Santa Rosa Plateau Rock Formations
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Rock Formations On The Santa Rosa Plateau
© W.P. Armstrong, 24 November 2007

  1. Woodson Mt. Granodiorite
  2. Santa Rosa Basalt
  3. Santiago Peak Volcanic
  4. Bedford Canyon Mudstone
  5. Santa Margarita Peak
  6. Santa Margarita Pk. Rocks
  7. Miscellaneous Images

Woodson Mt. Granodiorite

Covered with dense foliose lichens, incl. Xanthoparmelia

Engelmann oaks (Quercus engelmannii) and boulders of Woodson Mt. granodiorite.

Dark gabbro within granodiorite.

Santa Rosa Basalt

View of Mesa de Burro with Mt. San Jacinto in distance. In the foreground is Engelmann oak woodland with coyote trails through the grassland.

Ridge of Santa Rosa basalt along the edge of Mesa de Burro. Although Acarospora schleicheri is listed for the Santa Rosa Plateau, this species is now considered a to be soil lichen. The lemon-yellow crustose lichen on the vertical cliff may be A. contigua, a saxicolous species. Two similar lichens Pleopsidium chlorophanum and P. flavum have a marginally lobate thallus. They were fornerly classified in the genus Acarospora.

Ridgeline composed of ancient flow of Santa Rosa basalt.

Polygonal basalt columns at the edge of Mesa de Burro. These formations are similar to basalt columns at Devil's Postpile National Monument in the Sierra Nevada.

  Devil's Postpile National Monument  

Porous (vesicular) Santa Rosa Basalt showing air bubbles in ancient magma.

View of grassland valley west of Mesa de Burro looking north from Clay Hill. The Santa Rosa Basalt flow (upper right) once covered this valley. The basalt has been eroded away, exposing underlaying rocks of Bedford Canyon Mudstone and Santiago Peak Volcanics. These rock types are shown in the following photo images.

Santiago Peak Volcanic

These rocks have pitted surfaces and superficially resemble Santa Rosa Basalt with small air bubbles; however, the pitted surface is not caused by air bubbles. Plagioclase crystals in the rock have been weathered away, thus producing the pits. The pits are often elongate-rectangular and match the crystal shape and size. Generally, the largest crystals of Santiago Peak Volcanic are larger than Bedford Canyon Mudstone, although weathered rocks are difficult to differentiate.

Jurassic age Santiago Peak Volcanic (about 145 million-years-old). The fresh exposure (left) shows numerous white plagioclase crystals. In the weathered exposure (right), the crystals have been removed forming numerous rectangular pits. Rocks of this ancient basalt are exposed on Clay Hill, west of Mesa de Burro, where the overlaying Santa Rosa Basalt has been eroded away. The latter area is the type locality for Brodiaea santarosae. This rock is exposed from Santiago Peak in the southern Santa Ana Mountains through coastal areas of San Diego County.

A. Santiago Peak Volcanic from Owens Peak in San Marcos showing numerous white crystals.
B. Similar crytstal-bearing rock from the east side of Santa Margarita Peak near Roblar Creek.

Rocks from the type locality of Brodiaea santarosae west of Mesa be Burro. A. Jurassic age Santiago Peak Volcanic (about 145 million-years-old). Note the pitted surface where plagioclase crystals have been weathered away. The original basalt has been reheated and metamorphosed into a dark, crystalline rock. B. 10 million-year-old Santa Rosa Basalt from nearby Mesa de Burro. This vesicular rock overlays mesas of the Santa Rosa Plateau and is permeated by air bubbles resulting from the eruption of ancient magma.

Magnified view of Santiago Peak Volcanic from Clay Hill west of Mesa de Burro. This is near the type locality for Brodiaea santarosae. The elongate surface pits are due to weathered, rectangular plagioclase crystals in the rock up to 2 mm or longer. This Jurassic age metamorphic basalt dates back approximately 145 million years, in contrast to nearby Santa Rosa Basalt that is 10 million years old.

Magnified view of Santiago Peak Volcanic from Owens Peak in San Marcos, San Diego County. The rectangular plagioclase crystals appear very similar in shape and size to rocks from Clay Hill.

  Santiago Peak Volcanic Rock On Owens Peak  

Rocks from the type locality of Brodiaea santarosae west of Mesa be Burro: The left and middle rocks are Jurassic age Santiago Peak Volcanic. The right rock is from the nearby flow of 10 million-year-old Santa Rosa Basalt at the base of Mesa de Burro. The lower rock is covered with the foliose lichen Xanthoparmelia conspersa (See next image).

This exposed lichen-covered Santiago Peak Volcanic shows ancient crack that was filled by sediment (right). Note the rectangular pits where plagioclase crystals have been weathered away.

Bedford Canyon Formation (Metasedimentary Rock)

Covered with dense lichens incl. gray Xanthoparmelia & orange-red Caloplaca

Note: Metamorphic sedimentary and volcanic rocks are difficult to differentiate in the field. This is particularly true of weathered rocks with pitted surfaces. Ideally, it is best to carefully examine the crystal shape and size in fresh exposures. Although there is considerable overlap in crystal size, the largest crystals of Bedford Canyon Mudstone are smaller than the largest crystals of Santiago Peak Volcanic.

  See Lichens Of The Santa Rosa Plateau  

Outcrop of Bedford Canyon formation (foreground) showing fractured boulders splitting into angular sections. The distant ridgeline is composed of an ancient flow of Santa Rosa basalt.

Dense lichens incl. gray Xanthoparmelia & orange-red Caloplaca.

Left: Bedford Canyon mudstone has a pitted, weathered surface that resembles basalt; however, there are no air bubbles within the rock as in true basalt. See following image of the left rock split into three sections. Right: Santa Rosa basalt with air bubbles throughout the rock. Basalt from deeper in the flow has smaller bubbles.

Bedford Canyon mudstone has a pitted, weathered surface that resembles basalt; however, there are no air bubbles within the rock when it is split open.

Bedford Canyon mudstone is a fine-grained metamorphic sedimentary rock with numerous, tiny, glistening crystals. In general, the largest crystals are much smaller than the largest crystals in Santiago Peak Volcanic rock. See magnified view of Santiago Peak Volcanic. In addition, the rock breaks easier with a sledgehammer.

Congomerate phase of Bedford Canyon rock showing angular fragments (black arrow).

Acarospora socialis on Bedford Canyon rock.

Acarospora socialis on Bedford Canyon rock.

  See Acarospora schleicheri On Soil In Kern County  
See Acarospora socialis (formerly A. schleicheri)

Miscellaneoous Images

Palmer's Goldenbush (Ericameria palmeri var. pachylepis)

Rainbow Manzanita (Arcotostaphylos rainbowensis)

Spherical lignotuber of Arctostaphylos rainbowensis.

Oak Moss (Evernia prunastri)

See A Basket Of Oak Moss
  See Lichen Dyes & Perfumes  

Web-Lined Entrance To Tarantula Burrow