blackfoot-valley

the upper more recently deposited layers. This is confirmed also by the flood deposits downstream in eastern Washington where again, the upper and more recent layers become progressively thinner. It seems clear that each time the ice dam renewed itself, it held for less time than had the previous ones. The later floods were becoming smaller and smaller, but still great, as the newer ice dams were less high. The ice dams therefore could be floated by lesser depths of water each time. What was going on here? Well, the four millennia between 17,000 and 13,000 years ago was a time of rapid melting of the ice sheets in the upper latitudes worldwide. This was a result of a climate change that took place dur- ing this very short period of geologic time. When we remember that the joined pair of North American ice sheets that covered the whole northern half of the continent were up to 12,000 feet thick in their central regions, this was rapid melting indeed. As the ice sheets became less thick, they naturally exerted less pressure on the glaciers they had spawned that crept out from their edges. This included the Purcell Lobe moving south through the Idaho Panhandle, which simply moved less aggressively and was thinner each time. Each succes- sive dam that it formed became less high. Therefore, the filling of each new Lake Missoula to a level high enough to float the smaller ice dam took shorter and shorter periods. Conversly, the time it took to re-establish the ice dam 23