Portuguese Bend Landslide
During the Miocene, erosion of rocks in the area we now call Southern California were being deposited: Silts and diatomaceous oozes in deep ocean basins, sand along rivers, coastal areas and shallow marine environments; and pebbles, cobbles and sand along rivers. Volcanic activity was also common during this time, depositing layers of ash over the entire region. Over time, these sediments lithified into rocks.
Tectonic activity along the many faults in the Southern California region uplifted these rocks (called the Monterey formation in this area) above sea level. Weathering began to erode some rocks, forming the cobble beaches common along the Palos Verdes Peninsula, and altering others, such as the ash beds. Ash beds will often alter to form a clay called bentonite. Bentonite is very slippery when wet! Uplift of the Palos Verdes region along the Palos Verdes Fault tilted theses rocks further, causing them to tilt towards the Pacific Ocean. The term geologists use to describe when bedrock tilts in the same direction of the slope is called adverse bedding and is a major cause for mass wasting events such as landslides. |
