Mammals of Washington, Volume 2 / University of Kansas Publications Museum of Natural History

GEOLOGIC HISTORY OF WASHINGTON

The composition of the mammalian fauna of any area is dependent on several factors. These include the composition of the original fauna, species which have since invaded the area, and quantitative and qualitative changes that have occurred in the area. The latter two factors refer to changes in relative numbers or extermination of species through environmental changes or competition with other forms and evolutionary changes that have occurred in the species making up the mammalian fauna.

Our knowledge and understanding of the distribution and history of the species of mammals occurring in Washington decreases rapidly as we go back in time. The distribution of the modern fauna at the present time is fairly well known. The distribution of species 100 years ago is less well understood. This is especially true of certain game species and carnivores whose distribution has been altered by man. Our knowledge of the distribution of mammals in the Pleistocene and earlier times is based on fossil skeletons. Such knowledge must necessarily be meager, for conditions favorable to fossilization and the preservation of fossils until their subsequent discovery by man, were not of common occurrence.

In the Cascades and in eastern Washington, the Miocene was a time of orogeny and great volcanism. Great flows of lava, 4,000 feet thick in the Snake River area (Russell, 1893), emerging from fissures in the Snake River area, formed the Columbian Plateau. The Columbian basalt slopes inward centripetally from the eastern, northern, and western margins of the Columbian Plateau with an average descent of 25 feet to the mile (Flint, 1938). The dip of the lava flows results in the basalt-marginal course of the Spokane and Columbia rivers today, along the northern edge of the Columbian Plateau. The earlier part of the Pliocene was a period of erosion and deformation. In the early Pleistocene the five great volcanic cones of the Cascades, Mount Baker, Glacier Peak, Mount Rainier, Mount Adams and Mount St. Helens were formed. In eastern Washington a gentle folding of the Miocene lava flows occurred. The folding took place slowly and the Columbia River in its course along the eastern edge of the Cascades cut through the folds as they formed, making a series of water gaps. Farther south, the Simcoe-Frenchman Hills anticline seems to have arisen more rapidly and the Columbia River was forced eastward before it became impounded and rose over the barrier and plunged down, tearing out the great Wallula Water Gap (Flint, 1938). This gap is a mile wide, eight miles long and, in places, a thousand feet deep. The impounding of the Columbia by the Simcoe-Frenchman Hills anticline resulted in a lake several hundred miles in area. Sediments deposited in this lake form the Ringold formation. The Ringold formation possesses a very early Pleistocene mammalian fauna.

The Pleistocene was a time of great change in the mammalian fauna of the world. Unfortunately the beautiful glacial sequence revealed in Europe and the Mississippi Valley cannot be detected in Washington. In western Washington the deposits of the last continental glaciation and fluvial deposits of the last interglacial period almost everywhere obscure evidence of earlier glaciations. Deposits of an earlier glaciation, named Admiralty by Bretz (1913), have been detected in places. Deposits of greater age, that may represent a still earlier glaciation, have been noted. In eastern Washington the only definite proof of multiple glaciation is of one glaciation preceding the last. This is the Spokane glaciation of Bretz (1923). That multiple glaciation in the sequence reported from the Mississippi Valley affected Washington seems probable. The lack of evidence of a complete sequence is negative evidence. In western Washington the earliest glacial deposits might be beneath the later deposits or they may have been removed or reworked by subsequent glaciations, whereas in eastern Washington they may have been removed by subsequent glaciation and erosion.

The time interval between the two known glaciations appears to have been of greater duration than the Recent. The drift of the earlier period is sometimes found covered by the till of the later glaciation, and preserved by it. The early material is deeply weathered and all save the hardest pebbles and quartzites, for example, are rotten and disintegrate at the touch. In contrast, the later deposits are almost unweathered. Pebbles are hard, and ring when struck. A zone of leaching and oxidation of the finer materials reaches a depth of some 30 inches, below which the till is fresh.

Two names are currently applied to the last continental glaciation of the state of Washington. That west of the Cascade Mountains, studied and described by Bretz (1913), was termed "Vashon." The interglacial cycle preceding it was called "Puyallup." The glaciation of eastern Washington has been called "Wisconsin," after the Mississippi Valley terminology, by several writers. Papers by Flint (1935, 1937) describe and map it.

The Vashon and Wisconsin glaciations probably occupied the same time interval, although this has not certainly been established. In the present report I have used the term "Vashon-Wisconsin" in speaking of the entire period, or the glaciers both east and west of the Cascades together. Vashon, alone, is restricted to western Washington and Wisconsin to eastern Washington.

