The sea-level record preserved in ancient shorelines forms a basis for studies of tectonic uplift, glacial loading, and the changing volume of the oceans. The existing record is derived largely from depositional features such as beach ridges and coral reefs, which contain material suitable for radiometric dating. Erosional shorelines have proved more difficult to date. Direct age estimates for shore platforms can now be obtained with exposure-dating techniques based on cosmic-ray–produced isotopes. Here we report measurements of cosmogenic 36Cl on the Main Rock Platform in western Scotland that indicate its formation in a postglacial event spanning less than a few thousand years. Together with isostatic modeling, the 36Cl results suggest cutting during the Younger Dryas (in Britain, the “Lateglacial” or “Loch Lomond”) Stadial, when stable sea level and severe climatic conditions combined to enhance bedrock erosion.