2/20/2011 Email from Jim E. “possibly worth a note in EcoBlog (a geologist, Jim Benedict, I knew used the technique to date Ute rock barriers above timberline to funnel elk or deer for the kill):
Benedict: http://www.jstor.org/pss/1551260” Thanks so much, Jim, for sending this information, quite fascinating!!

Here’s the link Jim sent  General: http://en.wikipedia.org/wiki/Lichenometry and the first few sentences from WikiPedia…..

Lichenometry

Picture of the Map lichen or Rhizocarpon geographicum, the most used lichen in lichenometry.

In archaeology, palaeontology, and geomorphology, lichenometry is a geomorphic method of geochronologic aging that uses lichen growth to determine the age of exposed rock: lichens are presumed to increase in size radially at specific rates as they grow. Measuring the diameter of the largest lichen of a species on a rock surface can therefore be used to determine the amount of time that the rock has been exposed. Lichen can be preserved on old rock faces for up to 10,000 years, providing the maximum age limit of the technique, though is most accurate (within 10% error) when applied to surfaces that have been exposed for less than 1000 years^[1]. The use of lichenometry is of increased value for dating deposited surfaces over the past 500 years as radiocarbon dating techniques are less efficient over this period.^[2] The most common lichen used for lichenometry are that of the genus Rhizocarpon, for example the species Rhizocarpon geographicum, and those of the genus Xanthoria.

Lichenometry can provide dates for glacial deposits in tundra environments, lake level changes, glacial moraines, trim lines, rockfalls, talus (scree) stabilization and former extent of permafrost or very persistent snow cover.

Among the potential problems of the technique are the difficulty of correctly identifying the species, delay between exposure and colonization, varying growth rates from region to region as well as the fact that growth rates are not always constant over time, dependence of the rate of growth upon substrate texture and composition, climate, and determining which lichen is the largest.