New cosmogenic isotope measurements from the northeastern margin of the Laurentide Ice Sheet (LIS) lead us to conclude that: 1) highly weathered landscapes cannot be used to argue against the presence of LGM ice, and 2) ice streams drained the LIS along northeastern Baffin Island during the LGM.

A long-standing debate over the reconstruction of former ice sheets revolves around the use of relative weathering of landscapes, i.e., the assumption that highly-weathered landscapes have not been recently glaciated. New cosmogenic isotope measurements from northeastern Baffin Island shed light on this debate. Average ¹⁰Be and ²⁶Al ages for two tors on a ~500 m upland adjacent to the mouth of Clyde Inlet, a 150-km-long fiord that penetrates interior Baffin Island, are >66.9±5.1 ka and >62.1±1.8 ka (SL, >60 Be PR=5.1 atoms/g; Al PR=31.1 atoms/g). The tors, on local summits ~100 m above the highest lateral moraines and other clear signs of actively-eroding ice, are deeply weathered, exhibiting large weathering pits, grus, and quartz veins and knobs that stand in relief. Three erratics perched directly on the ancient tors have average ¹⁰Be and ²⁶Al ages of 16.7±1.3 ka, 11.9±0.6 ka, and 10.5±0.8 ka.

These results suggest that non-erosive ice overran highly-weathered upland surfaces along the northeastern margin of the LIS during the LGM. These data invalidate the use of relative weathering to define LIS margins in the eastern Canadian Arctic, and possibly elsewhere, especially where perched blocks have been described on weathered upland surfaces along the fringes of Northern Hemisphere Pleistocene ice sheets. The proximity of non-erosive plateau ice to erosive fiord ice requires strong gradients in basal thermal regimes, suggestive of an ice-stream mode of glaciation. We propose that this style of glaciation dominated the fiord coastline of northeastern Baffin Island where the topography and glacial geology is similar to the Clyde region. Ice-stream-dominated ice-sheet margins are dynamic and unstable, and consequently, the northeastern margin of the LIS was probably involved in abrupt climate changes, especially those including the coupling of ice sheets and oceans (e.g., Heinrich Events).