While the ice sheet existed in Washington there may have been relatively little movement of the mammalian fauna. There is definite evidence of a brief contact between the Rocky Mountain Fauna of the Blue Mountains and the fauna of the southern Cascades. For example, the pocket gopher of southeastern Washington (Thomomys talpoides aequalidens) is most closely related to the gopher of the Simcoe Anticline, and the long-tailed meadow mouse (Microtus longicaudus halli) of the Blue Mountains closely resembles the meadow mouse of the Yakima Valley.
Mammals of the Blue Mountains and those of the southern Cascades may have come into contact on the Simcoe-Horseheaven Hills Anticline, which now stretches 150 miles from the Cascades to the Wallula Water Gap. Excepting the easternmost 40 miles, it is timbered. East of the Columbia, a continuation of the anticline and other hills reaches to the Blue Mountains. Supposedly, in Wisconsin Time, this anticline possessed a more humid climate and the habitat was essentially the same as that of an alpine meadow today. The forms on the two ends of the anticline that are closely related inhabit humid, meadow habitat. In an earlier paper, [Dalquest] and [Scheffer] (1944: 316) named this connection the Simcoe Bridge. Its existence was so strongly indicated by the distribution of pocket gophers in Washington that we supposed that the study of many other species would show that they crossed this bridge. However, study of additional species shows that for them the Simcoe Bridge was of only slight importance; there appears to have been but little mingling of the fauna of the Blue Mountains and the Cascades by way of the bridge. The Columbia River probably acted as an effective barrier to many forms that might otherwise have utilized it. The forms that did cross on this bridge are species known to be active in winter and to emigrate over considerable areas through tunnels under the snow ([Davis], 1939: 257). The pocket gopher and long-tailed meadow mouse may have crossed the Columbia, under a cover of snow, when the river was frozen over. The Columbia has frozen over at the Wallula Water Gap in historic times.
Fig. 19. Pend Oreille River (or Clark Fork of the Columbia) from a point near Newport, Washington, looking south, June 13, 1938. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 67.)
Great movements of mammal species came after the retreat of the Vashon-Wisconsin ice. The greatest of these was the spread of the Rocky Mountain Fauna northward and eastward to the Pacific. In this process, northeastern Washington was inhabited by animals that probably came from Idaho and Montana. Some of the species from farther north, as for example the caribou, may first have been forced into Idaho and Montana by the glaciers. The invasion of northeastern Washington probably was not a mass movement of an entire fauna, because invasion seems to be still going on. As yet the woodchuck and striped ground squirrel of the Pend Oreille Mountains have not crossed the Columbia River, a relatively minor barrier in northeastern Washington. The mountains west of the Columbia are occupied instead by the yellow-bellied marmot, a member of the Great Basin Fauna, and there is no ecologic counterpart in these mountains of the golden-mantled ground squirrel.
Also the invasion of the Cascade Mountains by a number of Rocky Mountain species may have been an intermittant or gradual movement. The red squirrel and Rocky Mountain subspecies of jumping mouse now are present in the extreme northeastern Cascades, where they possibly arrived relatively recently. The squirrel is competing with the coastal species already present and may eventually supplant it. The same may be true of the two forms of jumping mouse.
The invasion by the Rocky Mountain Fauna was rapid as compared with that of the Pacific Coastal Fauna. Evidence of this was presented in an earlier paper ([Dalquest] and [Scheffer], 1944: 310), where it was shown that the gophers of the douglasii group, isolated during the Vashon time in the southern Cascades, made only a few feeble postglacial movements and then only when conditions were ideal. In this same time the fuscus group of gophers moved from Idaho and virtually surrounded the range of the douglasii group. This tendency to immobility seems to have been characteristic of every member of the Pacific Coastal Fauna. If the retreat of the Vashon and Wisconsin glaciers occurred at the same time, both faunas had an equal opportunity to invade the deglaciated area. Yet, the Pacific Coastal Fauna actually moved northward along the coast only to the Fraser River area, and slightly farther in the Cascades.
Possibly the two glaciers retreated at different times. The Wisconsin Glacier was a piedmont glacier, fed by valley glaciers to the west and east, and may have disappeared when the feeder glaciers dried up. The Vashon Glacier was instead an ice cap, supposedly self-supporting much in the same manner as is the Greenland Ice Cap, and may have persisted longer than the Wisconsin glacier. If it did persist longer it formed a barrier to the northward emigration of coastal species of mammals.
Possibly, also, the whole of the Pacific Coastal Fauna possessed an inherent sluggishness resulting from their long residence in the uniform climate and habitat of the Pacific Coast. Certainly the species show today great habitat specialization as compared with species of the Rocky Mountain Fauna. Also, there are fewer individual mammals per unit of area in western Washington than in northeastern Washington. The persistence of the Vashon Glacier, an inherent lack of incentive to emigrate, or retention of a favorable environment, may account for the relatively small area invaded by the Pacific Coastal Fauna.