The slow and gradual movement of the earth's crust can bend, fold, stand these rocks on edge, or even tun them upside down. That deformation has happened in the Ouachita Mountains. It gives structural geologists headaches. The pressure on the rocks can deform or break them, causing folds or faults .
Folds mainly go up and down.
The ones that go up are called anticlines , the ones that go down are called synclines . These features can be large enough to be hills and valleys in a mountain range, or can be squiggles seen in a hand-sized rock. In the Ouachita Mountains of Arkansas, the layers, or strata , of the rocks have been so corrugated and convoluted, that interpretation of events causing these deformations cause considerable debate among geologists. In other parts of the state, like north Arkansas, nice nearly horizontal layers of rocks can be seen to extend miles across counties. It shows that different forces work in different places.