| 10. The Little Slide | ||||||||||||
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Interpretive Stops
1. Along Aptos Creek Additional Information
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Landslide EvidenceEvidence of a landslide, most likely caused by the heavy winter rains of 1982, can still be seen on some redwood trees. If you look at the backsides of these trees (sides facing the upslope), you can see mud embedded in the bark, in some places 15 feet above the ground. Mud embedded so high on tree bark means that a forceful flow of mud and water rushed down the hillside at one time. Other landslide indicators are the mounds of dirt debris, now covered by vegetation, that are lodged against the upslope side of some of these trees.Debris Avalanches and Debris Flows: Two Common Landslides in CaliforniaLandsliding is the downhill mass movement of soil or rock, or a combination of the two, called debris. Landsliding is ultimately driven by gravity. Several types of landslides have been categorized by geologists, two of the most common in California being debris avalanches and debris flows. Debris AvalanchesDebris avalanches are relatively small, shallow slides that are common on steep slopes underlain by sandstone. One debris avalanche probably caused the collapse of the Loma Prieta Mill during the 1906 earthquake. Calling them "earth-avalanches," geologists of the time reported that debris avalanches were common in the Santa Cruz Mountains during the 1906 quake, and that one "earth-avalanche near Olive Springs demolished the Loma Prieta Mill and killed several men." Debris FlowsA second and more dangerous type of landslide commonly found in California is a debris flow. This type of slide occurs when rock and soil become completely saturated with water and start to move. Movement of rock and soil in a debris flow can reach surprisingly fast speeds, sometimes up to 60 miles (hundreds of kilometers) per hour. A small debris flow, probably generated during the heavy winter rains of 1982, is likely responsible for plastering the mud into the redwood bark seen at this stop. Starting near the top of the ridge, the debris flow moved down the slope toward the ravine and Aptos Creek. Why do we see landslides in the area?Two main events have created the potential for landslides: the uplift of weak, unconsolidated sediments and the fracturing of these sediments due to stress caused by faulting. The Purisima Formation, deposited by the marine embayment that once covered this area, is made up of weak, unconsolidated sediments that were uplifted during the late Pliocene when the Coast Range was formed. Over time, water and winds eroded these soft sediments, forming the steep-sided slopes and canyons seen throughout the park. Just as everything else on Earth is subject to gravity, so are the sediments on these steep-sided slopes. Gravity, along with other outward forces, continuously attempts to bring down these sediments causing what geologists call, "shear stress." But "shear stress" is balanced by the slope's internal ability to remain intact, called "shear strength." Occasionally, the balance between shear stress and shear strength is tipped, and gravity and its cohort forces win. This happens when extra mass is added to the slope. Extra mass can be added by any number of human-made and natural ways. One human-made way is to erect a building on the slope. A natural way for mass to increase is through the addition of water, such as during or after a heavy winter storm. When the slope's shear stress is more than its shear strength, the slope gives way to gravity and starts moving downhill. The second reason for landslides in the area is the fracturing of the underlying rock due to stress within the fault zone itself. The North American and Pacific Plates rub against each other along the fault zone creating stresses which cause rocks to fracture and weaken. The weakened rocks then become more susceptible to movement. At least five large slides have been mapped within the state park. One landslide, along the appropriately-named Big Slide Trail, scraped off trees and underbrush from an entire hillside after it was triggered by the 1989 Loma Prieta Quake. Landslides are Part of Natural DisturbanceLandslides are devastating to human life and property, but they are also part of the inevitable change to a forest. As part of natural disturbances such as fire, flood, and wind, landslides in this area have probably helped prepare a nutrient-rich soil for redwood seedlings. 1982 Floods Close Nisene Marks SP for Six MonthsDuring the 1982 floods, Nisene Marks State Park was closed for six months. One large logjam was reported on Aptos Creek just south of Marcel's Forest.
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