Castle Rock State Park – Part III

Figure 7                                                                                  Figure 8

This is the third post in a series about Castle Rock State Park.  It was submitted by a reader named Jonathan.  In the first post, Jonathan explained why he chose to write about Castle Rock State Park and he also showed where it’s located.  In the second post, Jonathan discussed the geologic history of the park.  In this post, Jonathan discusses Castle Rock and surrounding outcroppings.

Castle Rock (figure 7) is one most popular outcroppings, or bosses, in Castle Rock State Park. The Park’s brochure claims the formation is roughly 50 feet tall with caves that are as much as 15 to 20 feet wide, 10 to 20 feet high, and 8 to 10 feet deep. These caves, or tafoni (figure 8), are so large that in the late 19th century, when the Castle Rock School opened in this area, the first teacher, Miss Ida M. Jones, lived in a cave for 6 weeks as her cabin was being built.

Three processes of weathering play important roles in shaping these rocks: chemical weathering, mechanical  weathering, and biological weathering.

The tafoni are a product of chemical weathering whereby carbonic acid in rainwater seeps into the interior of rocks where it dissolves calcium carbonate, which cements together the grains of sand that make up Vaqueros Sandstone. Dry summer winds then wick this water to the rocks’ surface, bringing with it the dissolved carbonic acid and calcium carbonate. As the water evaporates, dissolved calcium carbonate is left to form a hard crust at the exterior of the rock. The interior of the rock continues to erode as the cementing calcium carbonate is removed, allowing for caves, cavities, or pockets to form when the outer crust is broken by a falling tree branch, hail, windblown debris, passing animals, etc (mechanical weathering).

Tafoni appear as both caves (figure 9) and intricate honeycomb-shaped latticework (figure 10). So unique are these formations that they inspired John Steinbeck to write in his short story, “The Murder,” “This happened a number of years ago in Monterey County… At the head of the canyon there stands a tremendous stone castle, buttressed and towered like those strongholds the Crusaders put up in the path of their conquests. Only a close visit to the castle shows it to be a strange accident of time and water and erosion working on soft, stratified sandstone.”

Figure 11
Mechanical weathering also impacts the structure of Castle Rock and its surrounding outcroppings. Take, for example, a Pacific Madrone that split an outcropping in half as it rooted itself in the rock’s cracks (figure 11). Finally, biological weathering is seen where lichens (combinations of fungi and algae) live on rocks and slowly eat away at the rocks’ surface (figure 12).

Figure 12

Chemical, mechanical, and biological weathering processes result in jointed outcroppings that sometimes appear as if they are not bedrock, but erratic boulders transported to their current locations by unknown forces (figure 13).

Some of these cannonball-like formations are likely the result of the same chemical process that form tafoni; the collection of calcium carbonate at a rock’s crust creates better-cemented areas called concretions (figure 14).