Fig. 17. Extent of Vashon-Wisconsin ice over Washington. The Wisconsin and Vashon glaciers have been connected through the Mount Rainier section of the Cascades because it is thought that few or no mammalian species lived in the Cascades north of Mount Rainier while the ice was in place. Data generalized from Flint (1937), Bretz (1913), Culver (1936) and other sources.

The Vashon glaciation seems to have consisted of an ice dome centering in Puget Sound (the Puget Glacier of Bretz, 1913) and flooding the lowlands from the Olympic Mountains to the Cascade Mountains. The southern edge of the Puget Glacier was slightly south of the present terminus of Puget Sound. Fingerlike projections of ice were forced up valleys of the western Cascades and the northern and eastern Olympics. Some of these upward moving fingers of ice met and coalesced with valley glaciers descending from the mountains. At the southern edge of the glacier, the Black Hills and Porcupine Hills remained above the ice although partially surrounded by it.

The Wisconsin glacier, according to Flint (1935), was a great piedmont glacier, fed by valley glaciers from the Cascades and Coast Ranges to the west and the Rockies to the east. It extended from the Idaho boundary to the Cascade Mountains. From the Canadian Boundary it sloped down to an approximate elevation of some 6500 feet at Republic and to 2500 feet on the northern edge of the Columbian Plateau which was the southern edge of the glacier. The Kettle River Mountains, in almost the center of the glacier, remained a peninsula or driftless area that divided the glacier into two lobes. The Pend Oreille, Huckleberry and other mountain ranges, formed nunataks, or islands above the ice, at the southern part of the glacier.

The behavior of valley glaciers in the northern Cascade Mountains during Vashon-Wisconsin time, seems to have been variable. Some depression of the snow line, at least in the north, seems probable.

The Vashon Glacier impinged on the eastern, northern and to some extent the western, slopes of the Olympic Mountains. Late Pleistocene valley glaciers in the Olympics, however, seem to have been inconsequential.

The time of the retreat of the Vashon-Wisconsin glaciers is a subject of special interest to the mammalogist in that it represents time for invasion and dispersal of species and in that it represents generations of individuals upon which natural selection might act. It is generally agreed that a period of approximately ten thousand years has elapsed since the retreat of the Vashon-Wisconsin glaciers from Washington.

Information on the climate of the state of Washington previous to the period of the last continental glaciation is understandably meager. Bretz (1913) considers the Puyallup period a time of excessive precipitation and erosion. Bits of lignite from Puyallup sediments seem to be of Douglas fir. Presumably the climate was slightly warmer and more humid than it is today. Vegetation possibly consisted of coniferous forests.

With the advance of the Vashon ice, mammals north of the ice border were all or mostly eliminated. Climatic conditions south of the border of the ice probably were strongly affected by it. Remains of mammoths have been found in Vashon till. The presence of many non-boreal species of mammals in southwestern Washington indicates their persistence there and that conditions therefore were not intolerable for them. Probably the climate of southwestern Washington was cool and dry. Fir, spruce, and Douglas fir may have been the dominant trees. Hansen (1941 A: 209) found evidence from studies of pollen that coniferous forests were growing in west-central Oregon in late glacial time. These pollen studies of postglacial peat bogs by Henry P. Hansen give evidence of postglacial climatic changes. Hansen points out (1941 B, 1941 C) that climatic changes west of the Cascades were probably slight because of the influence of the Pacific Ocean. Pollen profiles indicate an early, cool, dry climate followed by a warmer one and increasing humidity. The present climate may be considered cool and humid. Most of western Washington lies in the Humid subdivision of the Transition Life-zone.

Fig. 18.

Fig. 18. McDowell Lake, Little Pend Oreille Wildlife Refuge, Stevens County, Washington, September 29, 1939. (Fish and Wildlife Service photo by Victor B. Scheffer, No. 730.)

The pre-Wisconsin climate of eastern Washington is unknown. From the loessial nature of the Palouse Soil (Bryan, 1927), a preglacial deposit, the area would seem to have been arid, probably a grassland or a sagebrush desert. If the fossil fauna discovered at Washtuckna Lake, Adams County, is of this period, the forest conditions of the Blue Mountains were slightly more extensive than at present. The glacial climate of the Columbian Plateau in Wisconsin time was probably cool and arid. Pollen studies by Hansen (1939, 1940) indicate increasing dryness and warmth since the retreat of the Wisconsin ice. Present-day climate on the Columbian Plateau is warm and dry. The plateau is principally a sagebrush desert. The glaciated area to the north is cooler and more humid, supporting extensive forests of yellow pine and other conifers.

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