If you look in the northern most cove at Bonny Doon Beach, (yes, it’s the clothing-optional section), you will see a tall skinny dike of sandstone that cuts across the cliffs-which are made of Santa Cruz Mudstone. A dike is essentially loose material, in this case, the older and deeper Santa Margarita Sandstone, that was squirted up under pressure through the younger but harder mudstone. The dikes and the Santa Margarita Sandstone in general contain bituminous (asphalt-like) material in varying quantities. There are many similar dikes in the sea cliffs along the north coast of Santa Cruz. This one is unusual in how skinny and rectangular it is.
Some of the bituminous sandstones just a few miles south of Bonny Doon Beach, at Majors Creek, were quarried for paving material. Apparently about 600,000 tons, valued at over 2 million dollars, were quarried and sent by boat to pave San Francisco between 1888 and 1914.
In the 1950s the Husky Oil Company ran an experimental project to get oil and gas out by heating the sandstone in the drill hole and then recovering what melts. Over three years they pulled out 3,000 barrels of oil and some gas. In general, costs were too high to make it economically viable.
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- Living With the Changing California Coast. Gary B. Griggs, Kiki Patsch, Lauret E. Savoy. University of California Press; 2005.