Geology of the Canyon

Geology of the Canyon. Saint Francis Dam Disaster National Memorial and National Monument. , Geology That Contributed to the Failure , Landslides of the eastern abutment, oversaturated sandstone and conglomerate of the western abutment, and an active thrust fault under the footing contributed to the failure. The Sespe Formation became over-saturated, decreasing its cohesive strength. Typically, conglomerate has considerably high strength, but it quickly disintegrates into sand and clay when saturated. The strengths of saturated schist and red conglomerate were not high enough to handle the pressure of the water and the large dam infrastructure. , , Geology of the Dam , The dam overlaid the San Francisquito Fault, a boundary between different rock types: conglomerate and sandstone (west) and mica schist (central and east). Conglomerate is a sedimentary rock made of rounded gravel, pebbles, and sand that is usually held together by silica, calcite, or iron oxide. Sandstone is a sedimentary rock composed of sand-sized grains of mineral, rock, or organic material. It is like conglomerate, but the rock particles are sand rather than gravel. The conglomerate and sandstone in the area make up the Sespe Formation. Schist is a course-grained metamorphic rock which consists of layers of different minerals. Pelona Schist is the dominant rock type of the eastern San Gabriel Mountains. The Pelona Schist bedrock on the eastern embankment of the dam is the location of an ancient landslide. , , "Considering the technology available at the time, there was no way he or any of the people working for him would have known about this gigantic landslide." , -David Rogers, Geological Engineer , , Formation of the San Francisquito Canyon , The canyon you are standing in is in the Sierra Pelona Mountain Range, which is part of the Central Transverse Range. To the southeast are the San Gabriel Mountains, and to the northwest are the Tehachapi, San Emigdio, and Topatopa Mountains. During the mid-Pleistocene Era tectonic forces from colliding plates began to uplift the Central Transverse Range. Canyons were formed as water gradually eroded the uplifting mountains, eventually leading to the formation of the canyon and the San Francisquito Creek running through it.