The Geologic Foundations of Ring Mountain

Ring Mountain is set in the midst of California's Coast Range, where the western edge of the North American Plate meets the Pacific Plate. A general description of the coastal mountains of California can be found in this California Coastal Commission description. Some of the more important details of the modern geologic history as it applies to Marin County can be found here.

The geologic features of Marin are dramatically divided along a major feature of this boundary (the San Andreas Fault). The sharp division can be seen in a geologic map representation of the county produced by students. The dramatically contrasting geology of the Pt. Reyes peninsula, which forms the western portion of Marin County, is discussed in another College of Marin WWW project.

"Clickable" East Marin Geologic Column

Prior to the development of the theory of plate tectonics and information derived from the deep sea drilling project, the Coast Ranges of California were a geological enigma. The mixture of pillow lavas, cherts, limestone, interbedded sands and shale, serpentine and blueshist metamorphic rocks, many of which are disrupted and sharply sheared, with no recognizable top or bottom, had left geologists scratching their heads and developing elaborate explanations. With the development of plate tectonics, however, the disparate mass became suddenly explainable, and the Coast Ranges assumed significance as a reference area of worldwide importance. As a result of the extraordinary high pressure and low temperature blueshist metamorphic rocks exposed in its slopes the Tiburon Ridge and the Ring Mountain area has been a prime study site.

While the evidence itself has commonly been jumbled as if in a giant mixer and much of it has been lost, geological interpretation of the jumbled, piece-meal evidence in the context of the plate tectonics theory makes it possible to imagine (see) the worlds contained within the rocks on, and below, Ring Mountain. A brief text summary of the stratigraphic column can be found here.

During the age of the dinosaurs, it is interpreted that the geological foundations of Ring Mountain existed not as a mountain at all but rather as part of the deep ocean floor. The history begins at a sea floor spreading center where vulcanism produced pillow lavas and submarine hotspring activity left its altering mark on the pillows and other rocks touched by the escaping heat of the Earth. This oceanic crustal rock forms the foundation of Marin County east of the San Andreas Fault and much of the California Coast Ranges.

As the ocean floor slowly traveled away from the heat of the sea floor spreading center it was headed for collision with the westerly moving North American continent. During this period, prominent radiolarian cherts seen throughout Marin and other parts of the Coast Ranges of California, were slowly being deposited on the thousands of miles distant to the slowly converging continent.

Concluding its history as part of the ocean, the ocean floor moved into a deep offshore subduction zone where, over the course of millions of years, layer upon layer of mud and sand were deposited by turbidity currents or submarine landslides. These currents produced the frequently visible layers of sand and clay in the Marin County that are known to geologists as greywackes and shale. Derived from the erosion of distant land or islands far to the east of Ring Mountain these landslides rarely brought marine fossils over the vast distances, however they are reported from several sites in the county. As one moves east in the California Coast Ranges, these fossils become more and more common. Clearly the geography of California looked far different than it does at the present time and may have looked more like the current western margins of the Pacific Ocean.

As the Pacific and North American plates collided at the rate of about 2-6 cm per year, the dense ocean plate slid under the lighter continental plate, the ocean sediments were bulldozed off, together with chunks of ocean crust, and shoved up against the leading edge of the continent.

A particularly interesting cross-section consisting of, from right to left, basalt (in the Alexander Ave. road cut), cherts (on the right of the large highway cut) and turbidites (on the left side of the highway), can be seen in this photo of Marin immediately north of the Golden Gate Bridge).

Some parts of this sea floor section were subjected to rapid downward transport in what geologists call a subduction zone, where they were tectonically mixed and intruded by serpentinite, a rock believed to have been metamorphically derived from rocks from the Earth's upper mantle. Eventually, this subduction ceased, and the subducted mass rose back up to the surface bringing with it the rocks which were squeezed but not significantly heated (creating some of the rare minerals found on Ring Mountain).

The rock unit composed of this jumbled mass exists throughout the California Coast Ranges, and is variously referred to by geologists as the "Franciscan Formation, Group or Complex." Evidence of the chaotic nature of this history can be seen in our walk up Ring Mountain, in many sites throughout Marin County, and in the locale from which the rocks takes their name, San Francisco.

Recent Volcanic Activity in Marin

12 million years ago, molten rocks were working their way through the jumbled oceanic rocks and erupting on to a long disappeared landscape that may have been similar to that in the Cascade Mountains to the North. 

We see the remains today best illustrated in North Marin's Burdell Mountain, here looking north (1981) across Novato (1997 view of Burdell looking across Hamilton Air Force Base), and here in an aerial photo that shows the pronounced tailings pile from an andesite cobblestone quarry at the summit. 

We also see evidence of recent volcanism in the old Hutchinson quarry property just across the Corte Madera Creek embayment where Larkspur Landing is today. The light colored rocks in this photo are rhyolites used to fill the pond where Dirty Harry finished off the bad guy. A Courtyard Hotel now sits on the site of a rhyolitic dike that pierced the quarry. The magmatic gases caused significant emplacement of pyrite and other minerals in the intruded greywacke and shales. A unique pyritized vein that showed fault slickensides was exposed in the early stages of development of the Larkspur Landing property. 

Modern Landshaping in Marin

While humans have created significant changes in the landscape, streams and landslides have worked on the Franciscan rocks.  Stream processes have been significantly altered during the Pleistocene ice ages. The geological and biological impacts of the Ross Valley's Corte Madera Creek are explored here.

Landslides have played an important part in the history of Mt. Burdell and slides have shown up in a housing development on it's South East side. Consensus attributes the slide to construction activity on a large project above the housing development.  Images from the Buck Center/Partridge Knolls slide. More information on landslides in Marin County can be accessed by the USGS Summary Distribution Of Slides And Earth Flows In Marin County, California open file report.

A comprehensive geologic map of the eastern portion of Marin County and surrounding counties is available from the USGS c/o An image version(.jpg), a .pdf document, and a GIS version are available.

If you are just driving through Marin on Hwy 101, check out our developing roadside tour but lookout for the deadend!

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