| Summary: | The dynamics and paleo‐glaciology of ice sheet interiors during the last deglaciation are poorly constrained, hindering ice sheet model. We provide direct evidence of Fennoscandian Ice Sheet (FIS) interior behavior during deglaciation through surface exposure dating. Our results demonstrate early thinning of the FIS, prior to the Younger Dryas (YD, 12.8–11.7 ka). Interior thinning in central Norway was concurrent with retreat along the coastline, exposing ice‐free mountainous tracts, potentially as early as 20–15 ka. The FIS then formed moraines in these ice‐free tracts during the YD. This is contrary to current hypotheses advocating a landscape fully covered by cold, inactive ice during this period. Present empirical and model reconstructions fail to capture rapid interior downwastage, increasing uncertainties in ice sheet volume estimates and sea level contributions.
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