The Slag Plains are a region of North America after the Great Nuclear War. It extends from Hamilton, ON, to Oshawa, ON.
Pre-War, the Slag Plains were one continuous metro, one of the densest population centers in Canada. The area was near-completely leveled by nuclear weapons, the ground blackened. The rubble became rolling hills, channels of rivers, and swampy heaths. But the plethora of metals used to build the old world remained in the ground, and this region is a rich source of steel, copper, and other metals.
As such, this land was colonized by intrepid individuals who pulled this metal from the earth and sold it at steep prices. These individuals lived in small ramshackle communities called Steel Boomtowns. New trails came into use, and metal flowed from this region to Owen, Niagara, and hence to the rest of the Great Lakes.