Geology (cont. 6)


About 45 million years ago sea level rose, and the shoreline therefore moved inland, closer to the eroding mountains. Being the “dumping ground” for somewhat faster moving water (flowing down steeper hills makes water velocity increase) meant that the sediment being deposited was coarser than mud (faster water carries larger particles). In fact it was sand and it accumulated in offshore bars and submarine canyons, on top of the green muds. Today it is the beige rock known as the Torrey Sandstone. Its layers are thick, and it looks like a strong rock, but it has a system of fractures that help it to fall, sometimes disastrously, onto the beach, when it has been undercut by the waves. Because the Del Mar Formation lying beneath it is so much weaker, waves can cut deep notches in it, removing support from the Torrey above it. The Torrey sandstone is characterized by caves created by weathering, possibly scoured by wind. Because it frequently has round cannon-ball-size concretions (deposits of harder minerals around sand grains or fossils) in it, and these make rounded holes when they fall out, many people believe they are the origin of the caves.


Page 6

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