Wednesday, September 18, 2019

Toppenish Ridge Landslide near Mabton

Highway 97 south of Toppenish provides a nice view of a large landslide at the east end of Toppenish Ridge.   


Toppenish Ridge is underlain by Columbia River Basalt Group, specifically the Elephant Mountain Member - one of the younger lava flows of the CRBG. Toppenish Ridge is one of several folds of the Yakima Fold Belt that has rumpled the lava flows of the CRBG. There are multiple landslide complexes on the steep slopes of these ridges as a result of the fractured basalt lava with weak sedimentary units in between the lava flows.

Most of the slides are relatively old. By relative, the general sense is that they may have taken place during or shortly after ice age floods impacted the area. As such the edges of many of these slides  has softened with time and weathering. But also with time and weathering and erosion, some of these steep ridge slopes will become weaker and will fail. I do not know if anyone has taken a try at dating this landslide, but is appears much younger with sharp scarps still intact throughout the slide complex. 

Bare earth lidar imagery of slide

The lidar imagery is pretty classic and shows the slide deposit extending over the flood plain of Toppenish Creek and covering over flood overflow channels. The creek has yet to begin any erosion on the slide deposit - another factor in the case of the slide being a young slide. Also note in the lidar imagery that the eastern portion of the slide has only partially moved down the slope and that there is a gravel quarry excavation right at the base of this slope that has removed a portion of the slide deposit.

This slide got some brief media attention when the landslide at Union Gap initiated a couple of years ago. The conditions are similar with basalt overlying weak sediments. The slide is dissimilar in that this one appears steeper. 

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