I really don't know if there's any way I can explain it so that it helps you. It looks to me that it's a series of triangles radiating from the center. The midpoint of the base of each triangle radiates from the center. They look narrow/tall when they are at the highest and lowest part of the sidwall, as they make their way towards the side they become fatter so you'll see more of them.
Sorry for the crappy sketch.
