The Tahoe area has been covered by multiple glacial episodes. Much of the evidence of the older glaciations have been reworked by the more recent Tahoe glacial episode (70,000 to 150,000 years ago) and the Tioga glaciation (19,000 to 26,000 years ago). It is during one of these glaciations, a glacier flowed down out of the Sierras toward Lake Tahoe following the path of Glen Alpine Creek.
As it flowed down out of the mountains the glacial ice carried sediment eroded from the bedrock of the High Sierras. At lower elevations, the glacier began to melt, depositing the sediment where ever the ice released the sediment. The sediment that fell of the sides of the glacier built up the linear high hills on either side of Fallen Leaf Lake. These deposits are called lateral moraines. Similarly, sediment built up at the end of the glacier in a terminal moraine. The terminal moraine for the Glen Alpine Creek glacier is likely under Lake Tahoe.
When the glacial event ended, the glacier retreated back up the canyon. The retreat was not uniform. There were periods of relative stability where sediment built up at the end of the glacier. Then there were periods of temporary advances when the glacier would push previously deposited sediment down the canyon into ridges cutting across the valley, followed by continued retreat. These ridges across the valley are recessional moraines.
The size of sediment carried by a glacier varies widely, from large boulders to fine powder. Large boulders are be plucked from the bedrock and rock can be ground to a powder at the base of the glacier. All of this material is deposited in moraines in an unsorted mass.
One of these moraines dammed the flow of Glen Alpine Creek forming Fallen Leaf Lake.
Logging requirements:
Send me a note with :
- The text "GC4CB0P Fallen Leaf Lake Moraines" on the first line
- The number of people in your group (put in the log as well).
- Describe the composition of the inside of the hill at this location. Would you classify it as a moraine and why? If so which kind?
- How many more recessional moraines do you see up the valley?
- About how much higher was the lake when it first formed? Why do you think that height?
The above information was compiled from the following sources:
- GEOLOGY OF THE LAKE TAHOE REGION, CALIFORNIA and NEVADA September 9, 2011 Fieldtrip for AAPG Foundation Trustee Associate 34th Annual Meeting Truckee, California
- Richard A. Schweickert, Mary M. Lahren, Jim Howle, and Winnie Kortemeier, Emeriti, Department of Geological Sciences, University of Nevada, Reno, USGS, Western Region Remote Sensing and Visualization Center, Carnelian Bay, California, Western Nevada College, Carson City, Nevada
- Konigsmak, Ted. Geologic Tips Sierra Nevada. GeoPess 2007.
- http://clasfaculty.ucdenver.edu/callen/1202/Landscapes/Glac/GlacialAlpine/