GSA Today
Volume 32, Issue 10

Layering in granites resembles that in cross-bedded sedimentary rocks and is commonly interpreted to form from the same processes. Here, along the John Muir Trail near Pinchot Pass, Sierra Nevada, California, USA, layering is prominent near the roof of a Cretaceous granite; the same contact is visible on the western slope of Mount Perkins on the skyline, where dark red metamorphic rocks overlie the granite. Fluid dynamics analysis indicates that the parent magma was 6–10 orders of magnitude too viscous to permit the turbulence required to form cross-bedding. Width of foreground view ~2 m. This is an example of a conflict between field interpretation and physical analysis; dealing with such conflicts is discussed in the article on pages 4–10.

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