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PPENDIX
California Rocks and Minerals
A Brief Field Guide
for Rock Hounds and Natural History Detectives
This appendix presents an opportunity to explore in more
detail how California’s geologic history is revealed by the
distribution of so many kinds of rocks in the state. It is
provided for those with an appetite for more details about
the evidence California’s geologic history has left behind.
It would be impossible to find one storage room or
laboratory with the variety and quality of mineral and
rock samples found in California. Figure 2-6 in text illustrates the rock cycle that produces this variety.
The importance of the distribution of these minerals
and rocks ranges far beyond geologic curiosity. We have
already followed the clues they provide us about California’s tumultuous geologic history. In addition, these rock
types and structures often control local and regional
geomorphology—the California landscapes which evolve
from them. The specific chemistry of soils produced when
different rocks are weathered plays a significant role in the
distribution of plants and animals in California. Certain
types of rocks are associated with specific geologic hazards, especially when we build on them. Ores and gems
are also found in and extracted from particular rock bodies, and some are quarried for building materials. Even if
you are not a rock hound, you must appreciate the significant roles rock distributions play not only in the landscapes we view, but in California’s history and economy.
Most of the minerals and rocks scattered around the
state are left behind by past geologic processes. That is
why this appendix follows the chapter on geologic history, which described the geologic events that produced
California’s rocks. This discussion focuses on a search
for some of the most common, interesting, and important rocks, minerals, and earth resources in California.
Appendix Figure A-1 Granitic rocks near the top of Castle Crags in northern California are being weathered along a
series of vertical joints and fractures. The high silica/quartz
content of such granites makes them a little lighter in color
and weight and also makes them resistant to erosion.
the Basin and Range and Mojave. They make up a large
portion of the basement of the Coast Ranges, and they
are even buried by more recent volcanic material in
northeastern California and by recent alluvial deposits in
many California valleys. This “Sierra Nevada batholith” is
composed of more than 100 different types of plutons,
most referred to as granitic rock. Light-colored speckles
that usually dominate this more felsic (high in silica)
granitic rock are quartz and feldspar; darker speckles are
usually less common and include flat flakes of biotite
mica or other more mafic (higher in iron, magnesium,
etc.) minerals such as amphibole or pyroxene. Specific
names for these salt-and-pepper rocks (granite, quartzmonzonite, granodiorite, quartz diorite, etc.) refer to the
specific chemistries of each.
Recall that these rocks formed when an eastward
dipping ocean plate was being subducted below the continent, mostly during the Mesozoic Era and the Nevadan
Plutonic Rocks in California:
Lifted and Exposed
Intrusive igneous rocks are found throughout California;
they dominate the basement rocks, and they are frequently exposed in the Klamath, Sierra Nevada, Transverse, and Peninsular Ranges and in the mountains of
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Appendix California Rocks and Minerals
2
orogeny. Though some of this activity began even before the Mesozoic Era and some of it continued into the
Cenozoic Era, 100 million years will serve as a convenient, average age for these rocks. Millions of years
passed as enormous magma chambers formed below
California and gradually cooled and crystallized to produce these rocks with large crystals. Darker plutons are
composed of more ocean plate and mantle material from
below; lighter granites formed more frequently when
large volumes of higher silica continental crust were incorporated into the melt. Many of the once buried
batholiths have since been pushed and faulted upward
and exposed after overlying rocks were eroded away.
About 60 percent of the surface rocks in the central and
southern Sierra Nevada are made of these mostly granitic
plutons.
Lighter-colored dikes, sills, and veins in the batholith
represent more felsic (higher silica) magmas that were
the last to crystallize. The joints and fractures in the earlier-crystallized plutons served as conduits through
which these hot magmas, which were still melted and
often mixed with water, were squeezed and finally
cooled and crystallized. Rare and valuable earth minerals
(such as gold and silver) were sometimes incorporated
WHERE TO FIND PLUTONIC ROCKS AND THEIR LANDSCAPES
Klamath Mountains
Granitic plutons, mostly Jurassic Period, similar to Sierra Nevada batholith.
