I started playing around with terraforming after a few weeks break and noticed the changes.
Having dirt part of the process seems to be a good thing to me. Limits on terraforming stone seems logical but will we ever be able to transform stone ground through terraforming or some other skill? (Explosives for removing stone, perhaps? Masonry for building out of stone? Mining?)
There are large areas of ground that are nothing but stone and if we are prevented from changing these areas, no one will consider settling there. Most of the west shore is stone based from what I have seen in my tour around the lake.
Maybe working on rock ground requires a high level of terraforming and a pick axe instead of a shovel? Since we will eventually be limited to our tribal territory for building, I think it would be nice if we had the option to terraform all the ground within our zone of control and not be limited to only the patches of exposed soil.