Abstract

Massive igneous and metamorphic rocks commonly display a near-surface sheet jointing that is unrelated to other structures and textures but is related to topography in that it exists only at shallow depths and tends to parallel the surface of the ground. Most authors believe that this large-scale exfoliation accompanies expansion when erosion releases confining pressure on rocks which were once deeply buried. Confining pressure is released unequally, enabling residual compressive stresses parallel to the surface to cause the splitting. Little has been written about these structures in sedimentary rocks.

Massive sandstones on the Colorado Plateau flake off in sheets bounded by joints which are unrelated to other structures and textures, which are found only at shallow depths, and which are oriented parallel to the outcrops. This large-scale exfoliation is similar in all important aspects to that found in crystalline terranes and it has been produced in the same way—by differential residual stresses resulting from erosion, particularly canyon cutting, of rocks that were once buried. Exfoliation has contributed to canyon development and has led to such distinctive topographic features as exfoliation caves and domes. Most existing exfoliation joints are probably of Pleistocene age.

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