Castle Crags State Park
More resistant granitic rocks form ridges above older, less resistant rocks they once intruded.
More rapid weathering along joints isolates the pinnacles.
Coast Ranges west of San
Andreas Fault in Salinian Block
Mesozoic Era granitic plutons, similar to Sierra Nevada and Peninsular Ranges. Faulting has
moved them up next to the Franciscan melange (the other Coast Ranges basement).
Montara Mountain (Santa Cruz
Mountains)
Quartz diorites are strikingly similar to Farallon Islands.
Transverse Ranges
Granitic plutons similar to other California mountains especially near and above San Gabriel
and San Bernardino Mountain resorts.
Peninsular Range—San Jacinto
Mountains from Palm Springs
Tram and above Idyllwild;
Santa Rosa and Laguna Mountains
Southern California batholith dated at 70–120 mya. Slightly less felsic than northern
California batholiths, but weather to produce landscapes similar to Sierra Nevada.
Numerous pegmatite dikes noticeably cut through to produce gem sites. Upper San Luis
Rey River sites have magnificent tourmaline crystals in rock cavities.
Riverside
Common Nevadan plutonic quartz diorites and monzonites intruded into older limestones,
creating world-famous assemblage of 140 contact metamorphic minerals at Crestmore
limestone quarries. Such minerals are distributed throughout California along edges of
Mesozoic Era plutons, where older existing rocks have been heated and metamorphosed.
Sierra Nevada Mountains
Throughout the range, including eastern base around Alabama Hills (as seen in countless
commercials and films).
White-Inyo Mountains east
of Big Pine
Papoose Flat pluton (75–81 mya) squeezed through, thinned, and metamorphosed
Cambrian Period sedimentary rock formations.
Central and Eastern Mojave
Roots of the Ivanpah Upland are composed of granitic quartz monzonite from the Mesozoic
Era, but the rock and its erosional surface have been extensively buried by lava flows and
cones since the Miocene Epoch.
Granite Mountains
Named after outcrops of course-grained White Fang quartz monzonite.
Joshua Tree National Park
Prominent blocky granitic (monzogranite) boulders are weathered into spheres and rock
climbers’ playgrounds.
Colorado Desert
Plutonic rocks have wide range of ages, from pre- to post-Mesozoic Era. They include
the youngest plutons in the state, found in the Chocolate Mountains and dated at just
23–31 mya. Many of these youngest plutons were intruded into some of the oldest rocks in
the state.
A Brief Field Guide for Rock Hounds and Natural History Detectives
3
Appendix Figure A-2 Volcanic boulders are scattered
across the relatively young Sutter Buttes that erupt from the
Sacramento Valley. Pasty eruptions of andesite and rhyolite
left these rocks to be weathered into darker colors.
into the melt and emplaced within the vein. Some of
these intrusions are responsible for changing the human
history and economy of California.
Today’s vertical and horizontal joints and fractures
in these plutonic rocks are sometimes a result of compressional and/or extensional tectonic forces that have
stressed the buried rocks over millions of years. Other
joints and fractures were formed more recently as overlying rocks were eroded away (unloaded), releasing
pressure and allowing expansion of the rock masses.
This exfoliation breaks the rocks at the surface into
onion-like sheets and sometimes produces great outstanding domes. You might observe some or all of these
features in any exposed granitic plutonic body in Califor-
Appendix Figure A-3 Rocks were bur–
ied, and then exposed to extreme heat and
pressure that squeezed and contorted them
into metamorphic rocks. Hundreds of millions of years later, they were lifted up in the
Sierra Nevada and are now exposed to
weathering here at May Lake in Yosemite.
nia. Weathering processes are accelerated in the surface
cracks between these rock masses, a topic covered in
detail within Chapter 3.
Volcanic Rocks: California’s
Geologic History Flares Up
It may surprise some Californians that active and potential volcanic hot spots are scattered about in at least four
different areas of the state today. They include the majestic volcanoes and lava plateaus of the Cascades and
northeastern California, the area above the Napa Valley
near Clear Lake (including the geysers), the Basin and
Range east of the Sierra Nevada, and south of the Salton
Appendix California Rocks and Minerals
4
Sea in the Imperial Valley. Each of these areas has also
experienced some modern volcanic activity. In this section, however, we will examine only the distribution of
the volcanic rocks that have been left behind by past volcanic activity. Volcanic activity in human history will be
examined in Chapter 3.
