The work to reinforce Mount Soledad after the October 2007 landslide has provided a peek into La Jolla’s watery past.
Fossils of clams, snails and plants found at the slide site reveal this part of the mountain was a deep-sea canyon millions of years ago.
“It was a little bit of a surprise, but we were not shocked,” said Kesler Randall, a paleontologist at the San Diego Natural History Museum.
The museum’s field geologists and paleontologists discovered the fossils while monitoring the construction site as required by state environmental laws.
Finding a submarine canyon is not totally unexpected because the land La Jolla occupies today was formed as part of the continental shelf 48 million years ago - “so younger than dinosaurs but older than mammoths and saber-tooth cats,” Randall said.
The presence of fossils indicates the area was a canyon because they are usually found at the bottom of the shallow bowls, not in the flat layers of sediment that make up the rest of the shelf, Randall said.
Within the ArdathThe most interesting find is a one-ton rock slab containing petrified wood and fossilized cycad leaves. Cycad is an ancient tree that existed in prehistoric times and still grows today.
Other San Diego cycad fossils have been collected, but it is rare to retrieve plant fossils from the 48-million-year-old continental shelf, known as the Ardath formation, Randall said.
A unique set of circumstances would have been required for the leaf to not decompose by the time it was buried in sand hundreds of feet beneath the ocean’s surface, until it was uncovered in January 2008, Randall said.
The slab also contains trace fossils of teredos, a small, worm-like clam that bores into wood. The invertebrate still exists today and is commonly known as a “shipworm.” Sediment filled the borings and are now hardened blobs in the rock.
Regional pictureAll of the terrestrial organisms were washed offshore, so these fossils do not tell scientists as much about the ocean as they do the land.
“Couple this site with other sites on Mount Soledad and San Diego, and we have a nice, well-rounded regional picture of what San Diego used to look like.”
The fossils did not solve any great scientific mysteries-at least not yet. The museum will treat the fossils with hardeners and resins so they are protected, and record as much data about them as possible, Randall said.
“So scientists in 100 or 200 years interested in studying the Ardath layer, cycads or teredos come back to our write-ups and hopefully, have enough info to conduct their study,” Randall said.
The museum will continue to monitor the slide site during construction, so the discoveries may not be over quite yet.
For now, museum visitors will be able to see and touch the fossils collected to date. The large slab will be on display at the Natural History museum by the end of the month.
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