Structural evolution of an actively retreating glacier (1945 - 2025) modulated by bedrock steps and terminal overdeepening: Sandfellsjökull, east Mýrdalsjökull, Iceland.

Glaciological research has demonstrated that mechanisms such as fracturing, faulting, and foliation play a fundamental role in controlling ice flow patterns, debris entrainment, and styles of glacier retreat across diverse dynamic settings. While these relationships are well-documented for High-Arctic polythermal and cold-based glaciers, and increasingly for marine-terminating systems, the structural evolution of active temperate valley glaciers remains comparatively understudied. This study addresses this gap through an investigation of Sandfellsjökull, an active temperate outlet of Mýrdalsjökull Ice Cap. Previous research at Sandfellsjökull has focused primarily on proglacial geomorphology, leaving the evolution of its internal structure and margin dynamics largely unconstrained. Here, a multi-method approach is applied to quantify surface and sub-surface characteristics, combining geospatial photogrammetric analysis of historical aerial orthophotographs, UAV-derived imagery, ITS_LIVE (Inter-mission Time Series of Land Ice Velocity and Elevation) products and digital elevation models (e.g., ÍslandsDEM v1.0) with detailed ice facies, structural, and sedimentological analyses at the glacier margin. These datasets are used to develop a conceptual model of structural evolution between 1945 and 2025 and to assess the influence of bedrock topography on deformation patterns and debris entrainment.

Preliminary results reveal pronounced lateral and down-glacier variability in structure and debris distribution. The southern margin exhibits longitudinal compressional crevassing and confined ice flow between bedrock outcrops, with supraglacial meltwater channels cross-cutting debris cones and feeding an active outwash system. In contrast, the northern margin is characterised by a concentric tephra band deforming around an undercut bedrock step and transitioning abruptly into stagnant, debris-covered ice undergoing passive downwasting. Stratigraphic analyses from 12 sections indicate a dominant dispersed ice facies with stratified debris bands entraining fine-grained tephra - likely derived from the 1918 Katla eruption - as well as angular basaltic lithologies derived from freshly plucked bedrock. These observations highlight the critical role of topography in governing glacier structure, debris entrainment, and retreat style, with implications for basal ice formation (regelation and glaciohydraulic supercooling) on adverse slopes. Ongoing work in this study integrates structural mapping with changes in fracture density, surface elevation and velocity, as well as meteorological data, to resolve the spatio-temporal evolution of Sandfellsjökull in the context of recent climate warming.