I don’t remember writing I think veins are better (probably because I don’t think so 
).
You can always count for an unorthodox idea from me and I have one. In the Anno series there are non-renewable resources mined in huge quantities but the terrain is not damaged because mining spots are represented by objects which can be exploited (i.e. they resources come from mining spots, not from the terrain). A clay pit being an object which Lings can mine infinitely (in Anno 1404 the sources deplete over time and can be renewed for money, which is interpreted as hiring a geologist to find new veins) may be not very realistic but completely solves the problem of unintended terraforming for the sake of obtaining resources.
That being written, if I modded a clay pit as a workshop and added an ingredient-less recipe producing clay mound we could have an infinite clay pit without any major changes. Sadly this would limit the pit’s functionality to a specific crafter class (Potter makes sense).