A series of marine terraces can be found along the cliffs of Torrey Pines State Reserve that mark the ancient shorelines as the receded. The best parking is in the Torrey Pines State Reserve. The current parking fees (as of 28Jul2012) for the Preserve are: $12 Mon-Thurs, $15 Fri-Sun and Holidays. To reach the various wayponts, you will need to walk along a sandy beach, possibly change parking areas, and hike along relatively steep trails.
Collecting, altering, defacing, etc. natural features, fossils, etc. is prohibited at the State Park.
The cliffs at Torrey Pines State Reserve have evidence of both an advancing and retreating shoreline. The advancing shoreline is discussed in the Advancing Shoreline – Torrey Pines earthcache. The retreating shorelines are recognized by a series of marine terraces. First you will look at the modern marine terrace to identify the features of marine terraces, then you will move on to look at progressively older marine terraces.
A marine terrace is a flat area at about sea level often at the base of cliffs. This flat area is constantly being widened by erosion at the base of the cliffs. Ocean waves erode material at the base of the cliff. One of the most effective mechanisms of erosion occurs when strong winter storm waves pick up the rounded cobbles at your feet and pound them against the cliff base. This area is also sometimes called an abrasion platform.
While you may not see these cobbles all the way down the beach, they are usually covered with a layer of sand that is deposited by the gentler summer waves. You can sill find a few lying about.
This process creates notches that undercut the cliff (N32 55.192 W117 15.548). At this location you can see a notch that has formed. When the notch gets deep enough, a portion of the cliff becomes unstable and collapses. Collapsed portions of the cliff can be seen to the north. Waves then pull the collapsed material off the beach and redistribute it.
Image source: Wikipedia
Older marine terraces are a little tougher to see than the modern terrace since additional erosion has partially covered them. They can still be recognized by some of the features seen on the modern beach.
As you look up the cliff, there is a layer of rounded cobbles near the top just above the white rocks of the Torrey Sandstone. These cobbles are the rocks that were hidden under the sand of the ancient beach. Storms pounded those rocks against a cliff a little further inland.
The Beach Trail (accessed at N32 54.847 W117 15.509, the section between the beach and cliff top is currently closed until they repair it, so for now you need to walk down from the parking area atN32 55.230 W117 15.160) takes you up to the ancient abrasion platform (N32 54.906 W117 15.496). In addition to the rounded cobbles, there are fossil shells.
Today this ancient marine terrace is marked by a gradual slope from the edge of the current cliffs to a short steeper line of terrain about 200 yard inland that roughly parallels the current cliffs. Erosion has softened the abrupt cliffs that were created along the ancient shoreline.
Image source: SDNHM
Rock layers in diagram are illustrative and do not represent what the rocks at Torrey Pines.
As you continue up the trail toward the secondary parking coordinates you will come across another marine terrace at about 330 feet above sea level (I believe that is at N32 55.125 W117 15.326). In this area there is a short 5 foot or so elevation change and a relatively flat area of red sandstone (the Linda Vista Formation) at N32 55.146 W117 15.292). This location also gives you a good view up and down the coast. This marine terrace is thought to be about 700,000 years old and can be found across San Diego County.
As the age of the marine terraces get older, the features are harder and harder to recognize or even locate.
Now the question comes up of how did these ancient shorelines get so far above sea level. There are three possibilities.
1- Sea levels were much higher in the past and gradually came down.
2- The land was lower and gradually came up
3- A combination of the two
As for which happened here, it is hard to tell.
Logging requirements:
Send me a note with :
- The text "GCZ528 Marine Terraces – Torrey Pines" on the first line
- The number of people in your group.
- Walk to N32 55.298 W117 15.333 from the parking area at N32 55.230 W117 15.160 and examine the road cut. What features do you see in the cliff that indicate it was a marine terrace.
- Include what part of the marine terrace it was.
The above information was compiled from the following sources:
- Geology of Torrey Pines State Reserve By Don Grine, Geophysicist Emeritus, Torrey Pines State Reserve 2006, http://www.torreypine.org/geology/geology.html#rock
- Robert P. Sharp and Allen F. Glazner, Geology Underfoot in Southern California, Mountain Press Publishing Company, 1993
- Department of Geological Sciences, Salem State College, Salem, MA, http://w3.salemstate.edu/~lhanson/gls214/gls214_barrier_isl.htm
- San Diego Natural History Museaum (SDNHM), San Diego Ancient Shorelines, Thomas A. Deméré, Ph.D., Curator of Paleontology, illustration by Bradford O. Riney, http://www.sdnhm.org/research/paleontology/sdshoreline.html
- Wikipedia, Wave-cut platform http://en.wikipedia.org/wiki/Wave_Cut_Platform