The timing and extent of former glacial advances can demonstrate leads and lags during 2 periods of climatic change and their forcing, but this requires robust glacial chronologies. In 3 parts of southernmost Patagonia, dating pre-global Last Glacial Maximum (gLGM) ice limits 4 has proven difficult due to post-deposition processes affecting the build-up of cosmogenic 5 nuclides in moraine boulders. Here we provide ages for the Río Cullen and San Sebastián 6 glacial limits of the former Bahía Inútil-San Sebastián (BI-SSb) ice lobe on Tierra del Fuego 7 (53-54°S), previously hypothesised to represent advances during Marine Isotope Stages 8 (MIS) 12 and 10, respectively. Our approach uses cosmogenic 10Be and 26Al exposure 9 dating, but targets glacial outwash associated with these limits and uses depth-profiles and 10 surface cobble samples, thereby accounting for surface deflation and inheritance. The data 11 reveal that the limits formed more recently than previously thought, giving ages of 45.6 ka 12 (+139.9/-14.3) for the Río Cullen, and 30.1 ka (+45.6/-23.1) for the San Sebastián limits. These dates 13 indicate extensive glaciation in southern Patagonia during MIS 3, prior to the well-14 constrained, but much less extensive MIS 2 (gLGM) limit. This suggests the pattern of ice 15 advances in the region was different to northern Patagonia, with the terrestrial limits relating 16 to the last glacial cycle, rather than progressively less extensive glaciations over hundreds of 17 thousands of years. However, the dates are consistent with MIS 3 glaciation elsewhere in 18 the southern mid-latitudes, and the combination of cooler summers and warmer winters with 19 increased precipitation, may have caused extensive glaciation prior to the gLGM.