reminds me of when I was a bairn, we used to go bottle-digging at a local ash dump which dated from WW1 era. The dump had been fired in the past, quite vigorously owing to the high carbon content, and many areas showed bright red/orange soil as a result. You could certainly tell when you were into undisturbed tip.
Maybe explains my fondness for ochre....::)
Used to find some lovely melted bottles, too, and a few heartbreakers!
MARK