Editing Geology of the Appalachians (section)

=== Mesozoic era and later ===
Pangea began to break up about 220 million years ago, in the early [[Mesozoic]] (late [[Triassic]] period). As Pangea [[rift]]ed apart a new passive tectonic margin was born, and the forces that created the Appalachian, Ouachita, and Marathon Mountains were stilled. Weathering and erosion prevailed, and the mountains began to wear away.<ref name="usgs" />

By the end of the Mesozoic, the Appalachian Mountains had been eroded to an almost-flat plain.<ref name="usgs" /> It was not until the region was [[tectonic uplift#Isostatic uplift|uplifted]] during the [[Cenozoic]] era that the distinctive topography of the present formed.<ref>{{cite journal|first1=C. Wylie|last1=Poag|first2=William D.|last2=Sevon|title=A record of Appalachian denudation in postrift Mesozoic and Cenozoic sedimentary deposits of the U.S. Middle Atlantic continental margin|journal=Geomorphology|volume=2|issue=1–3|date=September 1989|pages=119–157|doi=10.1016/0169-555X(89)90009-3|bibcode = 1989Geomo...2..119P }}</ref> Uplift [[River rejuvenation|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 [[downcutting|downcut]] so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures.<ref name="usgs" /> The ridges of the Appalachian Mountain core represent erosion-resistant rock that remained after the rock above and beside it was eroded away.<ref name="usgs" />