Walking Guide to the Petrified Forest

Petrified Forest

Great redwood trees turned to stone attract thousands
of visitors every year to this private park in the hills of
eastern Sonoma County. Rich in history from the late 1800s,
the Petrified forest is a perfect example of explosive
volcanic activity and the effects that have been witnessed
in many modern eruptions, including Mount St. Helens.
Redwood trees up to 8 feet in diameter and 100s of feet high
were blown down like matchsticks in the direction of flow of
awave of ash and glass from a major volcano to the north and
east. They were subsequently deeply buried by ash and
preserved by replacement of organic material by fine-grained
silicon and oxygen (chalcedony). Modern uplift and erosion
has exposed the tips of the trees and final excavation by
man results in the display we see today.

The park itself was established in 1910 by Ollie Bockee
and her family, after purchase from a goathearder named
Petrified Forest Charlie. He was a friend of Robert Louis
Stephenson and Jack London and held forth at the natural
wonder to visitors from all over the world. The Petrified
Forest is now a privately owned attraction, with a fine
museum devoted to the origin of the fossils and geology of
the area. A gift shop with crystals, petrified wood and
books to help understand An admission charge of $3.00 is
charged for a 20 minute walk through the natural displays.
Interpretive signs at major points are aimed at the
layperson to help understand the processes that result in
the preservation of these spectacular fossils.

Geology of the area is dominated by the Sonoma
Volcanics, and this is a perfect area to study this unit and
its role in the geologic history of Sonoma County. This
unit includes many different types of rocks erupted from a
series of volcanos that extended from Mount Saint Helena to
the northeast south to Sonoma, east to Napa and west to the
mountains surrounding the Petaluma-Cotati-Santa Rosa valley.
A wide range of chemical composition and evidence of
different eruptive behaviors indicates that the cause of
these eruptions is complex, indeed researchers in the area
do not agree on the ultimate origin of the magma and its
history in relation to plate tectonics.

A short hike will allow you to see all the major tree
exposures in the park. A meadow hike, done every sunday at
2 pm leads to views of Mt St Helena and a white slope called
the Ash Fall. 8 major and 4 minors (gulley, wood pieles and
stevenson plaque) displays are available on the main walk.

The hike starts in the patio area behind the gift shop.
The first attraction is a collection of petrified logs under
an ancient oak. These have beeen collected from around the
area.. Most are gray to white, the typical color of
petrified wood here and display growth rings on closer
examination and ridges on the surface formed by the thick
bark of the original redwood trees.

The trail leads uphill to the left through a mixed
woodland of oaks, firs, bay and manzanita. Low rounded
outcrops of gray rock is volcanic ash or tuff, very soft
here, it is not welded or hardened and probably formed as an
ashfall.

The first petrified tree exposed to the left is called
th Pit tree, because it is exposed in a 15 foot deep pit.
It is the only pine preserved in the park, with finer ridges
in the bark, unlike the more typical redwood. It is 43' in
length and 2' in diameter, hardly a giant, but evidence that
this was a cool, moist environmnet when the treees were
preserved. White-gray ash in the walls of the pit is more
tuff. The trees must have been buried rapidly by a great
deal of ash in order for them to be preserved. The tops of
the trees here point to the southwest, indicating that they
fell in that direction. They also slope up toward the tips,
so it is ulikely that they fell down a slope. The uniform
direction of tops and the preserveation of the trees
indicates they were blown down by a volcanic blast like Mt.
St. Helens and rapidly buried deeply by an ash fall. This
tree has been left in its natural state, with mosses and
weathering intact. The major redwood exhibits have been
power-sprayed with water to restore the original pristine
crystalline condition.

Further past the shelter and down slope is the gulley
tree, a series of broken fragments of an originally larger
tree, this one a redwood. A modern analog of the petrified
trees is visible through the woods, this one however has not
been buried, so it will probably not be preserved.

The trail climbs up to the right and enters into a
meadow area surrounded by Manzanita. Great wildflowers
abound here in the spring. An exhibit on the right
describes the tuff and its origin as a cloud of gas glass
and ash, sweeping down the side of a volcano flattening the
trees in its path. Dark fragments of volcanic rock and
glistening shards of volcanic glass are visible in the
dominantly ash-rich rock.

