The Surface, Not the Stone — Horn on Lichenometry

The Surface, Not the Stone By Horn

“How old is this stone?” sounds close to “How long has this face been exposed?” Close enough to trip over. They are different questions.

The rock formed before the lichen arrived. A fracture, retreating ice, or a shifting stone may expose a face much later. Lichenometry tries to read that surface’s exposure history, not issue the rock a birth certificate.

Start with a diameter: measure across a lichen patch. In a common approach, look for the largest suitable patch of a chosen species on the surface. That measurement is a length, not yet a duration. A ruler has done its job; now the assumptions have to do theirs.

To turn size into an estimate, you need evidence for how that species grows under comparable local conditions, ideally calibrated against surfaces whose exposure dates are independently known. There is no growth rate that fits every lichen on every stone.

There is also a small but important trap in the arithmetic. Diameter runs across the whole patch; radial growth runs outward from its centre. Under a simplified constant radial-growth assumption, you would halve the diameter before dividing by that rate. But the neatness of the calculation does not establish that growth was constant. A locally calibrated growth curve may tell a less tidy story.

Even an estimate of the lichen’s growing time leaves another gap: the surface could have stood bare before colonisation. Growth conditions can change. Earlier patches can die. The largest patch still present is not guaranteed to have been the first.

Shaping a board teaches respect for what a measurement actually describes. Width is not buoyancy. Here, diameter is not time until a defensible growth relationship connects them—and the lichen’s growing time is not automatically the whole exposure interval.

The useful answer is therefore an exposure estimate, with its calibration and uncertainties attached. Not the stone’s geological age. Not an exact clock. A living trace on a surface, read with care.