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Basalt On The Santa Rosa Plateau: Home Of Brodiaea santarosae

© W.P. Armstrong 24 November 2007

PDF of B. santarosae Article Published in Madrono Vol. 54: 187-198 (2007)

Aerial view of the Santa Rosa Plateau showing several vernal pools on Mesa de Colorado. Remnants of ancient flows of Santa Rosa Basalt occur on the Plateau. The basalt dates back 8-11 million years ago, to a time before this region was uplifted. Populations of the rare Brodiaea santarosae are endemic to the basalt outcrops.

Lichen-covered basalt boulders on the Santa Rosa Plateau. The colorful crustose and foliose lichens include Caloplaca, Candelaria, Candelariella and Xanthoparmelia.

  Rock Lichens On The Santa Rosa Plateau  
Rock Formations On Santa Rosa Plateau

Grassland west of Mesa de Burro with the remnants of an ancient basalt flow in the distance. This area is the type locality for Brodiaea santarosae.

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  

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 surface 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.

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 type locality for Brodiaea santarosae. The elongate (rectangular) surface pits are due to weathered plagioclase crystals in the rock. 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.

Xanthoparmelia conspersa growing on Jurassic age Santiago Peak Volcanic rock. The upper surface of the thallus is roughened by numerous spherical isidia. Isidia (photo inset) are pimplelike protuberances containing both algae and fungi that readily disperse to new locations. The lower surface and rhizines are black.

  See Soralia, Soredia and Isidia.  

Lichen-covered basalt boulders on Avenaloca Mesa with Elsinore Peak in the distance. This is the extent of the native habitat for the rare species Brodiaea santarosae.

Brodiaea santarosae flower stalk and lichen-covered Santa Rosa Basalt.

The flower of Brodiaea santarosae has longer perianth segments than other Brodiaea species in southern California. The total spread of this flower is nearly 50 mm. This species has long filaments like B. orcuttii, but has much larger flowers. It has no staminodes or slender staminodes like B. filifolia that vary in length. It is similar to the fertile hybrid between B. filifolia and B. orcuttii, except the flowers are generally about 40 percent larger than the hybrid. This flower was photographed in June on Avenaloca Mesa during an exceptionally dry spring. Regularly-watered plants grown in clay soil derived from San Marcos Gabbro developed slightly smaller flowers.

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