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The interpretation of differentially weathered mountainous areas along the fringes of Pleistocene ice sheets is fundamental for determining ice-sheet behavior and thickness during the last glaciation. Two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice, but that it was differentially erosive as a function of its spatially variable basal thermal regime. Cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern Baffi n Island shed light on Laurentide Ice Sheet (LIS) dynamics and thickness. Tors on weathered upland surfaces have minimum 10 Be ages between ca. 50 and ca. 170 ka (n = 12), whereas the majority of erratics perched in the uplands range from ca. 10 to ca. 13 ka (n = 14), indicating that the whole landscape was glaciated during the Last Glacial Maximum (LGM). Glacially sculpted bedrock (n = 7) near sea level refl ects the age of deglaciation, and increasing amounts of isotopic inheritance in highelevation sculpted bedrock (n = 3), higher elevation intermediately weathered bedrock (n = 11), and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus. Cold-based glaciation of uplands was contemporaneous with warm-based glaciation in fi ords, suggesting the presence of ice streams. 26 Al and 10 Be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure, burial, and limited glacial erosion. The paired isotope data reveal a minimum duration of burial of 80-500 k.y., presumably by cold-based ice, and indicate that the tors have an age of at least 150-580 ka. These data, together with other reconstructions of ice streams in the eastern Canadian Arctic, suggest that ice streams were a large infl uence on northeastern LIS dynamics throughout the late Quaternary. Thus, the northeastern LIS was sensitively tied with North Atlantic thermohaline circulation and abrupt climate change.

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Radiocarbon dating of bulk sediment has long been used as a method of last resort when reliable wood, charcoal, or plant macrofossils are not available for analysis. Accurate dating of sediment is complicated by the presence of multiple organic carbon fractions, each with a potentially different 14C activity. Additionally, the presence of carbon bound by clay minerals can significantly reduce the accuracy of a sediment age determination, with the oldest 14C ages seen in samples with the highest clay content (Scharpenseel and Becker-Heidmann 1992).