Lichenometry

In lichenometry, a lichen is presumed to grow radially at a specific, constant rate: this rate is used to estimate the age of the largest lichen on a rock surface, and thus the length of time the rock has been exposed.

The sophistication of approach to lichenometry has increased markedly in the last decade or two.

Lichenometry is a surface-exposure dating method that uses lichen-growth rates to infer the age of young ([a] few thousand years old or younger) glacial landforms, typically bouldery deposits such as moraines.[…]Several approaches have been employed to measure lichens, and a distinct advantage of lichenometry over other techniques is its simplicity (Bradwell, 2009).

Lichenometry is a dating method which is based upon the direct relationship between lichen diameter and age.