Since the retreat of glaciers after the Last Glacial Maximum, rock avalanches have occurred intermittently in Yosemite Valley, California. We investigated the distal portion of the oldest of these, the Royal Arches Meadow rock avalanche, which has been partially buried by sediment aggradation. Cosmogenic ¹⁰Be exposure ages of boulders within the deposit indicate that the rock avalanche occurred at 16.1 ± 0.3 ka, immediately after deglaciation and thus prior to most aggradation. The interface between the rock avalanche deposit and the underlying glaciofluvial sediments therefore provides an elevation marker of the valley floor at the time of deposition. To identify the elevation of this interface, we collected eight Ground Penetrating Radar (GPR) and five Electrical Resistivity Tomography (ERT) profiles across the rock avalanche. Both methods are sensitive to contrasts between the granitic avalanche deposit and the underlying sediments. By constraining ERT inversions with GPR interfaces that are continuous across the profiles, we identified a single interface, interpreted as the basal contact of the rock avalanche, that separates resistive material from conductive material underneath. The elevation of this approximately horizontal interface is between 1,206 and 1,209 m, roughly 10 m below the modern ground surface, indicating ≈ 10 m of sediment aggradation since deglaciation. Based on topographic expression and depth to this contact, we determined a minimum volume estimate of between 8.1 × 10⁵ m³ and 9.7 × 10⁵ m³, nearly three times larger than what would be estimated from surface expression alone. Our findings allow reconstruction of the sedimentation history of Yosemite Valley, inform hazard and risk assessment, and confirm that geophysical methods are valuable tools for three-dimensional investigations of rock avalanches, particularly those buried by younger sediments.

自从上次冰川期之后冰川退缩以来，加利福尼亚优胜美地山谷间歇性地发生了雪崩。我们调查了其中最古老的部分，即皇家拱门草甸岩石雪崩的远侧部分，该部分已因沉积物沉积而被部分掩埋。宇宙成因¹⁰沉积物中巨石的暴露年龄表明，雪崩发生在冰消融之后，因此在大多数凝结之前，发生在16.1±0.3 ka的岩石上。因此，岩石雪崩沉积物与下伏的冰川河流沉积物之间的界面在沉积时提供了谷底的高程标记。为了确定该界面的高程，我们在整个岩石雪崩中收集了八个探地雷达（GPR）和五个电阻层析成像（ERT）剖面。两种方法都对花岗岩雪崩沉积和下伏沉积物之间的对比很敏感。通过使用在剖面上连续的GPR界面约束ERT反演，我们确定了单个界面，该界面被解释为岩石雪崩的基础接触，将下面的电阻材料与导电材料分开。这个近似水平的界面的高度在1,206至1,209 m之间，大约在现代地面以下10 m，表明自冰消期以来沉积物的沉积约为10 m。根据地形表达和该接触点的深度，我们确定最小体积估计为8.1×10⁵ m ³和9.7×10 ⁵ m ³，几乎是仅根据表面表达估计的三倍。我们的发现可以重建优胜美地山谷的沉积历史，为灾害和风险评估提供依据，并确认地球物理方法是三维研究岩石雪崩（尤其是被年轻沉积物掩埋的雪崩）的有价值的工具。