Description
The landslides discussed in this report are located in an area bounded by Highway 395 on the west, Interstate 8 on the south, Highway 67 on the east, and the community of Rancho Bernardo on the north. The rocks exposed in the area consist of a basement complex on granitic and metavolcanic rocks overlain by a thin sequence of flat-lying Eocene conglomerates, sandstones, and claystones. Landslides occur at or near the contact between conglomerate and claystone formations or at the contact between claystone and basement rocks. The rupture surface is planar and, slickensided, with several inches of remolded clay above and ' undisturbed sediments below the slip surface. The slides are classified as slumps or combination slump and blockglide landslides. Landslides with erosional inversion of relief and slides with very young features indicate a wide range of ages, with a maximum age of early Pleistocene. Many factors contribute to a hillside environment favorable for landsliding. Among these are heavy rainfall, seepage pressures, earthquakes, structure, and weak, overconsolidated claystones. Landslides may be easily reactivated because further movements are controlled by residual shear strengths, a factor of the residual angle of internal friction and cohesion. Three of the seven landslides discussed in detail are designated the West-, East-, and North San Carlos Slides. The West Slide is primarily a block-glide slide, with slumping occurring only at the head. Accumulation of slopewash at the head of this slide has resulted in slow creep or repeated movements of short duration. The East- and North Slides are composite slides also, but slumping has been the primary mode of failure. Each of these slides consists of a series of overlapping slump blocks with a common, nearly horizontal, main surface of rupture. The Poway Slides designated A through D are located north of Poway Road and east of Pomerado Road. The out-of-slope dip of the contact between the sediments and basement rocks under slides A, B, and C was a major reason for the failures. Slide A, the largest of the four Poway Slides, has a well-developed topographic bench at the head and a 30-foot deep gulley separating the slide into two unequal portions. The slip plane follows the basement contact closely. Slide B is a block-glide type that also failed over basement rock. Its structure is a result of subsidence of the head of the slide into a gap created by horizontal movement in the main body of the slide. Erosion has all but obliterated the scarp and lower limits of Slide B. Slide C exhibits erosional inversion of relief which indicates a probable late Pleistocene age of sliding. It failed in a manner similar to the East San Carlos Slide. Slide D has a classic rotational slump structure different from the block-glide types of failures of Slides A, B, and C. One important conclusion regarding the recognition of landslides is that many of them are very ancient features that do not exhibit the classic geomorphic criteria normally used to identify landslides.