A few of these ancient hot spots are near areas where
volcanic activity happens to have continued to today.
Where there is not a clear line, we will yield to Chapter
3’s section on modern volcanic processes and landforms.
To manage this organizational problem, we ask the question: Where are some of the more common and impor-
WHERE TO FIND ANCIENT
VOLCANIC ROCKS AND THEIR LANDSCAPES
Table Mountain north
of Oroville
A more resistant ancient lava flow (Lovejoy basalt) stands out in the foothills above weaker rocks that
have been eroded away (an inverted topography) west of Lake Oroville.
Sutter (Marysville) Buttes
Within a 16X16 km (10X10 mi) area and at more than 600 m (2000 feet) high, these buttes are the only
volcanic outcrops in Central Valley. Pasty eruptions with breccias and tuffs formed these plug domes of
andesite and rhyolite. Probably an extension of the southern Cascades, they are very young. (The
surrounding late Pliocene Epoch Tehama Formation was warped by these eruptions.) They are
included here because they are isolated and detached geographically from the Cascades.
Sierra Nevada Foothills
east of Sacramento;
Sonora
After erosion denuded the ancient Sierra Nevada to lower hills about 20–25 mya, great deposits of
volcanic ash invaded from the east to bury gold-rich streams. These are the rhyolitic pink tuffs of the
Spring Valley Formation used as building stone during the Gold Rush. Miners were required to dig through
it to reach gold deposits. After some erosion of that surface, extensive volcanic mudflows (including large
volcanic cobbles) and basalts covered it. This is known as the Mehrten Formation (4–20 mya). Latite
(intermediate between rhyolite and andesite) is found on Table Mountain northwest of Sonora (9 mya).
Basaltic flows were left by the third and youngest volcanic period in this area less than 3.5 mya.
San Luis Obispo; Morro
Bay
Along Hwy. 1 from San Luis Obispo to Morro Bay is the chain of fourteen resistant throats of
volcanic plugs. Weaker rocks were stripped away from these “insides of volcanoes,” which are about 22
million years old, mostly made of andesite. Smaller 160 m (500 feet) Morro Rock is dacite (intermediate
between rhyolite and andesite) and was once extensively quarried to make breakwater and fill. Refer to
Chapter 2’s section on Cenozoic Era geologic history for the origins of these volcanoes.
Pinnacles National
Monument east of
Salinas
On the Monument’s west side, overlooking Soledad and the Salinas Valley, are Miocene Erhyolitic
pyroclastic volcanic breccias. They have weathered and eroded into the spectacular caves and
landscapes that have made the area famous. Rhyolite and some andesite and basalt are found on the
east side of Pinnacles. Again, the origins of these rocks are examined in Chapter 2’s Cenozoic Era
geologic history section.
Transverse Ranges;
Santa Monica
Mountains
The Conejo volcanics of the Transverse Ranges (especially in the Santa Monica Mountains and points
north) make up many buff- and rust-colored slopes and cliffs. These Miocene Epoch volcanic rocks
are sometimes mistaken for sandstone from a distance. They were extruded before the mountains were
lifted out of the sea, and they represent one of the most widespread Tertiary Period volcanic formations
in California. They stand out along Mulholland Hwy. a few miles north of the ocean; they were so
severely fractured, they became a reservoir rock for oil at the Conejo Field east of Camarillo.
Mojave Desert; eastern
Mojave
Outstanding volcanic features are preserved in the Mojave Desert. Cima Volcanic Field on the Ivanpah
Upland is south of I-15 in the eastern Mojave. Here, at least forty cinder cones and lava flows up to 90 m
(300 feet) formed from the early Miocene to the Holocene Epochs (7.6 million–13,000 years old). Since
they formed protective caps on top of the older, deeply weathered granitic surface below, they stand out
quite nicely. In several locations, entire flows or their remnants have held their ground while surrounding
terrain was stripped away. Cima Dome itself is an upwarped granitic mass; scattered volcanic features
formed on top of it. Road materials and cinder block are mined from many of the cones.
