The East Bay nonprofit grassroots organization Save Mount Diablo recently purchased a 7.4-acre volcanic dome for $125,000, calling it Marsh Creek-V.

“This is our most recent purchase on Marsh Creek Road crossed by Marsh Creek,” said Save Mount Diablo Executive Director Ron Brown. The property is located east of Clayton and adjacent to the Marsh Creek Springs event facility, just downstream from two of the nonprofit’s other properties. According to the organization’s recent findings, Marsh Creek-V is especially rich in biodiversity, including what might be the world’s tallest manzanita. Part of the reason for this richness is that the property is a rare volcanic dome with unusual geology.

The habitat is one of several preservations made possible for Save Mount Diablo by the current market. “We were able to make a purchase deal at a very low price and in near record time,” said Brown. “This is the sixth 5-to-10-acre parcel Save Mount Diablo has acquired in the past year for $125,000 or less.”

Marsh Creek

The property corner is crossed by Marsh Creek, a major water source for wildlife in Contra Costa County as well as a rare-species habitat for the threatened California red-legged frog. More importantly, it is adjacent to a large box culvert where the creek crosses underneath the road, making it a key connection to allow wildlife to pass through, avoiding the road. One of a series of unusually steep knolls near the northeast corner of Mount Diablo State Park, it’s one property away from two more Save Mount Diablo parcels and also rises to another large property protected by a scenic easement.

In the upper and middle stretches of Marsh Creek (the area upstream of Marsh Creek reservoir in Brentwood), Save Mount Diablo has recently protected more than a mile of the creek in small parcels.

Staff and several members of Save Mount Diablo’s land committee visited the property to survey its habitat. In addition to mature sycamores, oaks and willows, several rare plants and apparently out-of-place rocks were located in just a few minutes.

Volcanic Plug or Dome?

According to Diablo Valley College geology professors Jason Mayfield and Jean Hetherington, the most common igneous rocks on Mount Diablo are old ocean crust formed as much as 165 million years ago deep out at sea. These included parts of Diablo’s main peaks. Also present are pillow basalts formed at underwater eruptions along a mid-ocean ridge. These pieces of igneous rock were carried to North America and added to the continent by the movement of tectonic plates.

At the northeast corner of Mount Diablo State Park, 18 spots have been mapped so far, by other geologists, of high silica igneous rock visible in mound or dome-like surface exposures, within a 4-mile northwest-southeast band, a mile and a half wide.

All or part of 11 of the 18 spots are on protected land – at Chaparral Spring, and Clayton Ranch, for example – but most of them aren’t yet accessible to the public.

Complex Geology = Complex Soils = Rich Botany

Vegetation on Marsh Creek-V is diverse due to its conical topography, comprised of 45+ degree slopes rising from 550 feet to 870 feet, which exposes the parcel to a variety of aspects.

The most interesting plant species seen at this property are a group of manzanitas at the summit. They may be among the tallest anywhere, at 25 to 30 feet. Though manzanitas can be a difficult genus of shrub to identify, as they hybridize easily, there are only a few possible species these large specimens could be, based on the leaf shape and lack of a basal burl. In this region, a manzanita with these characteristics could be identified as big berry manzanita (manzanita glauca) or Contra Costa manzanita (arctostaphylos manzanita subsp. laevigata). There is yet another possibility, though not one presently recognized by manzanita experts.

Writing in “The Four Seasons” (Vol. 5, No. 4, 1978), James Roof described a new manzanita from Contra Costa County. Called Bowerman manzanita (arctostaphylos bowermaniae), it was named after Dr. Mary Bowerman, Save Mount Diablo’s co-founder. He characterized this species as endemic to Contra Costa County, occupying ridge tops or north-facing slopes between 400 to 800 feet in elevation, in grassland or thinly distributed oaks and buckeyes, confined in the Black Diamond Mines Regional Preserve – more than 3 miles from Marsh Creek-V in 1978. He postulated that A. bowermaniae was of hybrid origin between Contra Costa manzanita and big berry manzanita, but current manzanita taxonomists believe that A. bowermaniae is a relict ancestor that evolved into the more common Contra Costa manzanita we know today.

Hike a Volcanic Dome

You can visit a similar volcanic dome nearby using Save Mount Diablo’s self-guiding hike to Lower Perkins Canyon. Save Mount Diablo will lead short hikes there on Sept. 17 and Oct. 8. For more information, check the hike schedule at www.savemountdiablo.org.