Lake Missoula: Missoula to Thompson Falls

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
First-time Visitors: Please visit Site Map and Disclaimer. Use "Back" to return here.


Acknowledgement

I need to make it crystal clear that none of this is my own research. The pictures on this and associated pages were taken on a GSA field trip in 2003 led by Norm Smyers of the U. S. Forest Service and Roy Breckenridge of the Idaho Geological Survey, and the interpretations presented here are largely those of Dr. Smyers and Dr. Breckenridge as presented on the field trip and its guidebook. I thank Dr. Smyers and Dr. Breckenridge for an outstanding field experience.


## ##
##Just south of I-90 Exit 101 on the west side of Missoula. There are obvious shorelines on the hillside. Also note the slump scarp just above the white building at center. The hillslope is made of Eocene sedimentary rocks.

Evaro Hill

##On U.S. 93 just north of I-90. Shorelines are fairly plain in this view.
##Left and below: shorelines are subtle but evident.
####
##An unplanned stop at Evaro Hill. At high lake stand this pass would have been under 60 meters of water. 
####
####
##

2.1 Glacial Lake Missoula Silt

##The best view of laminated Lake Missoula silt along U.S. 93 is unfortunately in a road cut along a notoriously unsafe highway with narrow or no shoulders.
##At the most accessible safe outcrop, the silt is pretty much non-laminated. It is also pure white in contrast to the pink silt near Alberton. Below are laminated silts on adjacent private land.
####
####
Below: views southwest across the valley.
####
####

National Bison Range and Mission Range

##Between Ravalli and St. Ignatius, looking northeast at the Mission Range.
##Left and below: looking west into the National Bison Range. The Belt quartzite here is steeply dipping, so horizontal features on the hillsides must be shorelines.
####
####
##Below: especially well developed shorelines on the east side of U.S. 93.
####
##Left and below: views of the Mission Range near St. Ignatius.
####
####
####

Left: map of the Mission Range. This is a marvelous range, which owes its grandeur to a fault scarp. The fault is recently active and considered a potential source of large earthquakes. The Mission Range Fault and a couple of cross-cutting faults at the south end of the range are in purple. Not all faults are shown.

Below: panorama of the Mission Range. This end of the range has glacial features to rival Glacier National Park.

2.2 Pingo Scars

##Pingos are raised mounds formed in permafrost terrain by freezing of ground water. When they melt and collapse they leave round holes with raised rims and slump textures in the rims. Large pingos on the map above are highlighted in yellow. Left and below are views of some of them. Large ones are water filled, smaller ones are often dry.
####
####
####
##Left: a small dry pingo scar.

Below: views of the Mission Range.

####
####
####
##Looking north along the Mission Range scarp.

2.3 Polson Moraine and Flathead Lake

Flathead lake is impounded by two moraines. The south end of the lake is dammed by the Polson moraine. Below is a panorama looking north from just above the town of Polson. At extreme ends of the panorama is the moraine. At center in the distance is Flathead Lake, and the scarp of the Mission Range is at right. On the map above the moraine is gray and the roads where the pictures were taken are in yellow.

##
##South side of the Polson Moraine from U.S. 
##Road cut and gravel pit in the moraine just south of Polson.
####
####
##Left and below: south side of the Polson Moraine.
##
##Left and below: vies of Flathead Lake and the moraine from above Polson.
####
####
##Left and below: views between Polson and West Arm.
##
####
####
####

2.4 Big Arm Moraine

##Left and below: the western arm of Flathead Lake is blocked by the Big Arm Moraine.
####
##Left and below: looking back toward Flathead Lake from the summit of the moraine.
####
## 
## 
####
##Road cut in moraine
##Moraine topography
##Large gravel pit at the summit of the moraine
##Gently sloping fan surface west of the moraine.

Route 28

##These pictures show shorelines and lake sediments between Flathead Lake and Hot Springs.
####
####
####
##Left and below: outcrops of lake sediment on the valley floor.
##In the two views below, the peaks of the Mission Range can be seen above the tops of the hills in the middle distance.
####
##Some fairly obvious shorelines accented by trees.

2.5 Markle Pass and Camas Prairie

##Markle Pass contains a tract near the summit that is a fairly credible imitation of scablands terrain, except that it's in quartzite rather than basalt.
##Closed depressions like this one occur in the Scablands and have been called "kolks." They have been attributed to intense eddy action and likened to gigantic potholes.
## 
##Below: looking north to the scabland and the large kolk right of the road.
####

Panoramic view of the pass from the south.

##

##Looking south from the pass. The low ripples at center are giant current ripples. I remarked that they didn't look very convincing. It looked like slightly gullied terrain like you might see anywhere. That set me up nicely. "Wait until you see the surface materials."
##
##
 
 
##
##
##
##
##
####
##
##
 
##
####
####
 
 
 ##
##
##Left and below: looking back toward Markle Pass. The low ridges running across the picture below the pass are current ripples.
####
##Left and below: views of the ripple marks south of Markle Pass en route to Perma.
####
####
####

I shouldn't leave this locality without duly noting questions raised by some trip members who saw real problems with how to get deep water moving at such high speeds. The physical evidence leaves little doubt that it did happen, but how? Large reservoirs that drain catastrophically generally don't achieve high current velocities far back from the outlet. The outlet at Lake Pend Oreille was quite wide and perhaps dropped lake levels rapidly enough to cause high speed flow in constrictions. Perhaps, too, the highest speeds occurred at some critical point in the drainage when lake levels dropped low enough to make constrictions most effective. Could ice rafted by the floods have temporarily blocked some channels, creating local mini-outbursts?

2.6 Gulch Fills

##Flathead River near Perma
##Curious structures resembling earthen dams occur high up in side valleys in this area. Bretz called these "gulch fills." These were eddy deposits created during the lake drainage.
####
####
####
Above: one of the clearest gulch fills about 4 miles NW of PermaBelow: Flathead River
####

Clark Fork River

Views between Paradise and Thompson Falls

####
####
####
####

2.7 Eddy Narrows

##
##
##
##
##
##
##
##
##
##
##
##
##
##
##
##
##

Geological Society of America Channeled Scablands Trip, 1994

Spokane to Soap Lake
Soap Lake to Chelan
Chelan to Othello
Othello to The Dalles
The Dalles to Seattle

Geological Society of America Glacial Lake Missoula Trip, 2003

Spokane to Missoula
Thompson Falls to Spokane

Other Missoula Floods Pages:

Wallula Gap
Frenchman Coulee


Return to Virtual Field Trips Index
Return to Professor Dutch's Home Page

Created 9 April 2003, Last Update 01 July 2012

Not an official UW Green Bay site