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Mount Soledad Sinkhole

Photo by Michael Hart

New Slide, Old Fossils

By Kesler A. Randall

On October 3, 2007, a landslide which covered an area little more than an acre occurred on Mount Soledad in the community of La Jolla, City of San Diego. The slope failed on the 5700 block of Soledad Mountain Road, and moved east and down the hill toward Rose Canyon and Interstate 5. This natural disaster destroyed a section of the roadway, damaged or destroyed nine homes, and forced the evacuation of several dozen people. The landslide occurred immediately north of an earlier landslide that destroyed homes along Soledad Mountain Road in December 1961.

The term landslide is generally used to describe any kind of mass wasting of native rock and soil. Specific types of landslides include slumps, rockslides, block glides, mudflows, and earthflows. These are each defined by the movement of Earth and its geometry relative to the undisturbed bedrock underneath. Slump landslides are the most common in southern California and are defined by a specific “anatomy.” In a slump landslide, blocks of bedrock break away and rotate backward along a curved surface or slip plane. The rotated blocks move downward forming a high-relief scarp (head scarp) at the top the slide, while at the base of the slide, rubble is pushed beyond the slip plane and form a narrow, elongate toe. Often the slide is broken up into several smaller parallel blocks which each move along their own slip plane and from smaller individual scarps. These slump landslides can also be characterized by other features such as crevasses, hummocky landforms, and transverse ridges.

The Mount Soledad landslide has all of the characteristics of a slump landslide. The head scarp was near the western side of Soledad Mountain Road with the main slump block rotating and moving eastward. The majority of the damaged properties occurred in this portion of the landslide. Some smaller blocks on the perimeter of the main block failed, with the slip planes forming along inclined bedding surfaces. The toe of the slide was relatively long and extended eastward from Soledad Mountain Road, buried portions of Desert View Drive, and just stopped short of several homes.

Landslides in southern California are fairly common, particularly in areas where sedimentary rocks are exposed in steep slopes. There are several factors that may contribute to the failure of a slope, and these may work individually or in combination with each other. Failure may occur as a result of rock being weak from an existing ancient landslide, water entering fractures in the bedrock either naturally or by artificial watering, the presence of clay beds on which slip planes can form, or artificial over-steepening of a slope by grading activities. The 1961 Mount Soledad landslide was most likely caused by grading operations for the construction of new homes along Desert View Drive. Landslides in developed areas are often caused by the removal of “sediment dams” at the base of steep slopes. This removal of the shallow base of slopes to build flat home pads often steepens slopes and makes the area above more prone to landslides. Studies of surface deposits on Mount Soledad have revealed the presence of numerous ancient landslides, particularly on the steep eastward facing slopes. In addition, as the Mount Soledad landslide occurred in a residential setting, irrigation and water utility leaks may also have contributed to the failure.

Remedial construction efforts have been ongoing since the current slide occurred in October 2007. Some homes have been repaired while others have been demolished; meanwhile, the City of San Diego has been engaged in a major slope repair and stability project. This project involves the installation of concrete caissons which will anchor the slope to stable bedrock beneath the slide and a re-sculpturing of the slope. Caissons are created by a drilling rig boring 80-foot-deep and 5-foot-wide bore holes, filling them with cylindrical rebar cages and then pouring in concrete. Once the caissons are in place, construction vehicles remove the failed rock and recompact it. The hope is that a new slope will be engineered with proper gradient and recompaction to prevent future slides.

Because of the extensive remedial grading required at the site, Museum paleontologists were hired by the City of San Diego to monitor construction activities for unearthed fossil remains. Field personnel monitored both sedimentary rock matrix brought up by the drill auger and bedrock that was excavated from the slope. These activities exposed strata of the 48-million-year-old Ardath Shale, a sedimentary rock unit that occurs over in central coastal San Diego County from Tecolote Canyon to Carmel Valley. Geologic evidence suggests that sandstone, siltstone, and claystone layers in the Ardath Shale were at the landslide site. The Ardath Shale has been historically interpreted as being part of a large, ancient, deep-sea fan system that had eroded through the continental shelf, deposited approximately 5 to 8 miles offshore, and settled within 1800 to 4500 feet of water. At the 2007 Mount Soledad landslide, the presence of exposed strata of alternating sandstone, siltstone and claystone deposited in shallow, low-relief-cut and fill channels further support this deep-sea fan model.

Museum paleontologists on-site were not only observing and recording new stratigraphic and sedimentological information, but were making fossil discoveries as well. The largest specimen recovered was a 5'x 3'x1' slab of concreted sandstone containing numerous plant fragments and marine invertebrate borings. This block, currently on display at the San Diego Natural History Museum, preserves portions of a single large tree trunk, numerous wood fragments, and fragmentary leaves of cycad plants. Cycads are primitive seed-bearing trees and bushes, often with large green pinnate (feather-like) leaves, and are found today primarily but not exclusively in the tropics. Some of the wood fragments found in the rock slab preserve borings made by the clam Teredo (ship worm). The evidence of the ship worm indicates the tree trunk floated in the ocean for awhile before settling to the sea floor where it was eventually buried by sediment. Other fossils discovered at the site include shells of marine invertebrates. The dominant invertebrates were clams and snails, which lived on or within the ancient sea floor. Many are similar to forms that live today in tropical waters. Preparation and identification of these specimens is still in progress at the Museum.

Although natural disasters like the Mount Soledad landslide are horrific in terms of loss of public and private property, some good does come from them. By understanding the natural process of mass wasting, scientists can either help prevent slides from happening in the future or help better prepare for slides when they do occur. In addition, the construction efforts on landsides temporarily expose bedrock that would normally be obscured, and has allowed geologists and paleontologists to collect more data for improving our understanding of the depositional setting of ancient strata like the Ardath Shale. Through the collection of fossils, we have increased our knowledge of paleoecosystems from the Middle Eocene of San Diego. For the Mount Soledad landslide, there is a silver lining.

Kesler A. Randall is Collections Manager of Fossil Vertebrates in the Department of Paleontology at the San Diego Natural History Museum.

Slump diagram



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