Origins of the Ice-contact Stratified Ridges (Eskers) of Ireland

Abstract

ABSTRACT An extensive system of ridged landforms composed of ice-contact stratified sediment was deposited in the central lowland of Ireland during the most recent deglaciation (< 18,000 BP). The ridges have been interpreted by others as deposits of an ice-sheet drainage system (i.e.,eskers) and as such have been used with other data to reconstruct the deglacial history. The traditional deglaciation model shows systematic retreat of ice from south to north. Our study, which involves an analysis of the ridged landforms using lithofacies, sedimentary structures, and paleocurrent data in conjunction with the geomorphology, indicates that the pattern and nature of the ridges are not compatible with this model and support a new model of deglaciation (Warren 1991) in which the system fo med in an interlobate area during the simultaneous shrinking of two main glacial outflow centers. An extensive lake system developed in the lowland between the two outflow centers. Ridges formed both perpendicular and parallel to ice margins, involved both active and stagnating ice, are both continuous and segmented (beaded), and were deposited in subaqueous and subaerial environments. Almost all of the ridges are associated with lacustrine sediments. Ice-contact ridges are polygenetic, and a genetic classification is proposed. Those that formed perpendicular to the ice margin are termed eskers; those that formed parallel are termed moraines. Distinction between esker types (continuous or beaded subglacial tunnel fills, fluvial ice-channel fills, and subaqueous fans) and moraine types (s baqueous or subaerial) is crucial to a reconstruction of the mode and pattern of deglaciation in the central Irish lowlands.