What are ... Varves?

The next entry in my What are Wednesdays series is to discuss varves (First entry here). I ran into this problem when I was teaching Petrology where the students either A) Didn't know what a varve was, or B) Didn't understand how it formed.

Varves from the Green River Formation used as framing in the Geology building at the University of Utah

Varves, as pictured above, are sedimentary features that alternate in color from light to dark, often in thin bands. Some of the most famous varves are found from the Green River Formation of Wyoming and surrounding regions. The Sutton Geology building here at the University of Utah uses a lot of the Green River Formation both for framing materials but also as wall decorations so it was fairly easy to get a picture of some nice varves.

Varves are formed in end glacial lakes. The lakes has a seasonality, where they freeze every winter and are defrosted every summer. What happens is that during the summer sediment run off from the melting glacial is allowed to settle on the bottom of the lake. This produces the light colors which is rich in quartz sand and other sand and silt grains. You can often find drop stones in this layer as well (like that seen in the lower left corner) where a larger grain gets stuck in ice. The ice chunk floats out into the lake and then melts, dropping the larger grains.

During the winter the lake freezes over. The freezing of the lake limits all sediment from entering the lake so no more large sediment grains are deposited on the lake bottom. What does happen though, is that the freezing of the lake kills much of the biological activity that is occurring in the lake and settles the currents in the lake. The settling of the currents allows for finer grained materials and the organic material to settle to the bottom. These are the darker layers.

So each year of a varve is represented by one couplet of bands: 1 light for summer and 1 dark for winter.

That is all I have to say on varves. If you have any questions feel free to ask and if there are any topics you would like me to present on What are Wednesdays let me know.