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How do folds and faults control the directions of valleys in the Appalachian Mountains?
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The rugged Atlas Mountains surround a valley in Morocco.
In the Appalachian Mountains, folds and faults exert a primary influence on the orientation and form of valleys by dictating the underlying geologic framework. The region is characterized by a "fold belt" where layers of sedimentary rock were buckled into parallel ridges and valleys during the Alleghenian orogeny.123
The specific direction and placement of these valleys are controlled by the following structural factors:
- Differential Erosion: The landscape is dominated by parallel ridges of resistant rock separated by valleys underlain by weaker, more easily eroded strata. This structural arrangement forces the development of longitudinal valleys that follow the trend of the folds.31
- Structural Weakness: Rivers often exploit zones of weakness or minimal resistance within the folded and faulted strata as they downcut.4
- Cross-Axial Drainage: While many valleys align with the folds, some rivers exhibit discordant or cross-axial drainage, cutting directly across structural axes. This is often attributed to processes such as antecedence, where rivers maintained their paths as the mountains rose, or superimposition, where rivers incised through overlying sedimentary cover into the older, folded structures beneath.4
These processes collectively create the distinct, regularly oriented topography of the Appalachian Ridge and Valley province.3