Geology Rocks September 21, 2014

Yesterday was the Point Reyes National Seashore Association’s fourth annual Dinner on the Pacific Plate.  PRNSA supports the efforts of the National Park Service.  It ran the Trails Challenge that I did two summers ago in honor of the Seashore’s 50^th Anniversary, as well as a myriad of nature and art courses, such as the plein air painting weekend I enjoyed in June a year ago.

This year I took up PRNSA’s offering of a rock tour, the geological kind.  Prior to the fund-raising dinner, we were given a choice of ten guided adventures in the Seashore, and I opted for “Discovering Drakes Bay Geology”.  Being geology-challenged, I relished the opportunity to learn something about those iconic bluffs that allegedly reminded Sir Francis Drake of the white cliffs of Dover.  The guide for our small group was John Karachewski, a fellow geek who clearly delighted in sharing his enthusiasm for all things geological.

John started with a basic tenet of geology: one must think in four dimensions, the fourth being time.  He took us through the book that described the evolution of the Point Reyes Peninsula, where each chapter covers a few million years or so, from a time when the Peninsula and all of the Western edge of California south of it – through San Francisco, Los Angeles and down to Baja California – came to slide northward on the Pacific Plate by about 400 miles.  After the great subduction of the Farallon Plate under the Atlantic Plate, the Pacific Plate started its slow migration northward, with fits and starts in the form of 80,000 earthquakes, each the size of the great 1906 quake.  He described how the granite bedrock in Point Reyes was once part of the southern end of the Sierras, near the Mohave desert and why we don’t see it just a few miles away here at Pi on the Atlantic Plate.

As for the cliffs themselves, they are formed from eons of clay, silt, sand, and other sedimentation when the land mass was still submerged under water.  He recommended that we could actually tell what each layer consisted of if we were willing to grab a bit and stick in our mouths.  None of us took him up on that - yet.