Title says it all but let's be more specific: I'm planning a post-apoc world where our current civilization has been wiped out in a large scale war. The story takes place centuries after the apocalypse and the twist is that the new civilization has almost unlimited access to minerals and such as the remains of cities have been burned into this metal-like layer beneath the surface.
Thing is, is this even possible? And if not, how much would I have to stretch reality to create this scenario?
Some questions I've been struggling with:
What kind of weapon could effectively "gooify" entire cities?
Is this new goo layer something that could be actually used to work metalworks and create things such as swords and armor etc? What would the "quality" of this layer be, and how would a civilization with near medieval technology utilize this layer?
What kind of effect would this weapon have on environment and what would the living conditions of earth and for example vegetation of this "new earth" be where these weapons have seen global usage?