As far as I understand it: There once was a layer of hardened clay that lay at the bottom of the sea. Over a long time coral reefs, mussels, fish bones, algea, sand and other detritus accumulated on top of it forming a layer of limestone. Then due to tectonic processes the region gets uplifted above sea level and dries up. The limestone layer hardens and becomes cracked due to tectonic movement (stuff like earthquakes). The softer clay layer doesn’t crack though. Then rain and rivers (over and underground) erode the split apart limestone chunks so the rigid blocks are reduced to conical hills until they disappear completely. The erosion sharply stops at the clay layer though, since unlike limestone, clay is largely waterproof and not as affected by the corrosive water. That’s why you have the last pieces of the limestone layer strewn about on a plain that is the clay layer. In the future there will only be a plain left here. Here’s a picture where this becomes apparent:

Also from the article, this seems a lot more plausible:

The flatlands of Carmen were once a playground for giant children. One day, they initiated a contest on who could bake the most mud cakes, gathering mud and “baking” them under coconut half-shells laid flat on the ground. Before the contest could end, however, the children were called home. After a while, they returned to the area to witness their finished creations and left them undisturbed out of admiration—the baked cakes thus became the hills.