Circular patterns and exfoliation in crystalline terrane, Grandfather Mountain area, North Carolina

Aerial photographs of areas of crystalline rock in the Piedmont and Blue Ridge of western North Carolina reveal numerous large-scale arcuate, circular, and elliptical patterns with diameters from 1000 to more than 4000 feet. They are commonly defined by curving streams, curving ridges, and curving belts of contrasting vegetation. Geologic mapping shows that the circular patterns are not related to primary geologic structures; circumstantial evidence indicates that they are related to exfoliation or sheeting. Rock spalls up to half a mile in diameter may be formed by dilation of the rock as the erosion surface is lowered. The spalls enable tectonic joints to open, forming thin layers of rock of differing permeability and susceptibility to erosion. Exfoliation is probably involved in the erosive process and is probably more important than has been realized, especially in crystalline terrane.