The Physiography of Western Washington

floods the Columbia Plateau. Two low passes, the Columbia gap on the south and the Fraser gap on the north permit this cold air to flow westward unhindered and fill the adjacent portions of the valley. The southern end has recorded lower minima than the northern end owing to the greater width of the Columbia River gap. The central part of the lowland is notably warmer owing to the absence of low mountain passes to the east, However, when the cold Polar air is of sufficient depth to pour westward over the top of the Cascades, the descending air is warmed rapidly by descent and the lowland enjoys a true foehn. The Physiography of Western Washington HOWA UI) ?. COOMBS University ol Washington, Seattle. Washington Western Washington is divided into three topographic units all trending in a north-south direction. The westernmost of these is an extension of the Coast ranges culminating in the mile high Olympic mountains. The known rocks of the Olympics are essentially Tertiary sediments and volcanics which have been thrown into a series of N.W.—S.E. trending folds. The more intensely metamorphosed rocks have not yet been correlated . Subaerial and glacial erosion have fashioned these rocks into an extremely rugged range. The central unit is the Puget Sound trough. This depression is approximately 350 miles long and 50 railes wide and the altitude of the floor seldom exceeds 500 feet. Strata similar to those found in the Olympics also underlie Puget Sound. The area north of Whidby island however, is a notable exception . Unconformably overlying these strata is a generous covering of glacial debris which locally attains thicknesses of 1000 feet. Lobes of the Cordilleran ice sheet, augmented by valley glaciers both from the Olympics and Cascade ranges, extended as far south as the divide [separating the drainage of Puget Sound from that of the Chehalis river. At least two successive advances of ice have been recorded. According to Bretz, nine-tenths of all the Pleistocene deposits in Puget Sound is norted glacial material derived from the waning ice of the first (Admiralty) glaciation. These sediments were ¡severely attacked by iiubaerial erosion in interglacial (Puyallup) time and the features we know today were fashioned for the most part during this interim. The second glaciation (Vashon) provided a veneer of drift ovbi.· most of the area except in the southern portion where outwash material is more plentiful. The third and eastermost unit is the Cascade range whose peaks average 6000 feet in elevation. At certain localities along the abrupt western side of this range there is strong evidence of faulting. Aided by' glaciers, the streams draining the western side of the Cascades have bitten deeply into this rather even crested upland. The resulting · topography is a diversely !sculptured and scenic range with a relief of dOOO feet. (20) ...