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 Interpretive Stops
1. Along Aptos Creek Additional Information
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 Epicenter of 1989 Loma Prieta Quake Here in the ParkDid you know that the epicenter for the 1989 Loma Prieta Earthquake is right here in The Forest of Nisene Marks State Park? That's right. Not far off the Aptos Creek trail in the northern section of the park, a marker points to the epicenter of the 6.9 quake.
When you visit the epicenter, you're actually standing at the spot on the earth's surface, above the point where the quake started. The rupture, created by released energy along the San Andreas Fault zone, actually started at the hypocenter, 18 kilometers (11 miles) below the point where you stand at the epicenter. The San Andreas is a zone of parallel faults between two tectonic plates: the North American and the Pacific. Both plates rub against each other along this fault zone, creating what geologists call a transform plate boundary. Instead of sliding past the North American Plate, the Pacific Plate is moving in a northwest direction against the North American Plate. As the plates move against each other, stress occurs. When the stress is too much, rocks buckle and shear off, creating the jerks or heavy rolls that move our cars, our furniture, and sometimes our houses. Sandstones and Siltstones Make Up Large Portion of Park's GeologyIf you were to rub your hands across the "rock" exposed on the cut hillside at this stop, you'd notice that it feels like sand. Where we are, in the southwestern portion of the park, the substrate is composed of soft siltstones and sandstones. The grayish-brown rock at this stop is weathered sandstone. Sandstones in California are often gray or brown, and most, like the sandstone here, have been deposited by water. Why is sandstone here in Marcel's Forest?The land that Marcel's Forest sits on has been covered off and on by water for millions of years. The sandstone we see in Marcel's Forest, and elsewhere in the park, is the sediment left from a shallow marine embayment that covered the area during the Pliocene Epoch about 5.3 million years ago. This marine embayment, which reached depths of only 8 to 10 meters (26 to 32 feet), left sedimentation that geologists call the Purisima Formation. (See also Time Line). Various rocks along Aptos Creek show evidence of this past sea. Some rocks hold the fossilized remains of small pelecypods (mollusks with two shells, similar to clams) and other small invertebrates that once lived in the shallow embayment (See pictures at right). Purisima Formation Prone to LandslidesBecause the sandstones and siltstones that make up the Purisima Formation are not solid or compact, they are easily weathered and prone to landsliding, especially during wet winters. The winter before the April 1906 quake, heavy rains fell in the area, making the soil water-soaked and unstable. Nine men in a logging encampment near Hinckley Creek were killed in the early hours of that spring morning when their cabin was engulfed by a landslide.
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