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Quarrying and abrasion are the two principal processes responsible for glacial erosion of bedrock. The morphologies of glacier hard beds depend on the relative effectiveness of these two processes, as abrasion tends to smooth bedrock surfaces and quarrying tends to roughen them. Here we analyze concentrations of bedrock discontinuities in the Tsanfleuron forefield, Switzerland, to help determine the geologic conditions that favor glacial quarrying over abrasion. Aerial discontinuity concentrations are measured from scaled drone-based photos where fractures and bedding planes in the bedrock are manually mapped. A Tukey honest significant difference test indicates that aerial concentration of bed-normal bedrock discontinuities is not significantly different between quarried and non-quarried areas of the forefield. Thus, an alternative explanation is needed to account for the spatial variability of quarried areas. To investigate the role that bed-parallel discontinuities might play in quarrying, we use a finite element model to simulate bed-normal fracture propagation within a stepped bed with different step heights. Results indicate that higher steps (larger spacing of bed-parallel discontinuities) propagate bed-normal fractures more readily than smaller steps. Thus, the spacing of bed-parallel discontinuities could exert strong control on quarrying by determining the rate that blocks can be loosened from the host rock. [ABSTRACT FROM AUTHOR]

The subglacial processes of abrasion and quarrying are thought to be primarily responsible for bedrock erosion by glaciers. While theory points to sliding speed as the dominant control on abrasion, rates of quarrying are likely scaled by a more complex combination of sliding speed, effective pressure, bed roughness, and short‐term water‐pressure fluctuations. Here we pair a model for quarrying based on statistical characterization of bedrock strength with a model for subglacial hydrology that describes the temporal evolution of cavities under the influence of variations in sliding speed and effective pressure. Using a finite element model, we simulate the evolution of the hydrological system at the base of a glacier and compute rates of abrasion and quarrying. Cavity lengths and channel cross sections evolve through time, causing temporal shifts in ice‐bed contact area, which in turn govern the differential stress that influences erosion over the course of a year. Our results demonstrate how variations in meltwater production amplify rates of subglacial erosion relative to the case of steady meltwater generation. The level of amplification depends on how the variations control the ice‐bed contact area. Seasonal variations are most effective in boosting mean rates of basal sliding and hence subglacial abrasion, whereas shorter‐term variations (monthly‐weekly) most strongly influence rates of subglacial quarrying through temporal amplification of differential bedrock stress around cavities. This influence of transient hydrology on subglacial erosion processes may explain why glaciers in temperate climates with strong variations in temperature and precipitation erode faster than similar‐type glaciers in polar environments. Key Points: We model the influence of glacial hydrology on rates of subglacial abrasion and quarryingThe model experiments record the ice‐bed contact area and stress over time and use these to predict rates of basal sliding and erosionOur experiments show that meltwater variability consistently accelerates glacial erosion [ABSTRACT FROM AUTHOR]