Along the ICE AGE FLOODS
NATIONAL GEOLOGIC TRAIL

Glacial Lake Missoula National Natural Landmark Ice Age Floods National Geologic Trail Glacial Lake Missoula was the largest of several lakes impounded by the Cordilleran Ice Sheet in the Northern Rocky Mountains during the Quaternary period. The lake was approximately 2,900 square miles in size and contained more than 500 cubic miles of water. It occupied the Mission, Jocko and Little Bitterroot valleys, drained by the Flathead River; and the Missoula, Ninemile and Bitterroot valleys, drained by the Clark Fork River. The lake was formed by an ice dam across the mouth of the Clark Fork River at Lake Pend Oreille, Idaho, a result of successive advances of a lobe of the Cordilleran Ice Sheet. The site is located 12 miles north of Perma, Montana. At least three and possibly more than 100 separate collapses of the ice dam, possibly coincident with periods of ice retreat, caused the lake to drain, producing catastrophic floods across the Columbia Plateau and down the Columbia River Basin. It is estimated that the waters of Glacial Lake Missoula, 1,000 feet deep at Eddy Narrows, were withdrawn at a peak discharge rate of 10 cubic miles per hour. Approximately 550 cubic miles of water passed through those narrows during withdrawal. Evidence of this catastrophic withdrawal can be found in scourings, high eddy deposits of flood debris, flood bars of boulders and course gravel, and successions of giant arcuate ripples or ridges of gravel resting on bedrock surfaces. Ripples appear as ridges 15 to 50 feet high, 100 to 250 feet wide, and from 300 feet to a half-mile long. While the form, structure and arrangement of these features are similar to that of ordinary current ripple marks, they are termed giant flood ripples, due to their large size. The giant flood ripples at this site are unusual due to their being on the down-current side of notches in a ridge separating two basins that were both subsidiary to the main course of Lake Missoula in the Clark Fork Valley. The best examples of giant flood ripples are found in Camas Prairie. Quick Facts Designation:National Natural Landmark OPEN TO PUBLIC:Private Land, closed to the public, but features throughout the area are visible from many vantage points.

Eddy Narrows Ice Age Floods National Geologic Trail Eddy Narrows is a 10-mile long canyon between Plains and Thompson Fall along the Clark Fork River. Here all of the waters from the northern and eastern arms of Glacial Lake Missoula converged and picked up speed as they were forced through this restricted area. The steep valley walls are bare of talus and soils up to the level estimated to be the highest level of the lake (1000 ft), and there are polished grooves in the bedrock on a bench 340-400 feet above the Clark Fork River that run parallel to it. Since there is no evidence of glaciation in this area, the only explanation is the rapid draining of Glacial Lake Missoula. Using the dimensions of this flume-shaped area of the canyon, which controlled the rate of flow of water surging through it, in 1942 J.T. Pardee was able to calculate maximum speeds approaching 80 mph, emptying Glacial Lake Missoula in as little as three days. There are pullouts along MT-200 at either end of the narrowest part of this section marked for observing big horn sheep with Glacial Lake Missoula signage at the one between mileposts 59 and 60 at: 47.537940, -115.071654  Quick Facts Location:Between Plains and Thompson Falls, MT