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Marsh Creek 5
The Volcanic Dome
Size: 7.4 acres
Location: Marsh Creek- Morgan Territory area

Save Mount Diablo's Marsh Creek-V view of  Mount Diablo

Mount Diablo continues to benefit from the recession and a buyer’s market.  “Save Mount Diablo has just purchased 7.4 acre “Marsh Creek 5” for $125,000, our most recent purchase on Marsh Creek Road and crossed by Marsh Creek,” said Ron Brown, Save Mount Diablo’s Executive Director.  The property is located east of Clayton and adjacent to the Marsh Creek Springs event facility, just downstream from our Marsh Creek 2 and 4 properties.  Even for a creek property, Marsh Creek 5 is especially rich in biodiversity.  Part of the reason is that it’s a volcanic dome with unusual geology.
           
“We were able to make a purchase deal at a very low price and in near record time,” said Brown.  “We were contacted about the property on May 17, signed papers fifteen days later and closed escrow on July 15.  This is the sixth 5 or 10 acre parcel Save Mount Diablo has acquired in the past year for $125,000 or less.”

Marsh Creek
Save Mount Diablo's Marsh Creek-V property, a volcanic domeWe didn’t need to know much to be interested.  At its simplest the property corner is crossed by Marsh Creek at the mouth of the Dark Canyon segment of Marsh Creek Road.  More importantly it is adjacent to a large box culvert where the creek crosses underneath the road, a very important wildlife connection.  One of a series of unusually steep knolls near the northeast corner of Mount Diablo State Park, it’s one property away from two other Save Mount Diablo parcels and also rises to another large property protected with a scenic easement.

It’s easy to be interested in large parcels, and they’re cost effective since they’re cheaper per acre but since the recession began Save Mount Diablo has also been pursuing small properties, especially along Marsh Creek.  It is an important wildlife corridor and habitat for many species in the area east of Mount Diablo.  The property’s section of the creek is lined with mature sycamores, oaks and willows.  It is habitat for the threatened California red-legged frog but is also used by a wide array of wildlife. 

Marsh Creek 4We’ve been numbering these small Marsh Creek parcels unless another name is warranted, like the Morgan Red Corral or Moss Rock properties, which are also crossed by Marsh Creek.  All told, 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.  Another 8.5 miles of the creek have been protected within five parks from its headwaters in Morgan Territory Regional Preserve down to the new Los Meganos State Historic Park, and still more segments in Brentwood and Oakley.

A few days after being contacted, staff and several members of Save Mount Diablo’s land committee visited the property.  Its northern exposure along the road is steep, lush, and includes a lot of poison oak but several areas had been cleared when the owner was considering a house site.  Another small area has been quarried.  In just a few minutes several rare plants were located.  One surprise was that there are large rock outcroppings and slopes of smaller rocks, many of them pink, layered, with shiny light and dark crystals, which we recognized as a form of rhyolite or dacite, igneous (from magma or lava, sub-surface (plutonic) or above ground (volcanic)) rock with a high silica content. 

Volcanic Plug or Dome?
We started looking at geology maps.  There’s not as much igneous rock in Contra Costa as in the North Bay but it’s not all that unusual, especially on Mount Diablo.  This was different.  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. 

In the Oakland Hills, more recent volcanoes erupted and spread lava and ash locally around 10 million years ago.  And in various places around the county, ash from eruptions further away are consolidated as “tuff” beds.  The 4.83 million-year-old pink Lawlor Tuff is an age marker around Diablo, laid down on a flat to rolling landscape before the peaks were exposed, and steeply folded as Mount Diablo emerged. 

Rhyolite and DaciteMarsh Creek 5 is different.  Diablo Valley College geology professors Jason Mayfield and Jean Hetherington helped us to understand it.  At the northeast corner of Mount Diablo State park, near the intersection of Marsh Creek and Morgan Territory Roads, eighteen spots have been mapped so far, by other geologists, of high silica igneous rock visible in mound or dome like surface exposures, within a four mile northwest-southeast band, a mile and a half wide.   The age and location of the domes is interesting considering how Marsh Creek doubles back on itself and heads east, threaded among them.  As Diablo emerged through overlying sedimentary layers over the past 3-4 million years, it forced drainages east and north, and the creeks helped erode some of the domes.

These domes are tertiary (65 million to 2.6 million years old) felsic (magma or igneous rock with at least 65% silica) porphyric deposits (formed when a body of rising magma cools in two stages.)  The two stages of cooling produces two distinct sizes of minerals that make up the rock: large visible crystals formed from slow cooling underground, and small crystals formed from fast cooling as the magma reaches the surface.  Previous mapping has identified similar dome-shaped features as volcanic domes.  Alternatively, it is possible that these features represent eroded volcanic necks, but the lack of field evidence makes this idea difficult to test.  The felsic dome features are surrounded by older Early Cretaceous shale and sandstone (146 to 100 million years old).

In other words, the domes are magma extrusions which might or might not have reached the surface, what’s known as a plug dome (if the magma erupted, flowed above ground and then solidified), or a plug neck (if the magma solidified underground and then was exposed by erosion). 

Mayfield visited Marsh Creek 5 and confirmed that the rock there is rhyolite or dacite (or rhyodacite, an intermediate form of rock between rhyolite and dacite).  Quartz and feldspar phenocrysts are present in most samples.  In addition, many contain biotite and/or hornblende as a dark colored, ferromagnesian mineral.  In some samples biotite is most abundant, whereas, in other samples rocks with the opposite properties are found.  He scrambled around the parcel’s slope looking at rock outcrops to see if he could find volcanic rock in contact with sandstone to help him get a handle on whether this was a volcanic plug dome or a plug neck.  That’s still up in the air (so to speak), but most likely the outcrops are volcanic domes, formed at least partially above ground.

