Skip to main content

Geology of Vallejo

Introduction to the Geology of Vallejo

In 1988 I (Mr. Allison) was given the task to create a science unit on geology.  Part of this unit was the study of the rocks and geology of the local area.  In the last screen of this section is an old essay on the Geology of Vallejo that is fairly technical.  It is scanned from a document made on a great old dot matrix printer.

Most of the exciting geology in Vallejo is evident at Blue Rock Springs Park.  The rock for which the park is named is called Serpentinite.  It is a blue slippery feeling metamorphic rock that is about 150 million years old and is associated with earthquake faults.  The fibrous lines in the rock are asbestos fibers.

Above the park is the Sulfur Springs Mountain Range.  The rock outcrops are 10 to 20 million year old Andesite.  This is an igneous rock (lava) that is weathering away quickly due to its high iron content.  Iron essentially "rusts" away with repeated rain fall.  The soil around the outcrops are deep red from this iron.  In another few years, these relatively young rocks will be gone.

The sandstone at the park and all over Vallejo is from the Domengine Formation about 60 or so million years ago.  It is a sedimentary rock that was formed underwater and then raised up through earthquake activity as the Pacific Plate raises up California. There are fossils occasionally in the rocks.

Blue Rock Springs is a natural sulfur spring.  The water smells of sulfur and evaporation crystals are all over the rocks.  The water comes from underground and has a high mineral content.  In the 1940s Vallejoans took mineral baths in the water.  This was not a good idea as the mercury content of the water is very high.

There were two mercury mines in Vallejo.  The Hastings mine was located above the park.  It was closed after an accident that took the lives of several miners.  The other mine is on Saint John's Mine Road.  This was an active mercury mine through the 1940s.  There are over 30 miles of tunnels under the mountains in the area.  Cinnabar is the ore from which mercury is obtained.  The ore would be crushed and baked.  The mercury would vaporize and condense to be collected.  This was a very unhealthy mining practice.  Mercury was primarily used to dissolve gold out of crushed gold ore.  The mercury would be then be vaporized and the pure gold would remain.

The Syar Industries Quarry is unbelievable.  It is huge beyond what you could imagine.  The Basalt that is quarried there supplies the rock for many freeways (I80 680, etc.)  The larger rocks are used to hold embankments in place.  This Basalt was formed about 150 million years ago under the ocean.  As the plates crashed into each other this chunk surface along with the sandstone and other rock formation.  California is a bit like a train wreck in terms of geology.

Across the Carquinez Straits in Rodeo are some amazing geological formations.  There are pictures below of these features.  First of all there is a layer of white powdery ash tuff that is apparently 1000 feet thick deposited from a volcano that has long ago disappeared.  There are some geologists who believe it may have been located in the place that is now San Pablo Bay and that after years of erosion has disappeared.

Along the railroad tracks in Rodeo at the point is an example of an angular unconformity.  There is a top layer of oyster shell fossils and dirt that are about 2 million years old.  Then there is a slanted layer of sedimentary rock that needed to be deposited and uplifted then covered by the oyster shell layer and then the oceans needed to recede.  But that is not all.  Below the slanted layer is a horizontal layer of a different kind of sedimentary rock.  Take a moment and ponder how many times the oceans came, made deposits, left and then the plates' movement causing uplift in the middle of this sequence.  It was this type of formation that led geologists to conclude that the earth needed to be much older than the biblical 4,000 years.  This formation led to the beginning concepts of deep time.

The last formation in Rodeo of interest is the bluff containing 10-15 million year old pectin shells.  Pectins are the group of shells we know of as scallops.  The bluff is filled with these fossils.

Sulfur on Rocks in Blue Rock Springs

Sulfur on rocks as Blue Rock Springs.jpg

Lichen on Sulfur Springs Mountain Andesite

lichen on andesite.jpg

Asbestos Fibers in Serpentinite at Blue Rock Springs Park (150 million years old)

asbestos in serpentinite.jpg

Rock House Built Out of Andesite from Sulfur Springs Mountains

rock house made of andesite.jpg

Sulfur Springs Mountain Andesite above Blue Rock Springs Park (10 to 20 million years old)

close up of andesite.jpg

Calcite on Basalt from Syar Industries Quarry

Calcite on Basalt.jpg

Domengine Sandstone at Blue Rock Springs Park (60 million years old)

sandstone at Blue Rock Springs.jpg

Rock Wall Made From Serpentinite at Blue Rock Springs Park

serpentinite wall.jpg

Serpentinite in path above Blue Rock Springs Park

serpentinite on trail.jpg

Close Up of Andesite (note the deep chemical weathering)

close up of andesite2 good.jpg

View From the top -- Andesite with Quarry in the background

view and andesite.jpg

View of Vallejo from Sulfur Springs Mountain Range

view from Blue Rock Springs Park.jpg

Syar Industries Basalt Quarry
Syar Industries Basalt Quarry

Ash Tuff

Ash tuff.jpg

Angular Unconformity at Rodeo

Angular Unconformaty.jpg

Angular Unconformity

Angular unconformity4.jpg

Pectin Fossil at Rodeo


Pectin Fossils at Rodeo Close Up


Archives Locker