Highway 221 heading south of Prosser, Washington
View from the crest of the ridge towards Prosser and the Yakima Valley
The lumpy area in the foreground is a old landslide feature.
Old landslides are a common feature along the steep north facing slopes of this ridge. There are several reasons: 1) the slope is steep often over 45 degrees, 2) The north steep side of the anticlines on the Yakima fold belt are tightly folded and fractured with some minor faulting thus creating lots of fractured broken rock blocks, 3) Basalt is the predominant rock type and vertical columnar jointing provides weak plains in the rock and allows water to readily infiltrate, 4) sedimentary units between some of the basalt flows are clay rich and some of the more fractured basalt has altered to clay, and 5) the lower portion of the slope was rapidly eroded during the Missoula Floods - flood waters reached an elevation of approximately 1,250 feet well up the sides of the lower slopes.
The highway curves around this old landslide feature and cuts across the steep slope just above the slide area. Other possible slide areas are easily observed along Interstate 82 below the slope or from above by continuing along a gravel road along the crest of the ridge or by a tour of Google earth. I suspect that most of the lumpy topography is the result of old landslides; however, secondary folds, faults and aseismic deformation features are present in these areas as well (Reidel and Fecht, 1994 and Reidel, Campbell, Fecht and Lindsey, 1994) and some of the lumpiness may be a combination of these features along with slide areas.
Lumpy areas outlined in red are potential old landslide areas.