The Appalachians

Introduction. Broad regional uplift and differential erosion during the Cenozoic has been an equal partner with the tectonic forces of the Paleozoic in shaping the Appalachians of today.

• 1. Structure of the Appalachians is a consequence of the Taconic, Acadian, and Allegheny orogenies.
  
• 2. Late Paleozoic Mts. and the subsequent Triassic (Newark Series) and Jurassic was eroded to a peneplain by the Early Cretaceous. This peneplain surface is called the Fall Zone Surface (now preserved under coastal plain)
  
• 3. Cretaceous transgression over eastern peneplain.
  
• 4. Early Cenozoic drainage was established across the peneplain and Cretaceous to Early Cenozoic cover. Since metamorphic and igneous rocks of Paleozoic were buried they had no effect on drainage. Drainage flows to the SE over buried NE trending buried structure.
  
• 5. Erosion began to be reduced as buried crystalline peaks were exposed (today these peaks have similar elevations-beginning of the Schooley Peneplain... cuts across Fall Zone surface). Slow rate of erosion and relief fully developed the peneplain, this time with domination by surficial Paleozoic rocks.
  
• 6. Continued Neogene uplift. Cretaceous and Paleogene sediments stripped away. Erosion carved down through Paleozoic, carving valleys into softer limestones and shales while more resistant sandstones became ridges in the Valley and Ridge.
  
• 7. New challenges to the drainage system... only the strong struggle on as they did before (in their previous SE direction) now carving deep valleys and gorges (e.g. Pigeon River). The weak become submissive to the new topographic challenges and go with the newly exposed structure (NE-SW).
  
• 8. Water and Wind Gaps
• a. water gaps- gaps through the range still occupied by streams that cut them (e.g. James River and James River Gorge).
• b. wind gaps- transverse gaps in the range no longer occupied by streams (e.g. Compton, Thornton, Swift Run, Powell Gaps).
• Shenandoah River, a tributary of the Potomac, cut headward (south) in non-resistant limestone and captured headwaters of streams flowing across the Blue Ridge between the Potomac and the James River.

• 9. Uplift History
• Since the final orogeny of the late Paleozoic, the Appalachian Orogeny, the Appalachian region has periodically been subject to episodes of uplift which have rejuvenated erosional processes to again sculpt the old tectonic fabric of the region.
• At least six such episodes are recognized : mid-Triassic, Late Triassic, pre-Cretaceous, Miocene, and two late Cenozoic episodes. All but the oldest of these episodes appear to be partially the result of isostatic disequilibrium.
• Prior tectonic episodes greatly thickened the crust of the region. Subsequent periods of erosional activity (varying in rate, according to climate variation) placed these roots out of isostatic equilibrium, resulting in renewed broad regional uplift.
• Could isostatic disequilibrium be the caused of young uplift in other physiographic provinces we have discussed?