Barstow to Amboy;
Ludlow
A chain of volcanic cones and their lava flows from Barstow east to Amboy exhibit classic examples of
pahoehoe (ropy lavas), tubes, tunnels, blisters, collapsed domes, and volcanic bombs—a variety of
features difficult to find south of the Cascades. Pisgah Crater, about halfway between Barstow and
Amboy, is an extremely young cinder cone formed on top of very fluid and extensive basaltic lava flows.
Dish Hill (also known as Siberia Crater) is also large, lies between Ludlow and Amboy, and is famous
among rock hounds for granitic and olivine-cored volcanic bombs. Amboy Crater, on the eastern end of
the chain, is only about 2,000 years old and may have the greatest variety of impressive features. From
the trail up the cone and to Amboy crater, even the nongeologist can appreciate definitive volcanic
features, including dark lava flows that spread from it across Bristol Playa.
A Brief Field Guide for Rock Hounds and Natural History Detectives
tant volcanic rocks that reflect California’s geologic history? See the adjacent box for answers.
Metamorphic Rocks Reveal
Geologic History
As their name implies, metamorphic rocks are older
rocks changed by geologic processes that usually require millions of years. Since most metamorphic rocks
form in high pressure and/or heat environments deep
below the surface, it takes millions of years of uplift and
erosion of overlying rocks to expose them. Therefore, a
discussion of the current distribution of metamorphic
5
rocks in California is also a discussion of some of the oldest geologic history of California.
Most of the metamorphic rocks in California were
formed from the late Paleozoic through the Mesozoic
Eras and into the early Tertiary Period by two different
processes. First, rocks caught between the two plate
boundaries of a subduction zone were squeezed and
metamorphosed by intense pressure. Second, other rocks
were changed, but not totally melted, by intense heat
during contact metamorphism. These contact zones and
their metamorphic rocks formed when deep magmas
generated by subduction rose toward the surface. Exceptions to these generalities include some of the oldest
WHERE TO FIND CALIFORNIA’S METAMORPHIC ROCKS
The Oldest Rocks The oldest rocks include gneisses and
schists dated at 1.72–1.82 billion years old and are basement
rocks of a continental crust with slightly younger granitic intrusions. They are scattered about the Transverse Ranges,
northwest Mojave, and the Basin and Range. (See the Precambrian Era geologic history section in Chapter 2.)
The Klamaths Metamorphic rocks of the Klamaths were
also crushed and molded onto the North American continent.
They are related to the Franciscan melange, but numerous
plate collisions must have been responsible for their formation because their ages range from mid-Paleozoic Era through
the Mesozoic Era, especially during the Nevadan orogeny.
The South Fork Mountain Schists (blueschists formed about
120 mya) are examples. They indicate that the Coast Ranges
were even attached to the Klamaths during part of the Mesozoic Era.
Coast Ranges The many rocks of the Franciscan Complex (or melange) can be found throughout the Coast Ranges.
These sedimentary and metamorphic rocks formed when sea
floor materials were dragged east from the western Pacific,
then caught and crushed against the continental plate during
the Jurassic and Cretaceous Periods. The rocks were carried
deep into the subduction zone, but they were pressed and
squeezed up toward the surface, like toothpaste. The bluegreen schists exposed in the north and south Coast Ranges
contain glaucophane, a mineral which forms when mafic marine rocks are exposed to high pressures, not extreme temperatures. Such minerals as jadeite are common in the green pebbles that have weathered off these melanges and washed up
on central California beaches.
Many other rocks (such as the common graywacke and
chert) in the Franciscan Complex, or melange, show how it got
the name melange, which means a mixture of different rocks.
