Umbilicaria americana, FROSTED ROCK TRIPE
This species and its relatives are known as “rock tripe” and said to be edible, though perhaps only in dire situations. For example, the related species Umbilicaria mammulata reportedly fed George Washington and his starving troops during the harsh winter of 1777 at Valley Forge. A distinctive and abundant lichen on moist, vertical cliff faces in the Front Range, it lacks apothecia and has a sooty black underside covered with rhizines, creating a velvety appearance.
Thalli are leathery (and green!) when wet, and are “umbilicate” (attached only at one central point); they can be 15 cm across. Largest and most notable of our umbilicate species, this often grows in huge colonies.Fun fact: Some species of Umbilicaria have intriguing black “gyrose” apothecia with concentric fissures. This species, however, is rarely seen with apothecia.
Don’t be fooled: Umbilicaria vellea is similar, but usually smaller and generally found at higher elevations or latitudes. Details of the rhizines distinguish the two.