Alder Ridge Landslide

Twenty gullies cut the Alder Ridge Landslide. The slide plane resides in the clayey Selah interbed (Ellensburg Fm) and involves Miocene basalt bedrock. Pleistocene surficial units ride on top and partially cover the slide material.

Sheeted dikes cut stratified, flood-reworked basaltic colluvium atop older landslide deposits. The angular material was originally deposited as scree beneath a basalt cliff (hillslope deposit) and later entrained and transported downstream by floodwater (flood deposit). Exposure is a 2.5m high vertical bank in a new gully formed during a heavy rain event and flash flooding. The deposit and sheeted dike are Pleistocene age.

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. "Debris flow" deposits are flood-reworked colluvium (coarse facies of Missoula flood beds) separated by silty-sandy material (fine facies). Clastic dikes intrude the entire stack and are the same age as floods.

Missoula flood rhythmites preserved near the mouths of gullies are composed of reworked sand and silt (formerly 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, which varies in thickness along the Columbia Gorge. Figure from Newcomb (1971).