Analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38% between the oldest and youngest boulders from each moraine. This range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform. The wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders. A diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders (n ϭ 3-7) will always yield a lower age limit for the moraine. The model indicates that for identical dating accuracy, six to seven boulders are needed from old and tall moraines (40,000 -100,000 yr, 50 -100 m initial height) but only one to four boulders from small moraines (20,000 -100,000 yr, 10 -20 m). By following these guidelines the oldest obtained boulder age will be Ն90% of the moraine age (95% probability). This result is only weakly sensitive to a broad range of soil erosion rates. Our analysis of published boulder ages indicates that Ͻ3% of all moraine boulders have prior exposure, and 85% of these boulders predate the dated moraine.

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