Ring Mountain

Located at the northern end of the Tiburon Peninsula, this area is of considerable interest to botanists and geologists.

A variety of soils in the area provide special environments in what is essentially grassland. On the serpentine grasslands along the ridge four rare species occur, Eriogonum caninum, Hesperolinon congesturn, Castilleja affinis ssp. neglecta, and Calochortus tiburonensis. The Castilleja is known only from the Peninsula. The Calochortus, which was only recently discovered, is known only from this area and is of particular import in establishing relationships within the genus. Here, too, is the only Marin occurrence of the Plagiobothrys stipitatus.

A particularly fine display of wildflowers occurs in the area. Among the many flowers are six lilies, Dichelostemma capitatum, Triteleia laxa, Brodiaea elegans, Triteleia hyacinthina, Dichelostemma congestum, and Triteleia peduncularis, which bloom consecutively; four species in the Scrophulariaceae which bloom at the same time, Castilleja densiflora, Triphysaria pusilla, Triphysaria versicolor ssp. faucibarbata, and Castilleja rubicundula ssp. lithospermoides, and five Phacelia species which, in some years, have bloomed simultaneously, Phacelia californica, Phacelia divaricata, Phacelia imbricata, Phacelia distans, and Phacelia malvifolia.

The grasslands also include a number of native grasses such as Danthonia californica, Melica californica, Poa secunda ssp. secunda, and Nassella pulchra.

Some marshy bowls, formed by old landslides, support Juncus bufonius and Juncus phaeocephalus var. paniculatus, Carex serratodens, Carex densa, and Carex praegracilis.

The only trees present, California bay, Umbellularia californica, and madrone, Arbutus menziesii, are found exclusively in the metamorphic rock area.

There is a rich and varied fauna in the area and it is the type locality for several species of insects.

Geologically, the ridge is famed for its unique association of minerals and complexity. The main rocks are a melange of the Franciscan formation. Serpentine caps the ridge, having been thrust over sandstone and shale exposed downslope on the lower flanks of the ridge. Exotic tectonic blocks of high-grade metamorphic rocks occur on and within the serpentine and within the intensely sheared melange matrix separating the serpentine from the sedimentary rocks below it. The metamorphic rocks, which include many types that are quite rare, are relatively resistant to weathering and they are exposed as scattered, dark-colored, monument-like masses. The wide variety of coarse-grained metamorphic rock of the rare eclogite and blueschist assemblages is unique for any one locality, and some of the rock types present here have not been reported elsewhere. The mineral lawsonite, a significant metamorphic indicator mineral, was first described from one of the exotic tectonic blocks near the edge of the area.

Several sites of archaeological importance are present.

Integrity: Despite the presence of nearby roads and houses, the area is undeveloped.

Use: Private

Ref: Hill, Albert J. 1973. A Distinctive New Calochortus (Liliaceae) from Marin County, California. Madrono Vol. 22, No. 2, pp. 100-104.
Dudley, Priscilla, 1972. Comments on the Distribution and Age of High-Grade Blueschists, Associated Eclogites and Amphibolites from Tiburon Peninsula, California. Bull. Geo. Soc. Amer. 83 (11), pp. 3497-3500.

September 1975