Twenty gullies cut the Alder Ridge Landslide. The slide is developed in the Selah interbed and involves Miocene basalt bedrock, but several Pleistocene surficial units ride on top and partially cover it.
Sheeted dikes cut stratified, possibly flood-reworked basaltic colluvium atop landslide deposits. Exposure is a 2.5m high vertical bank in one of the gullies.
Portion of a landslide inventory map by Fransziska Woelke. Alder Ridge is located between Crow Butte Island and McCredie.
Quick sketch of the geology at Alder Ridge.
Missoula flood rhythmites, preserved near the mouths of some gullies, are composed of reworked sand and silt (loess) in their upper portions and coarse, angular basalt cobbles swept from local hillslopes in their lower portions. Clastic dikes cut the entire section and penetrate the bedrock. Gully 3.
Gullies 1, 3, 6, and outlet of 7-12.
Ownership map of the area assembled from publicly available Klickitat County tax parcel information. "DNR" is Washington State Department of Natural Resources, "USFED" is Federal Bureau of Reclamation, "MHR" is McBride Hereford Ranches of Mabton, WA, "Yakima Nation" and "Indian" is the Confederated Tribes of the Yakama Reservation and/or Bureau of Indian Affairs.
The landslide is developed in the Selah interbed. Figure from Newcomb (1971).