What is this?

[HungLikeJesus]

Originally Posted by Uisge Beatha

You could be right (I originally dismissed the appearance in picture #1 as such), but the second and third photos do show trapezoidal pieces. They also seem to indicate a spread in the border of the marks on the patio.

Not optical illusion; optional illusion. It's not for everyone.

And is it an Ark or an arc?

With the exception of the split board, I think that they are all straight 2x8 or 2x10s.