The height of the ravine is similar to what we found, however the floor of the ravine is approx at 14 depth, might be 12 or 13. Haven't tried going down any further since the lowest level of my quarry/mine complex is 19 and it has a floor every 6 blocks, which would make the floor of that level filled with lava, not a risk I want to take.
At least one side and part of another are right in the side of the ravine (not over it though, thankfully), so I know that one area is likely unsafe to dig further down, but I have lava in various places in there and being so close to bedrock, I don't absolutely know for sure.