Borrowdale Banksy on Dalehead: does friction oppose gravity?

 

We thought there were only two pieces by the Borrowdale Banksy but we found a third on the path up to Dalehead from Honister Slate Mine. You can't miss it if you follow the fence. It got me thinking again about what keeps the stones up around the top of an arch. I was thinking friction because the slate is made from volcanic tuff which is a bit rougher than slate metamorphosed from mudstone. But then I remembered that the stones are always leaning in slightly on the ones above so that with gravity pulling the ones above down, there is a normal contact force from the lower stones pushing back. This is at 90 degrees to the surface and provided the stones are angled, they will have some vertical component that opposes the weight. In other words, this should work without friction. Even so, hats off! I can discuss the abstract Physics but wouldn't manage the practicalities.