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Lichens are very sensitive to pollution from the atmosphere which causes them not to grow in highly polluted areas.An artifact might not have any lichens on it, yet it could still be much older than expected.In our case we are using this method to date stonewalls.The Golden Age for stonewalls is said to be from 1775-1825Lichens, mainly crustose, grow.The three main types of lichens found in New England are: crustose, foliose, and fructicose.Each species of lichens has distinguishing characteristics which separates one from another.Lichenometry is often the only way to find out when a stone wall was built. On dry days, lichens can become inactive and their metabolism can change significantly.' Lichens can live and grow almost anywhere under very harsh conditions.
There are certain problems with this method of dating.
They vary in color like orange, yellow, green, brown, gray, or black (see Figure 1). They grow so tightly to the rock that it is nearly impossible to pull them off.
Crustose lichens are the most commonly used in dating a stone wall because they have the slowest growth rate of all lichen species.
Sometimes a wall will be built with rocks that already had lichens growing on them. There are certain ways in getting around these setbacks like looking to see if lichens are growing across the entire wall or just a small portion of it.
One way to be more precise is to see if there is more than one colony of lichens on the wall.