Roadside Geology of Southern California

Posted 21 April 2016 by A.G. Sylvester ©

This route transects the Santa Monica Mountains from south to north between Pacific Coast Highway (PCH) and US 101 in Woodland Hills. The geology is complicated by faults and much of it is shrouded by vegetation. Rocks in the roadcuts within the first half mile of the canyon are strongly fractured and shattered from deformation within the Malibu Coast fault zone.

Cretaceous (?) and Paleocene Sandstone and Conglomerate

South-dipping beds of brown sandstone of Cretaceous age and Paleocene conglomerate are exposed in low roadcuts here and there on both sides of the canyon in the first two miles of the canyon. Between PM 1.3 and 2.0, bold stream cuts expose impressive outcrops of upper Cretaceous cobble conglomerate, consisting mostly of granitic, metavolcanic, and quartzitic detritus. Such rocks are widely distributed in southern California and reflect the erosional stripping of the rocks of, and above, the Sierra Nevada and Peninsular Ranges batholith during their tectonic uplift in late Cretaceous and early Cenozoic time.

Topanga and Modelo Formations

From PM 2.7 to PM 4.3, CA 27 goes through light gray to brown sandstone of the lower Topanga Formation of early Miocene age. Beds are thick and dips are mostly north. Conejo Volcanics, consisting of olive gray and black dikes and sills of diabase and which are rather featureless in roadcuts, such as that at PM 3.0, have extensively intruded and overlie the Topanga Formation.

At the intersection of CA 27 and Old Topanga Road (Pm 4.3), CA 27 enters north-dipping upper Topanga Formation and continues through it for about three and a half miles (PM 7.8). It was deposited in marine or brackish water and consists of light brown to tan, thick-bedded sandstone.. A rather featureless intrusion of Conejo Volcanics diabase is in roadcut at PM 7.5.

The Topanga Formation is overlain unconformably by north-dipping Monterey Formation strata is exposed from PM 8.0, through all the switchbacks, to the intersection of CA 27 with Mulholland Drive at PM 11.0. Here in the Santa Monica Mountains, the Monterey Formation consists of moderately hard, white-weathering, platy, siliceous shale. The fresh rock is dark brown.

The straight and relatively flat part of CA 27 through the residential part of Woodland Hills lies on gently dipping Modelo Formation claystone and siltstone that overlie the Monterey Formation along the south edge of San Fernando Valley between Mulholland Drive to US 101.

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