We estimated the age of inscriptions on a rock outcrop by estimating the ages of lichens that had overgrown the inscriptions. The inscriptions are considered to be historically important, potentially representing some of the earliest European exploration of Neahkahnie Mountain, the highest point along the Pacific coast from Baja California to Strait of Juan de Fuca, Washington. The rock bearing the inscriptions was destroyed by road construction activities in about 1970–1980, but the inscriptions had been photographed with sufficient detail to allow diameter estimates for the lichens on the rock, affording an opportunity for dating based on lichen sizes. Aspicilia and Placopsis are currently the only lichen genera that are common on similar outcrops in the area and form large light-colored discrete individuals with a radial form. We therefore derived a calibration curve for lichen size in relation to age based on Aspicilia and Placopsis sizes on nearby surfaces of known age (road cuts and stone walls), then applied that curve to the diameters of lichens in the photo. Based on the sizes of the lichens on the rock outcrop with inscriptions, the rock face had been available for lichen colonization and growth for > 100 yrs and perhaps shows a pulse of recruitment following extensive wildfires on the immediate coast in the 1840s. Calculated lichen ages are within 25 years of the expected time of US Army exploration of Neahkahnie Mountain under Captain C. C. Augur in the mid-1800s.