Turf-Banked Terraces in the Olympic Mountains, Washington, U.S.A.

Abstract

Turf-banked terraces are either poorly developed or absent in most mountains of the Pacific Northwest. They are well developed, however, in the northeast section of the Olympic Mountains, Washington. Here rain shadow effects promote a continental climatic regime, seasonally frozen ground, and periglacial geomorphic activity. In addition, the northeastern Olympics consists of flattened and rounded ridges composed of sedimentary rocks. These upper surfaces were exposed as nunataks during Pleistocene glacial advances and prolonged weathering has produced fine material from which the turf-banked terraces are formed. Preferred sites are located above timberline at 1725 to 1950 m on northerly exposures with slope gradients of 12 to 24°. Three plant communities (Late-Snowbank, Tread, and Riser) were distinguished across the terraces. The features were generally completely vegetated with surface humus layers 5 to 20 cm thick, bound with extensive root systems. Soil textures, exclusive of rocks, averaged 62% sand, 20% silt, and 18% clay. Some terraces have buried organic layers. In situ rock measurements indicate an average soil movement rate of 0.8 to 6.0 mm y⁻¹ over an 8-yr period with an overall mean displacement of 2.8 mm. This is an order of magnitude less than measurements made in other active solifluction areas. It is suggested that the features are presently only marginally more active than in the past when more severe climatic conditions existed. There is good evidence for the former presence of permafrost which would have facilitated the origin and development of the features.