I've noticed, when digging a trench around a copper node, that I often reach a depth where I can't seem to dig deeper. This comes to a surface which is a rather tan in color and relatively smooth, although not all on the same level but rather somewhat varied. No matter how hard I continue pounding on this it goes no deeper. Is this an absolute barrier to digging or can one go deeper somehow? I've tried both the antler pick and bronze pick with the same results.