There are nine rock units represented in Poly Canyon: the Toro Formation, serpentinite, volcanics, the Franciscan Formation, the Cuesta Ophiolite, the Vaqueros, Rincon, and Monterey Formations, and diabase. Rock units are unique combinations of rocks and minerals. Each unit is best seen at a different location of the Canyon. This discussion offers first a transect of Poly Canyon with a map showing where one can see these rock units and their typical vegetational cover. Second, for those interested in more detail on the subject, there is also a brief description of the origins and composition of each of the rock units and other locations in the Canyon where they can be found. Definitions are provided in footnotes.

Throughout this discussion, refer to Figure 4, a map of the geology of the Canyon. Above the electrical substation at the beginning of Poly Canyon Road, the fire break on the hill to the east shows bare redrock . This substrate has been formed through the decomposition of volcanic rocks (metamorphosed basalt ). Growing adjacent to the fire break, also on redrock, is a thick stand of black sage (Salvia mellifera). Further up Poly Canyon Road, at the Poly Canyon landfill, is another redrock site. A unique exotic plant community is thriving there. The grassy slopes below the redrock are on a sandstone outcrop (of Franciscan Formation).

The main ridge of lower Poly Canyon (along the eastern edge of Poly Canyon Road) is primarily serpentinite . The sparse cover on this rocky landscape includes stands of Yucca scrub, leather oak, and bunch grasses. There are numerous rock outcrops where one can find Dudleya (Dudleya spp.) and resurrection plant (Selaginella bigelovii).

Following Poly Canyon Road further into the Canyon brings one to the Botanic Garden. It is located on Toro Formation which is mostly an olive-gray marine shale with some interbedded lighter colored sandstone . This formation provides the characteristically oak-wooded slopes on the eastern flanks of the serpentinite ridge and underlies Design Village. The plant communities that predominate in these areas are grasslands and riparian woodlands.

Poly Canyon Road continues through the gates of the Peterson Ranch. The open valley and rolling hills that extend from the Peterson Ranch houses up toward the railroad tracks are primarily Franciscan Formation. This is a unit of mélange - a mix of sheared shale with blocks of other rock imbedded within it like fruit in a fruitcake . Close to the railroad tracks the mélange is buried under unconformable layers of much younger diabase , Vaqueros, Rincon, and Monterey Formations. The lower valley slopes are grazed grasslands and open riparian communities. Uphill are pockets of riparian communities around various seeps and springs, as well as clusters of oaks (Quercus agrifolia). Many of the steeper upper slopes are covered by coastal scrub.

The steepest, highest ridge, above the railroad tracks, is on part of the Cuesta Ophiolite (a slice of the Cretaceous sea floor). Chaparral and coastal scrub are the primary plant communities found on these rocky slopes, along with fingers of grassland and, at springs and moist draws, riparian vegetation.

The geologic origin of most of Poly Canyon rock units is primarily marine. Most of the rock units were once part of an ocean floor. The Canyon's oldest rocks were once the igneous bottom of the ocean crust (gabbro and ultramafic rock). These were covered first by basaltic rocks (pillow lavas and diabase) formed at a distant ocean ridge, then by deep marine sediments (chert and shale), and finally by continental sediments (sandstone) as the sea floor was moved by plate tectonics closer and closer to North America. As the Pacific Plate pushed against the North American Plate just over 20 million years ago, the oceanic crust broke apart. Some of it was subducted at the oceanic trench, where part of it was crushed and mixed up. This was eventually uplifted and is represented in Poly Canyon by the Franciscan Formation. Portions that were subducted, but not crushed or mixed up, resurfaced in enormous blocks called ophiolites (Figure 5). Locally, part of the Cuesta Ophiolite is located in the uppermost reaches of the Canyon between the railroad and the serpentinite. Franciscan mélange is found west of this. The fault zone between the Franciscan Formation and the Cuesta Ophiolite is near the railroad tracks and is buried by younger rocks (Miocene - 20 million years old). Serpentinite originated as peridotite in the Earth's mantle. It was hydrated during subduction. Subsequently it has surfaced within the Franciscan Formation as a toothpaste-like mass forced to the surface through the linear fissures along fault lines located primarily in the main ridge in the lower part of the Canyon. The Toro Formation was formed in a sedimentary basin along the marine trench where subduction was taking place.

