Arenophile genetics, Jedi, and another submarine canyon

Geology doesn't exactly run in my family. My parents weren't geologists, I married someone to whom dip and strike will eternally remain an arcane concept, and neither of my kids are geologists. Geology has, however, touched, so to speak, them all, and rubbed off most on my daughter. She's the one who has always loved the outdoors and adventures. She became, involuntarily, intimately familiar with the major faults of the Atlas Mountains during our trek there together when she turned fifteen (she was, according to our guide, the youngest western female to climb Jebel Toubkal, the highest mountain in North Africa). And her recent activities would suggest that arenophilia may be genetic.

Earlier this summer, on a well-deserved vacation/holiday in southern California, where was her first  destination after arrival? The Imperial Sand Dunes - in the distinctly uncomfortable temperatures of July. The area, also known as the Algodones Dunes, is the largest landscape of sand in California, covering its intersection with Mexico and Arizona. And there she collected, secured, and labelled in detail, plastic bags of sand for her arenophile father - mad dogs, Englishmen, and their daughters. En route, she came across the strange sight of a small group of Star Wars figures, for it was here, in 1982, that a massive set for Return of the Jedi was constructed. And, in the same way that the archaeology of the Ten Commandments has become an attraction of the dunes at Oceano (see my earlier post), so the Imperial dunes are a place of pilgrimage for devoted Star Wars fans who re-enact and dig. This phenomenon was covered by a highly entertaining article earlier this year in Harper's Magazine, " Raiders of the lost R2: Excavating in a galaxy far, far away." As one of the pilgrims is quoted as saying, “As a kid, you can only go so far playing with action figures. As an adult, you don’t play with action figures anymore. You become the action figure.”

Escaping from the desert heat, my daughter then sought refuge on the beaches around San Diego - and continued to assiduously collect and catalogue sands for her deranged father. And, in the process, she made an interesting and geologically significant, observation. The beaches of La Jolla Shores and La Jolla Cove are immediately next to each other, the Cove being located in the rocky headland that juts out at the southern end of the Shores. Yet their sands are dramatically, visibly, different. Between your toes on La Jolla shores is a fine sand (upper image, below), quartz grains shot through with dark rock and mineral fragments; at the Cove (lower image, same magnifications), the sand is coarse but poorly sorted, made up of a wide range of grain sizes, with shell fragments clearly visible, along with dark minerals, oranges, pinks, and purples. This contrast seem bizarre - until we look at the geology and what's going on along this stretch of coast.

 

First, the setting of the Cove is distinctive. It's backed by sandstone cliffs shown in the photo, weathering and eroding away at a rapid rate. The sandstone was formed around 70 million years ago as the tectonic turbulence that has long characterised southern California raised the Sierra Nevada mountains. The Sierras were formed as a series of gigantic bodies of originally molten rock, plutons, emplaced deep within the earth's crust, but rapidly jacked up to the surface by the tectonic turbulence. Once there, they were worn away by the relentless forces of weathering and erosion, the sedimentary debris forming the sandstones of La Jolla Cove. Now those sandstones are themselves providing much of the sedimentary debris for this segment of the coast - you can almost sense the sand pouring off the lower part of the cliff onto the beach - local ingredients.

 

Second, and even more importantly, if you were to pull the plug in order to see the landscape beneath the ocean along this stretch of shoreline, then an extraordinary topography would emerge. Just a few hundred yards offshore from the southern end of La Jolla Shores is the head of a huge submarine canyon. Similar to the Monterey Canyon further north (that I wrote about here), this feature is deep and sinuous, winding its way far out across the Pacific Ocean floor. And it's a sand-sucker - any sand that is carried down the coast along La Jolla Shores from the north is inevitably and irrevocably swept down the canyon and out to sea. The La Jolla canyon forms the boundary between two segments of coastline that are distinct with respect to their sedimentary processes and the transport and deposition of sand - two different littoral cells (see my "Beach Nourishment and Sediment Budgets" post). 

 

No wonder the sands of the Shores and the Cove are so different. This is a dynamic stretch of coast and the submarine canyon has long been a laboratory in which we have learned something of the dramas that are played out in these landscapes. And it's perhaps no wonder that one of the world's foremost centres of oceanographic research, Scripps Institution of Oceanography, is located (on land) at the head of the canyon.

[The image of the canyon, along with many others, can be found at http://walrus.wr.usgs.gov/pacmaps/sd-index.html; Scripps put out a news release a couple of years ago that describes some fascinating research on sand movements and budgets along this part of the coast: http://scrippsnews.ucsd.edu/Releases/?releaseID=785; the department of geology at San Diego State University have, brilliantly, put together Google Maps overlays of the geological maps: http://www.geology.sdsu.edu/kmlgeology/kmz/la_jolla/la_jolla.html; and see the comprehensive study of California's coastal processes and littoral cells, Littoral Cells, Sand Budgets, and Beaches: Understanding California’s Shoreline. The photo of the Star Wars set is from http://www.desertusa.com/sandhills/du_sh_star.html. And, finally, thanks to my daughter for her genetic peculiarity and for prompting the basis for this story.]

 

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