Abstract

Satellite imagery of Donegal Bay, northwestern Ireland, reveals two streamlined subglacial bedform sets produced during the late Devensian glaciation (ca. 22,000-15,000 yrs B.P.). The first bedform set trends northeast-southwest and records fast ice flow from inland ice domes onto the eastern Atlantic continental shelf. The second bedform set, trending east-west crosscuts and partially reorientates the first. Morphological and sedimentary evidence show that the second set corresponds to the last phase of fast ice flow (drumlinization) in northwestern Ireland. Morphological characteristics of the Donegal Bay drumlin field, and sedimentary characteristics of the Mullinasole drumlin, Donegal Bay, support a two-stage interpretation of drumlin evolution. These are the following. (1) Deposition of glaciomarine mud and diamict facies at a tidewater glacier margin. Stratified diamicts record debris flow events and sediment reworking. This facies sequence is erosionally truncated. (2) Deposition of subglacial diamict and sand facies recording ice readvance and drumlinization. Drumlinization (sediment streamlining) reflects ice mass-balance destabilization, episodic fast ice flow and ice-marginal oscillation, and may be correlated with millennial-time scale climate changes in the circum-North Atlantic.