Modelling erratic dispersal accounting for shifting ice flow geometries: A new method and explanations of erratic dispersal of the British–Irish Ice Sheet

Glacial erratics are geologically distinctive rocks transported away from their source area by ice sheets and deposited in lithologically different bedrock areas. They have attracted much scientific curiosity with >24 000 observations across the British Isles. A common misinterpretation is that they took a nearly direct line of transport from source to resting position, neglecting to change ice flow directions during ice sheet growth and decay. To rectify this, we sequentially modelled erratic time-space trajectories at 1000-year timesteps using ice flowlines in an empirically constrained ice sheet model simulation to predict erratic deposition areas. We addressed the processes of entrainment and deposition by combining all potential trajectories into a single footprint of possible locations. Erratic dispersal is predicted for three geologically distinctive lithologies; Shap Granite of Northern England, Galway Granite of Ireland and the Glen Fyne igneous complex from Scotland. The footprint of predicted trajectories compared against 1883 observations of erratic locations was found to successfully explain 77% of the observed erratics. Most erratics were explained by flow directions during ice retreat; however, some required earlier ice divide shifts to produce potentially long-duration, multiphase pathways. Our analysis demonstrates the possibility of explaining many erratics without explicitly modelling the complex processes of entrainment and deposition.