I am remodeling my fireplace. The hearth and mantle were granite/marble slabs (huge ones cut around the firebox). I removed them from the mantle and hearth with a sledge and chisel, expecting thinset or mastic to be used to attach the hearth stone to the subfloor. Unfortunately, I found this instead:
It's very hard and very uneven. I hardly dent it with a chisel or sledge.
In terms of unevenness, it is 1" from flush at the back (where it meets the wall) and 9/16" from flush at the front (where it meets the hardwood). It's also full of dips and divots and high points. It also appears to be set nearly against the hardwood at the edges, with no spacing for a joint or anything. This home is from the 1950's.
What's the best way to remove and/or level this surface out, so that it is a uniform 3/8" deep from flush?