![]() While some natural bridges may last a century or more, others have appeared and disappeared within a few decades. ![]() Natural Bridges State Beach is perhaps the best known and the progressive loss of two of the three bridges, one in about 1905, and the second during a storm on the night of January 10, 1980. The natural bridges or arches fascinated early visitors as they do today, but the same rock weaknesses that allowed the waves to erode those arches, have also led to their demise. “The natural bridges or arches fascinated early visitors as they do today, but the same rock weaknesses that allowed the waves to erode those arches, have also led to their demise. Several years ago we published a book comparing old photographs of the Santa Cruz coast, many taken 75 to 100 years ago, with photographs we took from the same locations in 2006 (The Santa Cruz Coast-Then and Now). ![]() ![]() There have been a large number of picturesque natural bridges that were often photographed by earlier visitors, which preserved a permanent record for what was here and how the coastline has changed. These rocks vary in their resistance to erosion and also have been weakened over hundreds of thousands of years of stress within the San Andreas Fault system and also from exposure to waves, salt air, and gravity. The rocky coast from the city of Santa Cruz north to the old town of Davenport consists of a 30 to 75 foot high uplifted marine terrace, which has been eroded into relatively young sedimentary rocks, a mix of sandstones, siltstones and mudstones. Instead we see large slabs fall to the beach or natural bridges or arches collapse catastrophically. Cliff failure doesn’t occur in simple six inch or one-foot increments every year, however. For the much of the Santa Cruz County coastline the average annual erosion rates typically range from a few inches to about a foot/year. As coastal geologists we often take measurements from old aerial photographs to see how much retreat has taken place over time and then calculate cliff retreat rates. This is a high-energy coast with waves 10-12 feet high arriving regularly. The coastline of Santa Cruz County has undergone some dramatic changes over the past century or so of human observation and photography, and watching the waves batter the cliffs during any winter storm makes you realize why. It is typically these weak zones, whether faults, joints, fractures or other features, which the waves seek out and erode, gradually creating tunnels, caves, or natural bridges or arches. ![]() In addition to the rock types themselves, and differences in their cementation or hardness, there are also often structural weaknesses or irregularities that cut through or penetrate the rocks making up coastal cliff or bluffs. So while there are rock types that are very resistant to the impact of the sea, granitic and volcanic rocks, and even certain sedimentary rocks that are high in silica, being several examples, which often form the protruding headlands or points we recognize and give names to, there are others that don’t hold up so well. In baseball terms, Mother Nature always bats last. Whether the constant rise and fall of the tide and the accompanying wetting and drying, heating and cooling, and exposure to salt water, or the sheer impact of breaking waves, the ocean is a challenging force to resist. While most coastlines often appear to be stable and permanent over the short time span of our visits, and some are, there are many others where the materials making up the coastal bluffs or cliffs are no match for the forces the sea exerts. Natural Bridges ~ 1890, 1970, 2006 (Click on Images for Larger View)īy Gary Griggs, Distinguished Professor of Earth and Planetary Sciences, Director Institute of Marine Sciences, University of California, Santa Cruz, California ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |