And so I built the slumpstone wall upon a course of broken pink concrete. In retrospect, I wish I had built the entire wall out of slumpstone, but I say now that the lowest part of the wall represents the "basement" rocks, whereas the slumpstone and its layered look represent the overlying sedimentary strata. The wall at this point still needs a course of cap bricks. I was able to return the unused slumpstone to the building materials store.