blackfoot-valley

That event was only the beginning. Although the ice dam was swept away, the glacier that produced it was still very much active. Its source was the great Cordilleran Ice Sheet, thousands of feet thick, which covered all of British Columbia (and adjoined the even greater Laurentide Ice Sheet covering the rest of Canada and a large part of the northern United States.) The damming gla- cier, called the Purcell Lobe because it moved down the Idaho Panhandle valley known as the Purcell Trench, continued its slow but steady advance southward. The Lobe, with continual pressure from the Cordilleran Ice Sheet, crossed the channel of the Clark Fork and re-formed the ice dam after a few decades. Lake Missoula continually formed behind it until reaching a depth sufficient to lift and break that ice dam producing similar catastrophic results. This happened again and again - and again! David Alt of The University on Montana, has concluded from his studies of alternating lake and river sediments along Interstate 90 near Missoula, that the lake has filled and emptied multiple times. In his book “ Glacial Lake Missoula and its Humongous Floods” , David states that the events were cataclysmic and occured between 36 and 42 times. The ice dam repaired itself and the lake would refill over a 30 year period until the dam again lifted and broke off. The average cycle, as determined from sediment thickness in the road cut near Mis- soula, was 58 years - and thus, if you had lived in the area during those times you would have witnessed in one lifetime the full cycle of calamitous draining and the slow refilling of the lake. In terms of geologic time, this was properly seen as a rapid-fire process of repeated drainings and fillings. The record shown by the sediments and marked shorelines also shows that each successive cycle lasted a shorter time than the one before. The de- posited layers show that the shortest lifetime of the lake itself was only nine years - as shown by the thinness of the most recent layer deposited. This is because the lake reached a lower maximum level each time. That is confirmed by the many successive shorelines visibly marked on Mt. Sentinel overlooking Missoula and even more clearly on the slopes of Mt. Jumbo overlooking down- town. If the more recent lake levels had risen higher than previous ones, wave erosion would have erased the faint terracelike traces left by earlier shorelines. From this evidence it follows that the first great flood in the series, which oc- curred around 15,000 years ago, was also the largest. The time period of the floods and refillings has been determined as span- ning roughly 2,000 years - from 15,000 to 13,000 years ago. As Alt’s richly in- formative road cut also reveals, the layers of sediments deposited on the then- lake-bottom during each of its reincarnations got continually thinner towards 22