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The relict moraines in Argentine Patagonia archive major expansions of the Patagonian Ice Sheet throughout the Quaternary, and are one of the few terrestrial climate proxies in the middle latitudes of the Southern Hemisphere that extends beyond the last glacial cycle. Determining their individual ages has proved challenging but has important implications for our understanding of terrestrial climate change in southern South America over the duration of the Quaternary. Here, for the first time, we demonstrate that sediment on outwash terraces can be directly dated to determine the timing of early–middle Quaternary glacial advances in southern South America. Cosmogenic ¹⁰Be and ²⁶Al surface exposure ages were obtained from outwash gravels associated with two of the oldest glacial sequences in the Lago Pueyrredón valley (47.5°S), Argentina. The outermost Gorra de Poivre glacial sequence marks the …