All or part of eleven of the eighteen spots are on protected land—at Chaparral Spring, and Clayton Ranch for example—but most of them aren’t yet accessible to the public.  Once you’ve focused on their characteristic shape—unusually steep knolls—they’re easier to pick out, several of them along Marsh Creek Road, Morgan Territory Road, and Leon Way.  Marsh Creek 5 happens to be very steep and thick with poison oak.  Nearby however, one dome is easily accessible at the bottom of Perkins Canyon, west of Morgan Territory Road and just south of the mercury mine, within Mount Diablo State Park (see “Volcanoes in Clayton?!?” hike article).

Complex Geology = Complex Soils = Rich BotanyMt.Diablo fairy lantern
As rocks break down, soils are formed, and when the geology is complex, so are the soils.  Because they’re high in some minerals, chemicals and trace elements, and deficient in others, rocky soils exclude some plants and are speciation sites where others evolve into new forms.  Vegetation on Marsh Creek 5 is diverse due to its conical topography which exposes the parcel to a variety of aspects. The north slope is draped in a shroud of full shade as is much of Dark Canyon to the east. The multistoried understory of this side of the property comprises an upper canopy of California bay laurel, interior live oak, and coast live oak. The mid-story canopy smells sweet of western hop tree blossoms but is also armed with an abundance of poison oak and its ever present urisol for those who dare to tread—if the property’s 45 + degree slopes aren’t enough to discourage you. It rises from 550’ to 870’.  Other shrub species occupying this level of the canopy are coffee berry, our native blackberry, snowberry, and hollyleaf redberry. The woodland floor is abundantly populated with mosses and fern species such as wood fern, maidenhair fern, goldback fern, and licorice fern.

On the exposed western slope the dense canopy cover gives way and herbaceous species bask in the sunlight such as monkeyflower and some of our wonderful native grasses including blue wildrye, California fescue, and smallflower melic grass.  On the eastern side of the dome slopes are gentler in character allowing blue oaks to replace live oaks and begin creating the savannah we are accustomed to on this side of the mountain, accented with scattered buckeyes.

At the top of the dome there’s more sun and more exposed rock. Plants with an affinity for this lithic substrate include foothill pine where turkey vultures are fond of roosting, huge manzanita, Hartweg’s umbrellawort, and Mount Diablo fairy lantern. This locally abundant species of Calochortus is listed as rare by the California Native Plant Society (CNPS) and also covered by the East Contra Costa County Habitat Conservation Plan (HCP).

Bowerman manzanita?Bowerman manzanita on Save Mount Diablo's Marsh Creek-V property
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 25-30 feet in size. Though manzanitas can be a difficult genus of shrubs to identify, as they are notoriously promiscuous and 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 characters could be identified as big berry manzanita (Manzanita glauca) or Contra Costa manzanita (Arctostaphylos manzanita subsp. laevigata), which is also a CNPS and HCP listed/covered species. Differentiating these two species is further aided by their flowers and fruits. Yet there is still another possibility, though not one currently recognized by manzanita experts.

James Roof, an early Director of the Tilden Botanic Garden, wrote about a new manzanita from Contra Costa County which he published in The Four Seasons (Volume 5, Number 4, 1978) as Bowerman manzanita (Arctostaphylos bowermaniae), named after Dr. Mary Bowerman, Save Mount Diablo’s co-founder. He described this species as being 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 then recognized Black Diamond Mines Regional Preserve. (Park District lands were more than three miles from Marsh Creek 5 in 1978.  Around Black Diamond, District lands have tripled in acreage and in 2011 are now less than a mile away.) Roof states that A. bowermaniae can be tree-like, growing to approximately 28 feet tall and easily as large as any other species in the genus.  He postulated that A. bowermaniae was of hybrid origin between Contra Costa manzanita and big berry manzanita. This hybrid manzanita expresses itself through Contra Costa manzanita leaf characters, though variably large and small on a single individual, and primarily solid stone fruit of big berry manzanita. However, current manzanita taxonomists believe that A. bowermaniae is a relict ancestor that evolved into the more common Contra Costa manzanita we know today.
We explored several more domes, and found the same manzanita at one nearby, but not at the dome in lower Perkins Canyon.    Save Mount Diablo’s resident botanist Heath Bartosh will return to take a closer look at fruits, leaves, and inflorescences in the coming seasons to gain more insights on these impressive manzanita specimens living atop this old volcanic dome hidden in plain sight along the Marsh Creek corridor.

Learn more about our "Twin" Volcanic Domes, Marsh Creek 5 and 6 in an NBC Bay Area interview with Land Programs Director, Seth Adams.

Take a self-guided hike to a more accessible volcanic plug near Perkins Creek.

Read the press release. Learn more about Marsh Creek and see our other Marsh Creek Properties.

Save Mount Diablo's properties are closed to the public except by guided tour. Please see our Hike Calendar for upcoming dates and opportunities.Marsh Creek 4SaveMount Diablo's Marsh Creek-V map

 

 

 

 

Sources:  Botany:  Heath Bartosh; James Roof, The Four Seasons (Volume 5, Number 4, 1978); Geology:  Jean Hetherington; Ken Lavin; Jason Mayfield; the Mount Diablo Interpretive Association’s Guide to the Geology of Mount Diablo State Park; and Doris Sloan’s Geology of the San Francisco Bay Region.  

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  Credits | Legal StatementCopyright 2012 Save Mount Diablo. Designed by Alison Martin. Funded by Clif Bar.