Originally posted by D1rtyS04p:

My first thoughts are: severe storms cause erosion and deformation to structures and I'm guessing it can happen to terrain as well. And also, The lore seems to imply the earth is "alive" allowing change and movement. And maybe the troll walking around can stomp the ground cause the terrain to ripple slightly.

This seems pretty close and covers some of the factors but I think it is more simple. Storms cause for example flints to reappear in the tidepools and rivers near oceans. I think that possibly some of the stones under the terrain might be replenishing themselves. Large creatures definitely can crush the landscape and they will battle each other sometimes when you're not present. I have heard that copper veins and such might reset, so that includes large boulders too over time. Haven't really witnessed this myself though.