Some sedimentary sections of the melange are many thousands
of feet thick. Conspicuous redcherts in the slopes of Marin
County overlooking San Francisco are examples. California’s
famous waxy/greasy, greenish serpentinites also formed in the
Franciscan when deep magma from the upper mantle with
high magnesium content flowed into faults and cracks and
crystallized. These intrusives can be found on the San Francisco Peninsula because all of the hills of San Francisco are
made of Franciscan rock. Mount Diablo, east of Walnut Creek
(between the Bay Area and Central Valley), is also made of Franciscan rocks and is surrounded by serpentinite, California’s
official state rock.
Western Sierra Nevada Various metamorphic rocks are
found in the Foothill Metamorphic Belt in the western Sierra
Nevada, east of Sacramento and west of the Melones thrust
fault. They likely formed when colliding plates crushed island
arc material against the continent during the Jurassic Period,
at the beginning of the Nevadan orogeny.
Sheared Metamorphic Rocks from the Transverse
Ranges, South The Pelona-Orocopia schists are found scattered from the Transverse Ranges down to the Chocolate
Mountains east of the Imperial Valley. They formed when a variety of rocks from a deep ocean basin or trench were thrust
together during the late Mesozoic Era. The related Garlock
and Rand schists may have been broken off and dragged away
from the other side of strike-slip faults to their present locations in the northwest Mojave; these rocks have been the
focus of recent studies of the tremendous displacement that
has occurred along the San Andreas Fault Zone during the past
several million years.
Contact Metamorphism Examples of rocks metamorphosed millions of years ago by intruding igneous plutons are
throughout California. The Crestmore limestone quarries in
western Riverside County, where there are more than 140
contact metamorphic minerals, is just one example. You will
also find them scattered in road cuts along Hwy. 243 from Banning up to Idyllwild. These are just a few examples of the extensive metamorphic outcrops in the Peninsular Ranges.
These events and their rocks are reviewed in other sections
within this appendix and Chapter 2 where plutonic intrusions
and earth resources are discussed.
6
Appendix California Rocks and Minerals
more detailed descriptions of California’s sedimentary
rocks are designed for more serious earth scientists—
rock hounds who have set a goal to further extend their
understanding of the distribution of rocks in California.
Human geographers and others who have had enough of
rocks and geologic history may want to skip this material.
First, we will locate some of the oldest sedimentary
rocks. Then, we will skip around California to find the
most interesting and important sedimentary rock formations. We will investigate landforms these rocks have
helped shape and the clues they have left about California’s geologic history. This section also serves as an excellent source for planning field trips.
Appendix Figure A-4 Cretaceous Hornbrook sedimentary
rock formations are exposed in numerous locations near I-5 and
the Oregon border. Younger, resistant lava flows typical of the
Cascades are seen looming over these formations higher in the
distance.
rocks in California, which were formed by similar processes much earlier in the Precambrian. See the adjacent
box for some metamorphic rock details.
Sedimentary Rocks and Their Fossils
Reveal Geologic History
Sediments and sedimentary rocks make up the majority
of the surface of the earth’s crust and that is also true for
California. Sedimentary rock formations provide some of
the most dramatic and beautiful landscapes in California.
In addition, they and the materials that were deposited
and lithified with them also provide the most detailed
accounts of ancient California environments and the
processes that must have changed them. They often provide us with energy and mineral resources. Also, because new sediments continue to accumulate on top of
their older kin in nearly every California basin and valley, the most productive farmland and almost all of the
towns and cities of California are located on sediment
and sedimentary rock.
In the following section, we will examine some of
the more interesting and important sedimentary rock formations in California. This is an opportunity to differentiate between two very different landscapes in California.
There are the relatively flat, sedimentary basins where
deposition of sediments continues and where most of
the farms and people are located. Standing in contrast
are the higher relief, mountainous areas where older
sedimentary rocks have been tectonically uplifted and
are exposed to differential weathering and erosion.
Focus on the Distribution of
Sedimentary Rocks: An In-Depth Section
By now, you should have a general working knowledge
of California’s geologic history and rocks. The following
California’s Oldest Sedimentary Rocks:
Showing Connections
Some of the oldest sedimentary rocks in California are
the 800 million–1.2 billion-year-old Pahrump Group
rocks of the Basin and Range. These conglomerates,
sandstones, siltstones, shales, limestones, and dolomites
of the Crystal Spring, Beck Spring, and Kingston Peak
Formations (from oldest to youngest, they are known as
the Pahrump Group) are thousands of meters thick.
