We attempted to test Thorson’s theory about the dates (1775-1825) of the “golden age of stonewalls” by using the inexact dating method of lichen growth measurement. Although there are environmental variables that make this dating technique less than perfect, it has proven to be somewhat effective. Lichens are symbiotic plants composed of a fungus and an algae and grow on stones and tree trunks. The two types of lichen most commonly found on new England stonewalls are foliose and crustose. Crustose lichens grow at a rate of one millimeter per year (100 millimeters would equal ten centimeters, or 59 years) The measurement is from the center to the edge, rather from edge to edge. (see lichen chart )
To check the accuracy of this dating technique, we measured the lichens on the back of colonial gravestones and check our estimated date with the date on the front of the stone. This method seemed reasonably accurate.