Land and Water

Blueschist (glaucophane schist) is a metamorphic rock formed at high pressures and relatively low temperatures. These rocks are found only where rapid downward and upward movement occurs near subducting plate boundaries. The blue color is due to the mineral glaucophane. A schist is a rock that breaks along parallel planes called foliations formed when differential pressures cause minerals to align. Glaucophane forms at a minimum depth of nine miles below the surface, indicating the large amount of vertical movement that has occurred at Edgewood.

Franciscan greenstone is metamorphosed basalt that formed mainly at spreading centers. Because this rock type often is more resistant to erosion, it can create ridges. It forms the central ridge and the highest point at Edgewood. The red color is due to weathering of the rocks that oxidizes iron to the rusty red form. This can be seen in the deep red soil on the Live Oak trail, which has eroded from Franciscan greenstone.

Greywacke is a dark-gray, poorly sorted, and dirty sandstone. This rock forms in the ocean as sediments accumulate through normal erosion of the continents and subsequent transport of sediment by rivers. Greywacke is similar to a sandstone and contains small fragments of the minerals quartz and feldspar as well as small rock particles. This is a common rock of the Franciscan rock complex.

Serpentinite is the name given to the rock that is composed largely of the rock serpentine, with smaller amounts of other minerals, such as crysotile (asbestos), magnetite, and epidote. This rock is associated with ancient subduction zones where tremendous tectonic forces have helped squeeze it to the surface from the place where it was formed–deep within the earth’s crust near the border with the earth’s mantle.

Edgewood’s youngest rock, at 50 million years old, can be found between the Old Stage Picnic Area and the upper slopes to the west, which are Franciscan greywacke. Whiskey Hill sandstone was originally made up of ocean floor sediment that was subsequently dragged into a subduction trench. This sandstone is not well exposed and it is difficult to tell it apart from the nearby Franciscan greywacke.

There are over 10 miles of hiking trails within Edgewood’s boundaries. They wind through woodlands, grasslands, and chaparral plant communities and offer magnificent views of the San Francisco Bay and the Santa Cruz Mountains.

On average, Edgewood receives 21 inches of rain during the Winter rainy season. This rainfall sustains Edgewood’s freshwater marsh, creeks, seeps, and springs, which provide critical habitat for plants and wildlife. These waterways drain either into Crystal Springs Reservoir to the west, or Cordilleras Creek to the northeast. The water in Cordilleras Creek in turn drains into estuaries, returns to the Bay, and flows into the Pacific Ocean.