A couple of mining questions, if you please.

I just dropped down to level four in my mine for the first time. Where the drop down occurred it created a section of black tiles that will take 23 piles of wood to soften them. I am wondering is it possible this area so close to the last cave down would create another cave down? If so would it be in exactly the same spot on level four? Also at what level can I expect greater iron purity? Or is it only distance from ladder and mine and mountains skill that determines this? Thanks in advance for any insight on this.