“Every breaking wave on the shore
Tells the next one there’ll be one more”
Bono and Edge, U2 “Every Breaking Wave” 2014
I was in San Diego a few weeks ago attending the Evangelical Theological Society conference. I will report on that later but today I bring you a few pictures I took before hopping on a plane back to Ohio. Early in the morning I went down to Sunset Cliffs on Point Loma and watched the waves at high tide churning against the shore. Relaxing and beautiful. But I also couldn’t help but ask myself while watching these waves hit the rocks: how much pounding can those rocks take before they become the sand that washes up on the beach?
A few rocks in particular caught my attention. Check out these strange rocks protruding from this rock shelf. They are concretions. Concretions are hard masses of material that result from precipitation of minerals from the surrounding sediments. They form within sedimentary rocks. These are especially large ones and probably high in iron content. The material in the concretions tend to be much more tightly bonded together making them more resistant to erosion than the surrounding material. As a result the sandstone has been eroding around them leaving them exposed to the waves.
Here is a wider shot showing the context of where these concretions are in relationship to the cliffs.
It is pretty obvious that these concretions are really tough rocks. But how tough? They take a daily beating and yet there they are. But for how long? I searched the internet for historical pictures of the same location. I was a bit disappointed that I couldn’t find any images from more than 20 years ago but I did find a number of pictures people have posted from nearly the same vantage point.
Above is the oldest one I could find. you can see that 15 years ago the same two concretions are visible. The larger on doesn’t appear to be protruding quite as far above the surface as it is now but the entire rock slab can’t have eroded more than a few inches in the intervening 15 years.
Here is another from 1999. The image was taken from nearly the same point I took mine but unfortunately the wave is obscuring the concretions but they are clearly there. The rest of the cliffs barely look different than they did 15 years ago.Above is nice shot of the larger concretion. At one time there was probably another one sitting next too it that finally broke off and fell into the ocean.
The simple lesson here is that erosion is not the same for all rocks and erosion in some places can take a very long time. Even in a high energy environment like this shoreline, the cliffs here have been shaped over many thousands of years. Just the time to erode the sandstone from around these concretions alone could be measured in at least many hundreds of years and maybe thousands.
Just for fun I threw in this picture I found on Wikipedia of another example of concretions on a beach. Differential erosion can make some cool things.