I have a terraced edwardian house in North Wales, built circa 1904. I need to lower the external ground level and when digging down have come across a thick piece of slate (at least 3 inches deep and maybe 4 inches wide) that runs along the wall. I think the DPC is where I've circled in red and the slate where the blue arrow is. Any thoughts on what the thick slate is? Could it be part of the footings?