Another display on the left up the trail is a statue of
PF Charlie and his mule, and a woodpile of petrified logs
from around the property

The Giant is the next tree, this one a handsome
specimen of a redwood, with deeply furrowed bark. This is
an extinct close cousin of present giants living in the
coast ranges. Now 60' long and 6' in diameter, it must have
been much larger originally, because of the 3' thickness at
the top. You can also see the beginning of the root ball in
the deeper part of the excavation, under the shelter. is
and other displays here have been cleaned with a
high-pressure spray to show the original texture of the
bark. Again the light-colored rocks of the walls are tuff
made from the volcanic ash that buried this area.

Several large logs are on display between here and the
Queen tree, and you can see the fine detail of growth rings,
knotholes were limbs once were and the sparkle of "drusy"
quartz, tiny crystal faces of quartz which sparkle int eh
light. ese form duing the petrifaction process where more
silica is available and a longer time to form crystals
allows them to grow.

The Queen tree has an oak growing out of it , and is
one of the largest in girth. 8' in diameter and 65' long,
again with a prominent root ball. This tree was also
probably taller. Its age was 2000 years when it fell. It
was discovered in 1870. It also fell in a southwest
direction, uphill, so must have been blown over by a
volcanic explosion.

The New excavation is a continuation of a project that
started years ago. With modern tecnology ( a backhoe) and
the sweat of present workers, a 70 foot long section has
been exposed, with 2 trees, slightly askew to each other.
The root ball has not been unearthed as yet, but work will
continue until it is exposed. These alos are redwoods, with
tops pointed to the sw, the same direction as all other
trees. Closely spaced grooves in the tuff of the walls here
are the result of horizontal fault movement along closeley
spaced planes in the rock.

The Monarch or "tunnel" tree is the longest preserved
and discovered yet. At 105 feet long and 6 feet in
diameter, it is the largest intact petrified tree in the
world. Ribbed bark and detail exposed indicate it is also a
coast redwood. The red coloration is not original wood, it
has been totally replaced by quartz. The color is most
likely from iron staining, from iron in groundwater. Large
knotholes are preserved, showing that many branches of the
tree existed. These branches were thinner and did not have
the thick bark that the main trunk had, so were probably
burned during the initial volcanic blast.

A large outcrop of tuff to the right of the Monarch
displays layering slanting downward from right to left.
This layereing was originally horizontal, or on a slight
slope, when deposited by the volcano. Now it has been
uplifted and tilted by monster vice-like pressures in the
earth. Thes pressures also cause faults and earthquakes
inthis area todaty. The Maacama fault, just to the west, is
one of the most active faults in northern California.

The "Rock of Ages" display shows another tree, somewhat
fragmented here, which protrudes from a bluff of tuff. It
probably extends far under the ground.

The "Robert Louis Stevenson" tree has the best
display of growth rings and surface detail. Druzy quartz
also adorns the surfaces. This is the only tree with an odd
orientation. it is more west-trending than the others.
TThe trees blown down by mt st helens also has vari=ying
orientations, alathough most are aligned in one direction.
So this variation is within the margins of error of our
reallife example. A great cross-section of the tree 5' in
diameter shows the convolutions of growth rings visible in
cut lifing redwoods today.

The Stevenson-Petrified Forest Charlie plaque commemorated the
meeting of these two giants of local history in 1880 and the
discovery of the major trees by Petrified Forest Charlie in 1870. The plaque is set in a monument of
smaller logs.

Meadow Hike

On most Sundays at 2pm, Dave Storck, the manager of the Petrified forest, leads a hike to the Meadow area on a trail that leads northwest from the Giant tree through a locked gate. Here you can see natural occurrance of petrified wood, a meadow of wildflowers at the right time of year (late march-april and into may) and two main attractions, Mount saint Helena and the Ashfall. This area is too soggy to walk for 3 days after a major rainstorm, check with the office for trail conditions and schedule for this hike. (707-942-6667)

References and data:

Mattison, Elise, 1990, California's fossil forest:
California Geology, v 44 no 9, sept, 1990, p 195-202