Digital Varves

Varves of the Month for 5/3/2010 - 6/3/2010

Varves from Glacial Lake Great Falls in Montana

Click on image to download original file (~1.3MB JPEG).

This month's spectacular shots of varves were sent to us by Rod Benson, an Earth Science teacher at Helena (Montana) High School. Rod has a fabulous web site showing geologic features of Montana that can be visited at This month's pictures are from Rod's picture set #107.

Click on image to download original file (~500k JPEG).

Click on image to download original file (~500k JPEG).

The varves were deposited during the last glaciation in glacial Lake Great Falls near Craig, Montana at the southwestern margin of the Laurentide Ice Sheet. Lake Great Falls and other glacial lakes east of the Rockies at the margin of this ice sheet drained eastward and were frequently subjected to drainage events and floods as the ice front oscillated and spillways for the lakes changed. The large outcrop shows some erratic changes in varve thickness that may represent some of the flood and drainage events. Visit Rod's web site for more on the formation of Lake Great Falls and the other glacial lakes of Montana.

Thinner varves in the section are composed of tan summer (or melt season layers) layers that are silt to fine sand with internal layering that reflects changes in drainage to the lake as diurnal oscillations or precipitation events during the summer. The winter (or non-melt season) layers are dark brownish red clay. The sediment in this section has contracted and cracked due to drying in the semi-arid Montana climate making the varve section look almost like a sedimentary rock formation.

Past Varves of the Month...