@John Brown (no body)
The index fossil of human civilisation (*) will be the bottlecap. Archeologists will quarrel over the distinction between, what we call, the Cola-Cola and Pepsi-Cola variant. They will also notice an association with concretions of hollow metal cilinders of seemingly standardized sizes.
(*) Mahatma Gandhi answered ~1947 to the question what his opinion was on Western civilisation: What civilisation?.