228-6 Reconstructing Alpine Glaciation on Pinegrass Ridge, WA Cascades: Insights from Field Mapping and High-Resolution LiDAR

Session: Advances in Mountain Hydrology: Connecting Cryosphere, Surface, and Subsurface Processes (Posters)

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Abstract:

The timing and extent of alpine glaciation in the WA Cascades remain poorly understood, with surficial mapping in many regions lagging behind global efforts. This study presents a reconnaissance-level surficial map of glacial landforms and deposits on Pinegrass Ridge (PR), located on the northeast flank of the Goat Rocks volcanic complex. We identified and documented glacial features spanning from early to late Pleistocene using 1-meter resolution LiDAR, Google Earth Pro, ArcGIS Pro, and targeted field mapping. In the northern and central portions of PR, a ∼3-5 m thick layer of till is overlain by a large boulder till that is matrix to clast supported. This till rests atop the Tieton Andesite (Qta2), which is sporadically exposed in these areas due to subsequent erosion and glacial stripping. In north-central PR, recessional moraines were identified containing boulders ∼0.5-3 m in size. These till ridges, extending roughly 0.25 km, are aligned in the direction of glacial flow. In northwest PR, massive ∼3x8m boulders aligned at 095° form a low-profile ridge interpreted as a lateral moraine. Nearby, glacial striations on Tieton Andesite (Qta2) trend 026°, with parallel till ridges oriented ∼049°. Southward, till thickness decreases, bedrock exposure increases, and large boulders are confined to the southeastern margin. The southwestern sector has thin till near the rim above the North Fork Tieton River but lacks significant boulders. Linear ridges of till in the southern region trend ∼080° in the southeast and ∼054° in the south-central zone. Younger glacial features across the study area include cirques, rock glaciers, kames, recessional and lateral moraines, drumlins, and roche moutonnées. There are a minimum of three recognized glaciations in the PR area; oldest to youngest are associated with Marine Isotope Stage (MIS) 56, 40, and 2. These findings enhance our understanding of the glacial history, landscape evolution, and paleoclimate of the region and contribute to broader efforts to refine the chronology and geomorphology of alpine glaciation in the Cascades. 

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

doi: 10.1130/abs/2025AM-10858

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