They contain some very primitive single-celled algae fossils and stromatolites deposited in shoreline waters. Deposited on those rocks are the Noonday Dolomite, Johnnie Formation, and Stirling Quartzite that mark the end
of the Precambrian (see Figure 2-1). Thousands of meters of Cambrian Period rocks were deposited on top of
them. These rocks and their fossils (especially found in
the White-Inyo Range) were deposited when the first
multicellular animals, followed by the invertebrates
(hard-shelled animals), appeared. Cambrian Period limestones and dolomites are also found in the mountains
east of Death Valley (such as in Titus Canyon) and are
scattered in other Basin and Range and Mojave Desert
locations.
The roof pendants of the Sierra Nevada (examined
in a previous section of Chapter 2) probably include the
same sequence of rocks as found in the Basin and
Range, but they were more extensively metamorphosed
by the Nevadan orogeny. Additionally, the marine depositional environments that dominated in those locations
by the start of the Paleozoic Era were somewhat similar
to environments of the Peninsular Range. Evidence that
rocks were extensively metamorphosed is found within
the limestones near Riverside and in other Paleozoic Era
deposits of the region.
In the Mojave, multicolored mudstones were deposited during the Cambrian Period to form the Latham
shale with marine fossils. Deposits of iron ore are found
in the middle Cambrian Period Bonanza King limestone
in the southern Providence Mountains.
Following the Cambrian Period, which begins the
Paleozoic Era, the abundance and variety of sedimentary rocks increases with each younger geologic period.
A Brief Field Guide for Rock Hounds and Natural History Detectives
Returning to our standard method of reviewing rock formations, we will now discuss some of the most interesting sedimentary rocks from many different ages. We
start a sweep around the state with the Great Central
Valley region.
Sedimentary Rocks Found Throughout
Northern and Central California
Central Valley. Thousands of feet of sediments have
been accumulating since the Cretaceous Period in the
downwarped basin we call the Central Valley. The layers
generally dip down away from the Sierra Nevada and
Coast Ranges toward the valley floor, so we see the older
rocks only on the edges of the valley and in well samples.
Starting with the marine Cretaceous sediments, overlying
rocks become more frequently nonmarine into the Quaternary Period. Some produce oil and gas; other rocks are
tapped for groundwater. The sedimentary rock layers are
more than 7,600 m (25,000 feet) thick in the Sacramento
Valley. These rock layers dip toward the valley on its
western edge where the Coast Ranges rise above it and
have weathered into long, parallel “hogback” ridges.
It is clear that sediments have washed down from
the surrounding steep mountain ranges (especially from
the east) into the Central Valley since the Mesozoic Era.
The most recent unconsolidated Holocene Epoch sediments cover today’s valley floor.
The Kettleman Hills (south of Coalinga) represent a
stretched anticlinal dome of Central Valley sedimentary
rock folded up above the San Joaquin Valley floor.
Rocks of this young structural feature produce not only
oil and gas, but many different kinds of Tertiary Period
fossils. Here, fossils from a shallow marine environment
are found in the younger Pliocene Epoch Etchegoin and
San Joaquin Formations. Below, in the late Miocene
Epoch Santa Margarita Formation, Ostrea titan giant oyster fossils up to 15 cm (6 inches) long are found. The still
deeper and older Miocene Epoch Temblor Formation,
which is also exposed in the nearby Coast Ranges, yields
Desmostylus sea cow and Merychippus horse fossils.
Northwest California. In the Trinity Forest, limestones between Red Bluff and Humboldt Redwoods
State Park on Hwy. 36 and near Hall City caves contain
some of the youngest Permian Period animal fossils in
North America; they must have been dragged east to this
location on an ocean plate because they match fossils in
the western Pacific.
