Lake Varve in Montana
May 07, 2010
This photo showing lake varve layering was taken near the Missouri River, between the cities of Helena and Great Falls, Montana. The alternating layers of light and dark sediments were deposited on the floor of Glacial Lake Great Falls over 10,000 years ago. The light-colored layers were formed during warm months as abundant melt-water transported sand and silt to the lake where it settled on the bottom. The darker layers are beds of clay formed from much smaller clay particles that were kept suspended in the lake during the brief summers by currents and wind-driven circulation. During winter, the lake froze over, and the tiny clay particles to sank to the bottom. Flocculation, the clinging together of clay particles due to static charges, helped keep the clay intact. Typically, winter layers have about the same thickness; whereas the thicknesses of the summer layers tend to vary more due to differences in the length of the melt season. Photo taken on April 26, 2010.