Capitals - Paired columns which usually have a square top and round base. These usually depict biblical scenes or legends of local saints.
Latin cross - most churches or abbeys would follow the shape of a Latin cross. The Latin cross is a universal symbol of Christianity.
Piers - columns were replaced with piers which were often used to provide support to arches. the piers are usually cruciform (cross-like) in shape when they intersect two big arches.
Probably the first thing that comes to my mind when I see that drawing is a cloud. Maybe draw a cloud and add details like shading or etc. Another idea that I have is a paw of a dog. I don't know, it's really up to you. I'm just stating what I would draw if I were you.
