ORIGINS OF THE ICE-CONTACT STRATIFIED RIDGES (ESKERS) OF IRELAND

An extensive system of ridged landforms composed of ice-contact strati fied sediment was deposited in the central lowland of Ireland during t he most recent deglaciation (< 18,000 BP). The ridges have been interp reted by others as deposits of an ice-sheet drainage system (i.e, ''es kers'') 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 anal ysis of the ridged landforms using lithofacies, sedimentary structures , and paleocurrent data in conjunction with the geomorphology, indicat es that the pattern and nature of the ridges am not compatible with th is model and support a new model of deglaciation (Warren 1991) in whic h the ''esker'' system formed in an interlobate am during the simultan eous shrinking of two main glacial outflow centers. An extensive like system developed in the lowland between the two outflow centers. Ridge s 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-contac t ridges are polygenetic, and a genetic classification is proposed. Th ose that formed perpendicular to the ice margin are termed eskers; tho se that formed parallel are termed moraines. Distinction between esker types (continuous or beaded subglacial tunnel fills, fluvial ice-chan nel fills, and subaqueous fans) and moraine types (subaqueous or subse rial) is crucial to a reconstruction of the mode and pattern of deglac iation in the central Irish lowlands.