Lake Missoula: Missoula to Thompson Falls

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

Acknowledgement

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

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 isdammed by the Polson moraine. Below is a panorama looking north from just abovethe town of Polson. At extreme ends of the panorama is the moraine. At center inthe 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 takenare 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.

Lake Missoula

	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 sometrip members who saw real problems with how to get deep water moving at suchhigh speeds. The physical evidence leaves little doubt that it did happen, buthow? Large reservoirs that drain catastrophically generally don't achieve highcurrent velocities far back from the outlet. The outlet at Lake Pend Oreille wasquite wide and perhaps dropped lake levels rapidly enough to cause high speedflow in constrictions. Perhaps, too, the highest speeds occurred at somecritical point in the drainage when lake levels dropped low enough to make constrictionsmost effective. Could ice rafted by the floods have temporarily blocked somechannels, 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 Perma	Below: Flathead River