The first time I visited my single player survival mode world after the updates, more than half of my house had been replaced by a gigantic mountain. The half of my house that has now vanished has been replaced by solid stone. No trace of the rest of my house remains under the mountain, including all my my chests containing inventory (the change to the terrain runs all the way down).
The mountain is mainly comprising stone blocks. Part of a mine has also been filled in. Again these are stone blocks, as they spawn in fresh biomes, not cobblestone.