The lichen can be measured and then calculated to see how long it has been growing.
One measures the lichen from a center point to the outside edge.
The three main types of lichens found in New England are: crustose, foliose, and fructicose.
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.
They can even live in places like the Arctic and almost any other solid surface.
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.
Then the centimeters can be multiplied by five to get the latest date that the wall could have been built.
There are certain problems with this method of dating.Each specie looks different and has distinguishing characteristics which differentiate the different types.Crustose lichens comprise of about 75% of all the lichens in the world.There are three main types of lichens in New England: crustose (see Figure 1), foliose (see Figure 2), and fructicose (see Figure 3).Within these three categories of lichens scientists have found about 13,500 species of lichens on earth.Somewhat recently scientists have come up with a new technique for dating certain artifacts by means of measuring lichens.