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Malibu Physical Geography (Physiography)
About 30 million years ago the relative movements of the North American Plate and the Pacific Plate changed from a head-on contact to a lateral slipping against each other. This zone of slippage, extending nearly the length of California, is called the San Andreas Fault. Along this zone, folding of the sea floor along the margin of the North American Plate resulted in the creation of the Coast and Transverse ranges, which are composed of the crushed, crumpled, and folded sea floor sediments.
The Transverse Ranges are the only east-west-trending ranges in California. The Santa Monica Mountain Range, which is the southernmost of the Transverse Ranges, is 46 miles long and averages 7.5 miles in width. The geologic structure of the Santa Monica Mountains is exceedingly complex after tens of millions of years of often violent geologic activity, evidenced by many ancient volcanoes and a long history of earthquakes. The main mass of the Santa Monica Mountains is comprised of complex folded and faulted structures of Late Cretaceous (approximately 100 million years bp) through Middle Miocene (approximately 18 million years bp) as well as sedimentary and volcanic rocks. This complex structure was caused by major thrust faulting, and was made more complex by localized faulting.
In the eastern, upper-most reaches of the mountains, a wide valley floor consisting of Holocene alluvium up to 60 feet deep overlies and is surrounded by Miocene Conjeo volcanics. The middle northern reaches of the mountains are Cretaceous and Tertiary sediments. Shallow alluvium fills in the relatively steep canyons. Tertiary basaltic and andesitic flows, pillow breccias, intrusives, and dikes are found in the eastern reaches of the watershed. Malibu Canyon cuts through Tertiary sandstones, siltstones, and breccias interbedded with Tertiary volcanics. Quaternary landslides occur throughout the watershed but are especially noticeable near the coast and in the Cold Creek subwatershed.
The Malibu area is a rare coastal Mediterranean ecosystem, found in few parts of the world.
The Malibu Beach Quadrangle
The Malibu Beach Quadrangle, as defined by the US Geological Survey, covers an area from Dan Blocker County Beach east to Las Flores Beach, and from approximately the southern boundary of Calabasas south to the Pacific Ocean at Malibu. The onshore portion of the Quadrangle covers an area of approximately 45 square miles in southwestern Los Angeles County and includes parts of the cities of Malibu and Calabasas and the unincorporated communities of Malibou Lake, Monte Nido, Malibu Bowl, and El Nido. The Malibu Civic Center is located in the south-central part of the map area, about 25 miles west of the Los Angeles Civic Center. Santa Monica Bay occupies the southern quarter of the quadrangle.
The Malibu Beach Quadrangle is dominated by steep and rugged terrain of the central Santa Monica Mountains. Local elevations range from sea level to 2828 feet at Saddle Peak in the east-central part of the map area. The main crest of the mountain range trends generally east-west across the center of the quadrangle, although the actual drainage divide is located north of the quadrangle boundary in the Simi Hills. Numerous southtrending broad-crested ridges and canyons with narrow channels extend from the range crest to Santa Monica Bay. The east-west-trending Malibu Coast Fault Zone forms the southern boundary of the mainland portion of the mountain range.
Malibu Quadrangle Drainage Systems
The most important drainage system in the quadrangle includes Malibu Creek and its tributaries, Cold Creek, Las Virgenes Creek, Stokes Canyon, and Liberty Canyon, which drain a large area south of the Simi Hills and flow via Triunfo Canyon - Malibu Canyon through the entire mountain range to Santa Monica Bay. The larger canyons in this drainage area are wide and flat-bottomed and form gently sloping to flat-lying terrain near their confluence with Malibu Creek in the northwestern quarter of the map. Malibu Creek flows southeast and then south in Triunfo Canyon - Malibu Canyon through a deeply incised channel near the center of the quadrangle. The Malibu Creek floodplain and delta form a gently sloping to flat-lying surface underlying the Malibu Civic Center near the coast.
The Quadrangle coastline west of Malibu Creek is characterized by broad, gently sloping, relatively continuous terrace surfaces that terminate in moderately steep bluffs above a narrow beach. East of Malibu Creek, the coastline consists of a moderately steep to steep mountain front with a few discontinuous terrace surfaces and a narrow beach.
Malibu East of the Quadrangle
The adjacent area to the east of the Malibu Quadrangle is in the USGS Topanga Quadrangle. The coastline continues with moderately steep to steep mountain front with discontinuous terrace surfaces and narrow beach, to the floodplain of Topanga Creek at the eastern end of the area. Other drainage includes the watersheds of Tuna Canyon, Pena Canyon and Piedra Gorda Canyon (Big Rock). Topanga Creek drains the second largest watershed of the Santa Monica Mountains, 18 square miles lying east of the Malibu Beach Quadrangle. Topanga Creek flows from its upper watershed into a deeply incised channel ending at its floodplain and small lagoon.
Malibu West of the Quadrangle
The adjacent area to the west of the Malibu Quadrangle is in the USGS Point Dume Quadrangle. The Santa Monica Mountians continue west of the Malibu Beach Quadrangle all the way to Point Mugu with the city limits of Malibu extending to the Ventura County line and Malibu Post Office service to the few residents west of that line. The south faces of the Santa Monica Mountains are drained by coastal creeks flowing in canyons including Solstice, Zuma, Trancas, Arroyo Sequit, Little Sycamore, Big Sycamore, and La Jolla canyons.
Arroyo Sequit Canyon is located approximately fourteen miles from the western end of the Santa Monica Mountains, draining the south side of Sandstone Peak, the tallest peak in the Santa Monica Mountains (3,111 feet) and the Malibu Springs area. Sandstone Peak is located approximately two miles north of Circle X Ranch, a former Boy Scout camp owned by the National Park Service since 1989. The Arroyo Sequit watershed generally flows in a south to southwest direction to the Pacific Ocean. Along most of its course, Arroyo Sequit has a steep gradient and is entrenched in a deep, somewhat sinuous canyon with a V-shape profile. Its headwaters are composed of a north fork and a south fork that merge about two miles upstream of Leo Carrillo Beach. The west fork portion of the drainage includes the southern slopes of Sandstone Peak. Both Boney Peak (2825 feet) and Sandstone Peak are located on Boney Mountain, a massive volcanic rock formation that belies the name of Sandstone Peak.
Arroyo Sequit Gorge and the meadows and woodlands of Nicholas Flat lie above Leo Carrillo State Beach. Yerba Buena Canyon (Little Sycamore Canyon) drains the areq west of Arroyo Sequit leading to the ocean in the County Line vacinity. Big Sycamore Canyon, and the La Jolla Valley are at the western end of the Santa Monicas near Pt. Mugu.
Acknowledgement: The material in this section was partly adapted from Seismic Hazard Zone Report 050, Malibu Quadrangle, 2001, State of California, Division of Mines and Geology.
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