What are the purple tiles?

Question

In Gnomoria, some of my land tiles have turned purple. They weren't previously. What are they? I cannot plant crops on them and it seems to be spreading.

Screen shot of the 'purple doom':

Answer 1

You can see the same purple tiles in this question. Which incidentally tells you how the mud is created.

Quote:

Mud plots are generated by flooding a room that is adjacent to water

You're probably experiencing flooding, that looks like a cave so make sure there are no bodies of water on top of it.

Answer 2

From looking at the placement, I am guessing that you placed a grove over the tiles. Based on what it looks like you are doing, and the fact that it looks like you are working at elevation 1, since the water tiles are one below the active level, I would have to say the purple tiles could be from having a non-grass covered dirt spot and making it a grove before the grass could grow on it. Should the tile get planted with a tree, it will stay purple. I am not sure if it will return to grass if nothing gets planted, as I am a bit obsessive about having my groves have grass tiles under them, so I wait for it to grow.

As for looking like the purple is growing in area, I can't figure that out.

Edit: Here is an example: I had a little cotton farm going, I decided I wanted to square it up. After digging out the hill, I flagged it as a continuation of the existing farm. It turned the tiles purple, and as you can see, the gnome was busy tilling the field. The tile she already tilled is to the very left, and it is now brown.