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Slip Sliding Away
Talking with Dr. Pat Abbott
Julie Maxey-Allison | 10th Street

At least four cliff collapses occurred in this area between August and December 2018. This photo was taken in October 2018.
Photo Julie Maxey-Allison.
Click to enlarge.

Dr. Pat Abbott, Professor Emeritus, San Diego State University specializing in Sedimentology of Mesozoic and Cenozoic rocks, the correlation of tectonically displaced terrains and natural disasters, sees our bluffs in geological terms, looking at the big picture. Dr. Abbott points out that Del Mar’s bluffs are composed of two different materials. The base, the lower two-thirds of the bluffs with the greenish hues, is made up of old sandstone and mud stones that are 45 million years old. The upper third, the rusty red material, is younger, some thousands of years old, and weak. “One can crush the hunks of sandstone on the top by the handful into sand. It is loosely bonded; it disintegrates. And, this is where the train tracks and people are.” As to stability, Dr. Abbott made clear in an interview aired on TV after the October 2018 bluff collapse that the bluffs “in the area between 9th and 11th streets could go at any time”.

Given the geological setting, cliffs meet ocean, the work of mother nature is ongoing. With the emergent physics, the entirety of what is going on with the great variety of stresses on our cliffs, Dr. Abbott cites the inevitability of our ocean’s waves advancing east, moving inland, and eventually eroding the bluffs. “You can’t stop ocean waves or gravity. The cliffs are coming down. It can’t be stopped.” But, this is in geological terms. When? Engineering statistics, he says, are not able to be averaged.

He also sizes up the little daily impacts. Each rainfall erodes the sandstone. Year after year, decade after decade, it shapes the cliffs. “One can see how the cliffs are sculptured by rain” but daily landscape watering also invades and soaks the soil below ground. It moves through the cliffs, sapping away strength below the surface. This is a cause of the collapse at Anderson canyon, a cliff area that has small internal underground caves. As more water from rain and landscape water entered the caves they grew and grew larger, attracting more and more water. Thus, the collapse.

Add the human effect: people walk atop the cliffs and the train rolls over them. Each day an estimated 40 or so trains run through Del Mar. Multiplied by 365 days a year the total is 14,600, and for a decade, 146,000. Of course there is an effect on the cliffs. “In the longer term the train tracks will have to move or they will end up on the beach. Any armoring or supports of the cliffs delays the inevitable.”
Back to water: “If Del Mar were serious, and I wonder if the city is serious, the city would control all water erosion that works its way below the surface and undermines the cliffs. All sprinklers, all plant watering would cease. Native plants that don’t need much water and the Torrey Pines that belong here would replace the current vegetation.” Also, “The shallow rooted non-native ice plant that adds tremendous weight to the cliffs would be removed.”

Dr. Abbott has clearly explained the future of our cliffs. They are eroding from the ocean waves, rain and irrigation water seeping below the surface, and the vibrations from the train runs. Sandstone is fragile. Appreciate these realities as you plan your next cliff walk.



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