The geologic history of Poly Canyon begins approximately 150 million years before present (see Tables 1, 2, and 3). The Toro Formation is the oldest coherent sedimentary rock unit in Poly Canyon: late Jurassic or early Cretaceous in age (120 to 150 million years before present). The primary component is a dark, olive-gray, brittle marine shale. There is also some intercalated sandstone of a similar, but lighter color. Toro deposits look dirty because of the color of the clay minerals and the numerous dark rock fragments contained in the shale. There are also fossils of the bivalve genus Buchia (Figure 6). Much shell debris is visible. Toro Formation is found on the east side of the main ridge (to the east of Poly Canyon Road south of the Botanic Garden), in the Botanic Garden, and near Design Village in the creek bed.

The serpentinite unit dates from the early Cretaceous, possibly even very late Jurassic (120 to 140 million years before present). Serpentinite derives from ocean mantle, primarily from metamorphosed peridotite. It is visible along the main ridge that backs the eastern edge of the campus to the vicinity of the Botanic Garden, as well as in the Botanic Garden, below Design Village. It is also seen on West Cuesta Ridge, beyond the metavolcanics-dominated ridge by the railroad tracks. There are numerous other serpentinite outcrops within Poly Canyon.

Table 1
GEOLOGIC TIMETABLE IN RELATION TO POLY CANYON

Table 2
ROCK UNITS OF POLY CANYON AND THEIR COMPOSITION

Table 3
ROCKS OF POLY CANYON

The volcanics, like the serpentinite, are primarily metabasalts of Cretaceous age and possibly date to very late Jurassic (120 to 140 million years before present). They are basaltic in origin and locally also have a large component of diabase, and are locally serpentinized. Redrock, which has been decomposed from basalt, can be seen at several sites. Volcanics are located adjacent to the fuel break above the electrical substation uphill from the beginning of Poly Canyon Road; at Poly Canyon landfill; at the first cattle crossing along Poly Canyon Road; and on the northern-most hill in the upper part of the Canyon, where Miocene-age volcanic rocks are found over Franciscan Formation and Cuesta Ophiolite.

The Franciscan Formation dates from the Cretaceous and, possibly, the tail end of the Jurassic (65 to 140 million years before present). It is also called the Franciscan mélange. There are no Franciscan rocks younger than Cretaceous in Poly Canyon. This unit has numerous igneous, sedimentary, and metamorphic components that vary from site to site as well as in relative size and abundance. They include rocks of oceanic and continental origin: basalt, gabbro, peridotite, diabase, chert, shale, sandstone, graywacke , and blueschist . Franciscan rocks are common and highly visible east of Peterson Ranch. They underlie the bare, grassy, boulder-strewn hillsides where landslides are common. One such place is part of the hillside below the "P," where blueschist pebbles are found in Franciscan conglomerates . The sandstones seen at the start of Poly Canyon (below the redrock ridge) are actually within a large block contained by the mélange.

The Cuesta Ophiolite dates to the Cretaceous (65 to 140 million years before present). It is characteristically less fragmented than the Franciscan Formation and contains serpentinite, peridotite, gabbro, basalt (including pillow basalt), chert, shale, and sandstone (Figure 7). The Cuesta Ophiolite underlies the highest ridge in Poly Canyon, above the railroad tracks.

The Monterey Formation is middle Miocene in age (13 to 16 million years before present). It is composed of diatomaceous and siliceous shales and siltstones (Figure 10). In Poly Canyon, it is found in the upper part of the Canyon among the rolling hills.

The diabase unit is middle Miocene in age (13 to 16 million years before present). It is an approximately 45-foot thick sill in the Monterey shale and Vaqueros sandstone. Diabase is igneous in origin, basaltic in chemistry. It is fine- to medium-grained. It underlies the scrublands and grassy hillside bordering Serrano Canyon just west of the railroad tracks.