Portland State University. Department of Geography
Larry W. Price
Date of Publication
Master of Arts (M.A.) in Geography
Mountain ecology -- Washington (State) -- Olympic Mountains, Terraces (Geology) -- Washington (State) -- Olympic Mountains
1 online resource, (93p.)
Well-developed turf-banked terraces are found in the Olympic.Mountains of Washington, but not found· in the other mountains of the Pacific Northwest. My study is based just on the Olympic Mountains and the·major thrust of my investigation is aimed at answering questions: What is the nature of turf-banked terrace development in the Olympic Mountains? Why does it occur, and what is its local ecological significance? The major field investigations were carried out during the summer of 1975.
A two hour reconnaissance was made over the Olympic Mountains to plot the distribution of turf-banked terraces; the most striking discovery was that terraces are limited to only a small area on the northeast side of the Olympics. Field measurements were made of slope orientation, slope angle, height. of risers, and length ?f treads. Vegetation transects were run across the terraces in order to 2 ascertain ecological significance. Internal characteristics of the terraces were investigated by making excavations through three representative terraces. Soil samples were collected and· laboratory tested for texture analysis.
Turf-banked terraces occur in the Pacific Northwest . only where the climate is of the continental-type and there are numerous freeze-thaw cycles, 'not where excessive moisture provides a mild, maritime climate. Terraces develop only on soil derived from sedimentary rock which has a high rate of decomposition and breaks into fine material and not on the· coarse soils derived from basalt or granite which have a slow rate of decomposition.
Within 'the Olympic Mountains, turf-banked terraces occur only in a small area in the northeast section which has a relatively dry continental climate. They do not occur in other areas of the Olympic Mountains which are very moist and have a maritime climate.
Within their area of occurrence, terraces develop on north-facing slopes which are protected from southwesterly winds and where soil is present. Terraces are found on the gentle, rounded alpine slopes, not exceeding 24° and not less than 12°, where only occasional outcrops of bedrock are found. These rounded ridges have apparently· not been glaciated but have undergone cryoplanation over a long period of time. Terraces do not occur on south-facing slopes which are generally steep and have little soil development. Terraces are not· found on the steep, jagged cirque heads nor on the active cirque walls.
Terraces are well-developed on north~facing slopes ' immediately below late-lying snowbanks which provide I ' moisture that saturates the soil
Animals frequent the terraces and cause downslope movement when the soil is saturated. Grazing removes vegetation and promotes frost activity in the soil. Hikers crossing the terraces crush and uproot flowers and stems, but the concentration of traffic on trails is better than being dispersed use since it limits the damage.
Movement takes place during spring when a frozen layer in the soil prohibits downward percolation of melt-water .from late-lying snowbanks. Higher velocities in the center of the lobe results in the typical lobate appearance of the terraces.
It is hypothesized that terrace development began between 2500 and 3000 years B.P. The terraces appear active today. Hummocky treads, bulging risers, collapsed lobes give evidence of movement.
Hansen, Katherine Jane, "The Nature and Distribution of Turf-banked Terraces in the Olympic Mountains, Washington" (1976). Dissertations and Theses. Paper 5400.