During the Cenozoic, the geology of the different regions diverged. The Appalachians started to uplift, while the Ouachita and Ozarks did not. The uplift rejuvenated the streams, which rapidly responded by cutting downward into the ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many millions of years earlier. Other streams downcut so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures.