Exam Connection — How This Topic Is Tested

Level 3 answer (linked analysis, 5-6 marks): "Plucking and abrasion work together in a positive feedback loop that accelerates glacial erosion over time. Plucking operates primarily on the lee (downhill) side of bedrock obstacles, where reduced pressure allows meltwater to refreeze in rock joints. As the glacier moves forward, these frozen-on blocks are quarried away, exposing fresh, irregularly fractured rock surfaces. These angular blocks become incorporated into the base of the glacier as subglacial debris — and immediately become tools for abrasion. The more debris plucking produces, the more tools are available for abrasion; the more abrasion occurs, the more rock flour is generated, which lubricates the glacier base and increases basal sliding velocity, in turn enabling more efficient plucking. The contrasting surfaces left by the two processes — the smooth, striated stoss face and the rough, jagged lee face of a roche moutonnée — provide diagnostic field evidence that both processes operated simultaneously at the same location." This answer links processes in a feedback mechanism, introduces an intermediate concept (roche moutonnée), and explains how field evidence connects to process understanding.