Thunder Mountain is getting worn down into dirt in several ways. You have already learned that acids in ground water can dissolve away rock.
Water can also seep into cracks in rock layers and then freeze.
The freezing water acts like a wedge and splits the stone into smaller and smaller pieces. Here is a simple experiment you can try at home and discover the effect that plants have on the rocks of the Ozark Mountains. Next, place the pre-soaked beans on top of the potting soil and add water till the soil is moist. Then cover the beans with a layer of plaster of Paris and place the cup near a window.