I live in a house that's been a rental for eons. I've recently started renovating some preexisting beds for the purposes of veggie and herb gardening (mostly). In one bed, there was a layer of hard, rocky stuff about 3" thick, about 6" below the surface. It looks like crumbling asphalt/blacktop (not so good with my pavement terminology), and broke apart very easily. It took us a while to figure this out, so we had pretty much mixed it in with the rest of the soil and all of our amendments (including precious home-grown compost).
My question is: should we be worried? Is there a possibility that nasty stuff remained in this layer and could add trace nasty stuff to our veggies?
Any insight appreciated!