224-9 Landslide Distribution and Chronology in Pinegrass Ridge, WA: Insights from LiDAR, Drone Imagery, and Field Mapping

Session: From the Cosmos and Back: Quantifying Processes and Rates of Landscape Change (Posters)

Presenting Author:

Authors:

Abstract:

Steep, rocky terrain and dense vegetation in the Cascade Mountains make it difficult to identify and map landslides using traditional methods. In this study, we use high-resolution LiDAR (WA DNR and NOAA databases), Dji Air 3s drone imagery, and field mapping to characterize the morphology, timing and distribution of landslides within the Pinegrass Ridge (PR) area of Goat Rocks volcanic complex, WA Cascades. PR is bounded by the N and S Forks of the Tieton River and hosts over 60 documented rotational slides, earthflows, slumps, lateral spreads, and complex slides. Several younger earthflows developed on older slumps and rotational slides, suggesting multiple slope failure episodes over time. A notable example includes an older earthflow dissected by a large slump near the mouth of Bear Creek. Additionally, W-facing slopes appear to be dominated by deeply weathered complex landslides, while E and S-facing slopes exhibit deep-seated rotational failures. Five large rotational slides formed on the E-facing valley wall of the S Fork Tieton River, with at least two of these later developing earthflows. Four of these landslides block the S Fork of the Tieton, while the oldest slide is located on the west-facing slope. In the Camp Creek area, complex slides have dissected older glaciated rotational slumps. E of Section 3 Lake Cirque, glacial deposits have been dissected by landslides in Corral, Bear Creeks and along the S Fork Tieton River. In the Grey Creek area, older eroded landslides partially blocked and diverted the S Fork Tieton to the SE, marking the transition from a broad U-shaped glacial valley to a V-shaped river valley. Landslides and minor rock falls on the northern PR are highly eroded however, in the NW some eroded slumps with associated earth flows are recognized. Previous mapping along the western PR interpreted faults on the slope over the N Fork Tieton; however, LiDAR analysis re-interprets these as lateral spreads. Slope failures likely occurred over extended periods and continue today due to weak bedrock or poorly to unconsolidated sediments (e.g., tuffs). Landslides are still active along the downcutting N and S Forks of the Tieton River. Mapping in landslide-prone areas requires caution to avoid overthickening bedrock or volcaniclastic units, as may be the case with the Devils Horn Rhyolite.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10904

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.