ABSTRACT. The endolithic lichen Lecidea auriculata is known to enhance rock surface weathering on the Little Ice Age moraines of the glacier Storbreen in Jotunheimen, central southern Norway. This study demonstrates the reduction in Schmidt hammer Rvalues that followed the rapid colonization by this lichen of pyroxene-granulite boulders on terrain deglaciated over the last 88 years. In the absence of this lichen, the characteristic mean R-value of boulder surfaces is 61.0 ± 0.3; where this lichen is present, R-values are lower by at least 20 units on surfaces exposed for 30–40 years. A similar reduction in rock hardness on rock surfaces without a lichen cover requires about 10 ka. The rapid initial weakening of the rock surfaces is indicative of rates of biological weathering by endolithic lichens that may be two orders of magnitude (200–300 times) faster than rates of physico-chemical weathering alone. If not avoided, the effects of this type of lichen are likely to negate the effectiveness of the Schmidt hammer and other methods for exposure-age dating, including cosmogenic-nuclide dating, in severe alpine and polar periglacial environments. The results also suggest a new method for dating rock surfaces exposed for <50 years.