A Dynamic Range
Many of the Santa Monica Mountains rock formations were created underwater. Learn about each formation here. Photos courtesy of Malibu Creek State Parks docents.
Imagine standing on the bank of Malibu Creek around 10 million years ago. There would be no Santa Monica Mountains surrounding you! The creek is thought to have flowed in its present course long before the range existed. The sedimentary layers you now see along the ridgelines were once underwater. About 5 million years ago compressive forces folded and faulted the land, uplifting it out of the sea. Its east-west formation is known as a Transverse Range, which is characteristic of Southern California. Transverse Ranges are caused by a bend in the San Andreas Fault which shifts the mountain ranges east-west instead of a north – south direction.
An artist rendition of the coast before the Santa Monica Mountains existed. Painting by John Iwerks, courtesy of Department of Geological Sciences, Cal State Northridge
The Santa Monica Mountains is a complex and dynamic range that includes active fault lines, fossils, remnants of volcanoes, and many formations created underwater. The distinctive black-gray and reddish volcanic rocks you see throughout the watershed are known as the Conejo Volcanics. They date back to around 13 million years ago and form the backbone of the range.
The range’s sediment yields susceptible to high erosion rates are attributed to its Mediterranean climate, topography, vegetation, and soil structure. Erosion of the volcanic and sedimentary rocks are carried by flowing water, filling valleys and streambeds with alluvial soil. Malibu Creek offers a transport system that carries sediment downstream to the coastline, providing beach sand for the recreation that Malibu is known for worldwide.
The trapped sediment behind the dam was intended for nourishing Malibu beaches.
For over a century, much of the Malibu Creek watershed’s high sediment yield has been trapped behind Rindge Dam. Just after the dam’s spillway was completed in 1926, a heavy storm indicated the unanticipated high sedimentation rate that would eventually fill the dam. Although built to store 500 acre-feet of water, by June 1945 the reservoir was down to holding only 76.5 acre feet of water and by 1967 it held less than 15 acre-feet. The abandoned dam now holds approximately 780K cubic yards of sediment (which tends to weigh more than water per cubic foot).
Since September 2024, the MCERP Geotech Team has been sampling, analyzing, and assessing the trapped soils taken from several depths behind the dam. Characterizing the grain sizes and other important material properties is a key component of the pre-construction, engineering and design phase; this data will establish the ecological and hydrological blueprint for restoring the watershed’s natural transport system of delivering much needed sediment to the beaches.
The Santa Monica Mountains' geology is a range with diverse rock types, including sedimentary, volcanic, and metamorphic rocks. Photo Bernard Yin