while finer, darker material is left in winter. One light band and one dark band make up a varve. Just as tree rings may be counted to reveal the age of a tree, varved layers of clay may be counted to determine the age of a glacial deposit. Butler Lake Nature Reserve features an eight-metre section of varved clays deposited by the postglacial Lake Agassiz. There are no visitor facilities. Access is via Wabigoon Lake.