Using lichen growth as a rough clock
Some crustose lichens grow at rates measured in fractions of a millimetre per year, and the rate is consistent enough over long spans that the diameter of a thallus can estimate how long a rock surface has been exposed. The technique is called lichenometry.
It works best in the range of roughly fifty to five hundred years, which is awkward for radiocarbon and useful for dating things like moraine deposition, rockfall and old stone construction. The method is calibrated by measuring lichens on surfaces of known age — gravestones are the classic reference, being dated, stone and undisturbed.
The caveats are substantial and the honest practitioners lead with them. Growth rate depends on moisture, aspect, substrate chemistry and how much the surface is disturbed. Calibration is local and does not transfer between regions. There is a colonisation lag between exposure and the first lichen that is itself uncertain. And the usual practice of measuring the largest thallus is statistically fragile, since the largest one may simply be lucky.
Treated as a rough clock with wide error bars, it is genuinely useful. Treated as precise, it is not.