Well others beat me to it but I really did know that it was the left overs from mining...
As in:
"The children on the Pine Ridge Indian (Native American for you PC assholes) reservation often are left with just slagheaps from the uranium mines to play in because the capitalistic pig energy companies, being aided and abetted by the United States government, have raped their land so extensively."
But, that of course would only be one context....
