Pecked curvilinear nucleated petroglyphs on Ring Mountain

Ring Mountain is an elevated landform on the

Tiburon Peninsula The Tiburon Peninsula (french: Péninsule de Tiburon), or The Xaragua Peninsula, simply "the Tiburon" (''le Tiburon''), is a region of Haiti encompassing most of Haiti's southern coast. It starts roughly at the southernmost point of the Haiti-D ...

Marin County Marin County is a county located in the northwestern part of the San Francisco Bay Area of the U.S. state of California. As of the 2020 census, the population was 262,231. Its county seat and largest city is San Rafael. Marin County is acros ...

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

. This mountain was named for George E. Ring, who served as a Marin County Supervisor from 1895 to 1903. A number of rare and

endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and inva ...

flora Flora is all the plant life present in a particular region or time, generally the naturally occurring (indigenous) native plants. Sometimes bacteria and fungi are also referred to as flora, as in the terms '' gut flora'' or '' skin flora''. E ...

inhabit Ring Mountain. The mountain's twin summits consist of

serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''ser ...

, a rock which is very high in

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

, producing soils of unusual chemistry (

serpentine soil Serpentine soil is an uncommon soil type produced by weathered ultramafic rock such as peridotite and its metamorphic derivatives such as serpentinite. More precisely, serpentine soil contains minerals of the serpentine subgroup, especially anti ...

). The landscape is strewn with many sizable boulders which exhibit a variety of

lithologies The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

including high-pressure

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...

s of

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...

blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The blue co ...

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite ...

, and

eclogite Eclogite () is a metamorphic rock containing garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dol ...

grade. Native American

pecked curvilinear nucleated Pecked curvilinear nucleated (PCN), in archaeology, is a form of prehistoric rock carving. The term was originally proposed by Teresa Miller and Reed Haslam in 1976 to describe a widespread type of rock carving in western North America. The form ...

petroglyph A petroglyph is an image created by removing part of a rock surface by incising, picking, carving, or abrading, as a form of rock art. Outside North America, scholars often use terms such as "carving", "engraving", or other descriptions ...

s created by the

Coast Miwok Coast Miwok are an indigenous people that was the second-largest group of Miwok people. Coast Miwok inhabited the general area of modern Marin County and southern Sonoma County in Northern California, from the Golden Gate north to Duncans Point ...

people are also found here.

Geology

Ring Mountain is a unique geological site, where rocks that formed in ancient

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

s can be observed. Serpentinized

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...

crops out on the two summits of the mountain, and the steep upper slopes are underlain by serpentinite-matrix

mélange In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically cons ...

. The melange contains blocks of high-pressure, low-temperature metamorphic rocks associated with subduction zone metamorphism. Melanges of this general style are known from the

Franciscan Complex The Franciscan Complex or Franciscan Assemblage is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew ...

, but this melange is particularly notable for the size and variety of the metamorphic blocks. Dating of metamorphic minerals in the blocks indicates that they were produced over a protracted history of subduction which began ~175 million years ago. The blocks preserve mineral assemblages characteristic of greenschist facies, blueschist facies, amphibolite facies, and eclogite facies metamorphism and is the type location of the mineral

Lawsonite Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common. It forms transparent to transluce ...

. The lower slopes are underlain by greywacke sandstones and shales of prehnite-pumpellyite metamorphic grade, but the contact between the sandstones and the serpentinite-matrix melange is not exposed.

Landslides Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...

and their deposits are abundant on Ring Mountain, for example at

Triangle Marsh Triangle Marsh is a wetland of the San Francisco Bay, situated at the base of Ring Mountain at the north end of the Tiburon Peninsula in Marin County, California. Archaeological Pecked curvilinear nucleated petroglyphs, and recovery on Rin ...

, and they carry serpentinite and metamorphic blocks far downslope from their ''in situ'' positions. Turtle Rock blueschist block on Ring Mountain

The origins of the serpentinite-matrix melange, and the mechanism of mixing the metamorphic blocks of different ages and apparent thermal-burial histories, has been a matter of debate. Some authors argue that the metamorphic rocks were exposed at the surface, eroded and re-deposited into a subduction trench to form the melange as an

olistostrome An olistostrome is a sedimentary deposit composed of a chaotic mass of heterogeneous material, such as blocks and mud, known as olistoliths, that accumulates as a semifluid body by submarine gravity sliding or slumping of the unconsolidated sedi ...

. Others interpret the melange as having formed in a subduction plate boundary where blocks of meta-basalt from the downgoing plate were mixed with serpentine from the upper plate mantle. Ring Mountain is one of the featured field trips found in the Streetcar 2 Subduction online field trip guide series released in December 2019 by the

American Geophysical Union The American Geophysical Union (AGU) is a 501(c)(3) nonprofit organization of Earth, atmospheric, ocean, hydrologic, space, and planetary scientists and enthusiasts that according to their website includes 130,000 people (not members). AGU's act ...

Animal life

Though Ring Mountain is an island ecosystem surrounded by Highway 101 and suburbs, it hosts a variety of wildlife, including coyotes, deer, skunks, and many birds and reptiles.

Plant life

Ring Mountain is the home of the only population of

Tiburon mariposa lily ''Calochortus tiburonensis'', the Tiburon Mariposa lily, is a rare member of the genus ''Calochortus'' in the family Liliaceae. It is Endemism, endemic to Marin County, California, where it is known only from one population on Ring Mountain (Cali ...

in the world. This flower grows near the summit of the mountain in the grassy areas. Ring Mountain is also home to other rare plants such as the Tiburon jewelflower and the Tiburon paintbrush. The Nature Conservancy bought the land around the mountain and has been responsible for preserving the rare native plant species of the area.

Recreation

Ring Mountain is a popular hiking area and provides spectacular 360 degree views of the northern Bay Area.

References

External links