On the rocks

Andrew Edwards

The rocks don’t speak, so someone has to tell their story.

At Crystal Cove State Park, that someone is John Wilkerson, a

retired high school science teacher and park docent. On Sunday, he

led a beachside tour to explain the park’s unique rocks and fossils.

On a stroll across the sands near the park’s Los Trancos area,

Wilkerson and the tour group stopped to observe a jagged boulder that

looked out of place among the rocks rounded by the sea.

The rough, boulder-like objects are concretions, Wilkerson

explained. A concretion is formed around logs that become the

formation’s nucleus. Iron migrates toward the log, petrifying the

wood and hardening the surrounding sediment. Eventually, concretions

formed in Crystal Cove’s cliffs tumble toward the beach.

“When that thing gets eroded out, it will come rolling down and go

‘plop,’” Wilkerson said.

Later on during the tour, the group found a concretion that had

recently fallen from the cliffs. Wilkerson said the log, probably an

ancient sycamore tree, had been removed and taken to a park ranger’s

station. A black, grainy carbon residue still lingered in the cavity

where the log used to be.

“I thought it was fascinating,” tour guest Herb Rabe of Laguna

Beach said. “There are probably thousands of people who walked by and

never knew what they looked at.”

Another unusual formation at the beach is a stretch of dark,

contorted rocks that resemble what it might look like if someone

poured out a bucket of asphalt, letting the substance pile up in

haphazard layers and allowing it to dry, forming a maze-like pattern

of twists and turns.

The beginnings of the formation go back thousands of years, when

the beach was thousands of feet underwater, Wilkerson said. Silt that

had been carried out to sea settled below the depths, forming a

clay-like substance. Somehow, most likely through fault action or by

slumping downward into an underwater canyon, the clay distorted into

a warped shape and eventually hardened into rock.

Before leaving the beach, Wilkerson showed thousands of clam

fossils that lay embedded in the rocks at the bottom of Crystal

Cove’s cliffs. What happened to the area’s clam population is a

mystery, Wilkerson said.

After finishing the tour of the Los Trancos area, Wilkerson got

ready to take the group south for a look at Laguna Beach’s geology.

“I thought it was very exciting, because it’s very special and

neat to know your past; it makes you appreciate where we’re at,”

Corona del Mar resident Alice Nelson said after finishing the tour’s

first leg.

* ANDREW EDWARDS covers business and the environment. He can be

reached at (714) 966-4624 or by e-mail at andrew.edwards@latimes.com.