Eskers are elongate, straight to sinuous sedimentary ridges deposited in subglacial, englacial or supraglacial drainage networks (e.g. Warren & Ashley 1994). The following four basic esker types are distinguished (after Warren & Ashley 1994; Brennand 2000; Benn & Evans 2010): tunnel fills, formed in englacial and subglacial conduits and exposed by ice ablation; ice-channel fills, deposited in sub-aerial, ice-walled channels; segmented tunnel fills, deposited during stepwise glacier retreat; and beaded eskers, composed of subaqueous fans deposited during stepwise retreat of a water-terminating glacier. The planform of eskers can vary from uniform, simple ridges to complex, dendritic networks (Shilts et al. 1987; Warren & Ashley 1994). Eskers are usually aligned sub-parallel to the direction of former ice flow and the presence of subglacial eskers reveals the locations of conduits at the glacier bed. Furthermore, some tunnel-fill eskers may grade into ‘Nye’ channels cut into the substrate, reflecting a transition from deposition to erosion along the conduit axis (Benn & Evans 2010).