I've included a picture of the St. Machar Green Man, which Trey scoped out after hearing the lecture, and I've included an informative description from my favorite website Wikipedia to help explain the historical significance of these cathedral fixtures:
A Green Man is a sculpture, drawing or other representation of a face surrounded by (or made from) leaves. Branches or vines may sprout from the nose, mouth, nostrils or other parts of the face, and these shoots may bear flowers or fruit. Commonly used as a decorative architectural ornament, Green Men are frequently found on carvings in churches and other buildings (both secular and ecclesiastical).
Superficially the Green Man would appear to be pagan, perhaps a fertility figure or a nature spirit, similar to the woodwose (the wild man of the woods), and yet he frequently appears, carved in wood or stone, in churches, chapels, abbeys and cathedrals, where examples can be found dating from the 11th century through to the 20th century
To the modern observer the earlier (Romanesque and medieval) carvings often have an unnervingly eerie or numinous quality. This is sometimes said to indicate the vitality of the Green Man, who was able to survive as a symbol of pre-Christian traditions despite, and at the same time complementary to, the influence of Christianity. (Rather than alienate their new converts, early Christian missionaries would often adopt and adapt local gods, sometimes turning them into obscure saints.)
Whatever his original significance may have been, many modern churchgoers characterise the Green Man as "the archetype of our oneness with the earth". The symbol is also popular with modern Wiccans and other Neopagans because it depicts an earth-centered concept of male divinity.