In the Klamaths, Cretaceous Period marine deposits
were accumulated when the area was slightly below sea
level just after the Nevadan orogeny. Marine fossils include ammonites and pelecypods in the Great Valley Sequence that were deposited before the Klamaths were
uplifted. Conglomerates of these formations are exposed south of Weaverville and in the eastern Klamaths.
Shallow-water marine fossils are found in the late Creta-
7
ceous Period Hornbrook conglomerates, seen near I-5
from Hornbrook to the Oregon border. In contrast, plant
fossils in the conglomerates, sandstones, and shales of
the Weaverville Formation (found near Weaverville)
show it to be nonmarine, deposited on a swampy flood
plain during the Oligocene Epoch. A variety of Miocene
Epoch Wimer Formation loose clastic rocks are seen in
the extreme northwestern Klamaths.
Structurally downwarped basins may be smaller on
the northwest coast compared to the expansive coastal
valleys farther south, but some have accumulated similar
rock formations of the same ages. The Eel River Basin is
one of the largest. It extends from Cape Mendocino to
Eureka and is filled with sedimentary rocks deposited
during the last few million years. Pliocene Epoch marine
sedimentary mudstones and other rocks of the Wildcat
Group include formations with names such as the Eel
River, Rio Dell, and St. George. One excellent exposure
of the Eel River Formation has been uplifted and is seen
at Scotia Bluff along the Eel River.
Cascades and Modoc Plateau. Since the Cascades
and Modoc Plateau are dominated by volcanic activity
and rocks, they provide relatively few opportunities to
study sedimentary rocks. However, there are some
basins that have been filling with sediment that includes
lake deposits and weathered and eroded volcanic material. At Lava Beds National Monument, fossils of a
mastodon and prehistoric camel were found in the lava
tubes. Will the recent sedimentary deposits be buried by
future ash and cinder deposits or lava flows as some of
their ancestors were?
Sierra Nevada. In Chapter 2 we examined the metamorphosed, mostly Paleozoic Era roof pendants of the
Sierra Nevada, which lie on top of the granitic rocks, especially in higher terrain. They represent the sedimentary rocks that formed when a depositional environment
ruled in the Sierra, but they were later squeezed and
faulted on a plate boundary. Most of them have been
eroded away. In the northern Sierra Nevada, rock formations of the Silurian Shoo Fly complex include the cherts
and limestones of an old ocean crust with accumulated
material along a plate boundary.
As expected, there are few deposits left in the Sierra
Nevada from the mountain-building episode called the
Nevadan orogeny. Today, scattered remnants of sediments accumulated after the Nevadan orogeny and its
erosion are more common. Marine sediments and sandstones with fossils from the late Cretaceous Period are
found on top of basement rocks along the current boundary between the Central Valley and the Sierra Nevada. In
some northern Sierra Nevada foothills, the younger
Eocene Epoch Ione Formation consists of sand, clay, and
coal beds that have been mined to produce glass, bricks,
wax, and fuel.
8
Appendix California Rocks and Minerals
Sedimentary Rocks in the Coast Ranges
Our search for sedimentary rocks continues in the Coast
Ranges. Here, the geology is so complex and the rock
formations (dating from the Mesozoic Era to the Pleistocene Epoch) are so diverse, we can only pick out a few
examples. For more detail, refer back to the section in
Chapter 2 on California geologic history, read articles in
other publications, or take field trips to the Coast Ranges
region.
The Coast Ranges are dominated by mostly Cenozoic Era sedimentary rocks deposited on two types of
basement, one of which is the previously examined
Franciscan melange. Mixed with the metamorphic rocks
of the melange are graywackes. They were deposited in
chaotic currents and mudflows within deep marine
basins below steep volcanic highlands, all near the subduction zone that existed during the Mesozoic Era. Thick
deposits are exposed in numerous Coast Range locations
along with the red cherts which contain single-celled
deep sea fossils (radiolarian) and deep sea limestones.
Along the margin between the Coast Ranges and the
Great Central Valley are exposures of late Jurassic to late
Cretaceous Periods marine conglomerate, sandstone,
and shale of the Great Valley Sequence. These several
thousand meters of sediments were washed from the
east off the continent and lithified. They have been extensively folded and faulted and are now found on the
eastern Mendocino and Diablo Ranges and the west
edge of the Sacramento Valley.
