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Mother Lode caves have formed in marble, essentially metamorphosed limestone. The limestone is about a quarter billion years old, dating to a time just before the rise of the dinosaurs. A few fossils escaped the baking and crushing of the limestone as it turned into marble. Fossil "horn" corals that resemble miniature cow horns and rice-grain-sized Fusilinids (animals similar to tiny clams) are scattered sparingly throughout the marble. More common are once-thin sandy beds that have been "baked" into hard quartzite. These are often bent into fanciful rippled structures that show just how contorted the original limestone and sand beds have become. The originally horizontal bedding planes (layers) of the limestone have been uplifted and distorted so they are now nearly vertical and trend roughly northeast-southwest. This has guided the development of the caves towards vertical pits that require advanced cave exploration skills and specialized rope techniques.   Photo by Dave Bunnell. Geology text by Bruce Rogers, Ralph Squire. Pages and their contents © Copyright 2000, 2001 by Elizabeth Bunnell, except where otherwise noted.