Field Trip to Glacial Lake Missoula

Last weekend (September 12) I enjoyed the first of many weekend geology field trips scheduled for the fall semester.  This field trip was part of the Winston-Thompson symposium, honoring UM Department of Geosciences professors emeriti Don Winston and Gray Thompson.

So, those of you in the know (um...GeoGuppy?) will wonder if I've taken possession of a time machine, as Glacial Lake Missoula no longer exists.  Much like the former Glacial Lake Hitchcock in the Connecticut River Valley or Glacial Lake Cape Cod in Cape Cod Bay, Glacial Lake Missoula was created during the Pleistocene period when the advancing Purcell Lobe of the Cordilleran ice sheet dammed the Clark Fork River (which runs through Missoula) near Pend Oreille, Idaho. The impounded river, as well as some of the melting ice, filled the valleys from Hamilton to Polson.  At its greatest extent, GLM had a depth of 600 meters and occupied a volume of 500 cubic miles!

http://vulcan.wr.usgs.gov/Volcanoes/ColumbiaPlateau/summary_columbia_plateau.html

Of course, most of you are probably more familiar with GLM as its waters were the source for the floods that created the channeled scablands of Washington and Oregon.  See Nova's "Mystery of the Megaflood."  Of the many scientific controversies surrounding GLM, the number of "megafloods" produced is probably the greatest.  It is thought that the lake filled and then catastrophically drained multiple times.

So, our field trip was to visit the many landforms and deposits found in the area once occupied by GLM.  Sadly, we are too many hours away from the scablands to have visited those - a different trip, perhaps.  We also did not visit the mega-ripples created as waters catastrophically drained westward, but I understand that they are more spectacular in aerial photographs and maps than from the ground.

What we did visit: diamictite deposits (clay, silt, and gravel) interpreted as glacial till due to the presence of striated boulders, overlain by paleosol (ancient soil), then silts with climbing ripples and layers with syn-sedimentary deformation, and finally laminated layers (interpreted) as varves, with dropstones.  Was that a sentence that made you go, "hmmm....?"  What we saw: the gravelly deposits of an advancing glacier, followed by soils, and then by thin layers of sediment that were deposited in a deep, quiet lake.  Those layers have "dropstones," literally pebbles and cobbles that dropped through the water from icebergs floating in the lake.  We also visited an area of the lower Flathead River thought to be where floodwaters burst through a canyon, depositing rock debris.

Photos: Top, left: The group at an "outcrop" of unconsolidated sediment north of Arlee, Montana.

Top, right: Diamictite overlain by fine sediments; note soft-sediment deformation at elbow level; also climbing ripples; grading up into varves.  Bottom: Striated boulder in diamictite allows for interpretation of glacial till.

The weather was lovely (80+ degrees, clear, blue skies) and there were dozens of happy geologists to share it with.  The field trip was followed by a barbecue at Professor Winston's legendary home along the Jocko River.  As I have summarized for those of you on facebook, we enjoyed: beer, bison burgers, a bonfire, and banjo music.  It was great company, a beautiful, warm night, and good music (the banjoist was accompanied by guitars, bongos, flute, and cello!).

Photos: Left: The Lower Flathead River as it flows away from the Kerr Dam, south of Flathead Lake.

Middle: Dropstones in varves.  Right: Geologic hoe-down around a campfire.

The next field trip: A geomorphology class data collection trip to the Mattie V Creek.