Extensive thick outcrops of nearly every age of
Cenozoic Era sedimentary rocks have been severely
faulted and displaced up to 300 km (200 miles) from
one another since the Miocene Epoch in the Coast
Ranges. They include the Paleocene marine sediments
in the Santa Lucia and Diablo Ranges, in San Francisco,
and at Point Lobos and Point Reyes. Eocene Epoch coal
beds and clays indicate that California experienced
tropical conditions; this includes the coal fields northeast of Mount Diablo, which were commercially mined
until 1920.
The Emerging Coastline. Deep ocean deposits of the
Miocene Epoch Monterey Formation have abundant
micro-fossils. This light-colored shale is found from
Santa Rosa to near Monterey and from south of Morro
Bay to Shell Beach. It is found as far south as the Peninsular Ranges; it is known as the Modelo Shale at Point
Dume on the Malibu Coast and in the Santa Monica
Mountains. The mostly marine Pliocene Epoch rocks
give way to alluvial and lake deposits because the sea retreated for the last time toward the beginning of the
Pleistocene Epoch. More recent deposits continue to accumulate to great thicknesses in subsided coastal basins
of the Coast Ranges.
Meanwhile, numerous marine fossils are found in
rocks along the coast, such as in Pliocene Epoch siltstones
Appendix Figure A-5 Travertine has accumulated over the
rocks at Nojoqui Falls in Santa Barbara County and plants are trying to grow over it. The deposits of calcium and magnesium carbonate leave this cliff looking like solid limestone, though the
underlying rock is not.
at Drakes Beach (Point Reyes) and sandstones at Moss
Beach. Additional fossils at other locations, such as near
Año Nuevo State Beach, also confirm that the coastline is
being lifted, exposing rocks once deposited below the
sea.
Sedimentary Formations in Southern
California Coastal Mountains and Valleys
In the Peninsular Ranges. You will notice late Cretaceous Period mainly marine sedimentary rocks in the
Peninsular Ranges from the Santa Ana Mountains and
scattered south into Baja. Among these, the mostly nonmarine Trabuco Conglomerate is common in the western Santa Ana Mountains. In contrast, marine sandstones
and shales of the Rosario Formation contain huge ammonites (flat, coiled mollusk fossils from the Mesozoic
Era). Exposures of the Rosario sandstones can be found
along the coast at La Jolla Cove, Point Loma, and points
south into Mexico.
A Brief Field Guide for Rock Hounds and Natural History Detectives
9
Appendix Figure A-6 Deposited in the
ocean and lithified only several million years
ago, these sedimentary shales at Montana
de Oro State Beach have been lifted, folded,
and exposed. Differential weathering and
erosion sculpt the tilting layers. Marine terraces rise in the background.
Sedimentary rocks deposited after the Cretaceous
Period in the Peninsular Ranges are widespread. Early
Tertiary Period rocks include the Paleocene Epoch nonmarine Silverado Formation in the northern Santa Ana
Mountains, where coal, clay, and mixed silicates used in
glass making have been mined. Eocene Epoch deposits
of rounded pebbles at the mouth of the ancient Poway
River became the Poway Formation in San Diego (examined in the Cenozoic Era discussion in Chapter 2). The
Miocene San Onofre breccia, with its chunks of broken
schist, is found in spots along the southern coast.
More extensive are the rocks formed since the later
Tertiary Period in the Peninsular Ranges. They include
the nonmarine conglomerate, sandstone, and siltstone
of the Pliocene Epoch Mount Eden and San Timoteo
Canyon Formations. Abundant and impressive marine
fossils are easily recognized in the Pliocene Epoch San
Diego Formation north of Mission Bay (see Fig. 2-1).
Some of the thickest sedimentary deposits are found in
the Imperial and Coachella Valleys, Elsinore trough,
Perris Plain, Los Angeles Basin, and offshore basins, as
well as in the coastal basins to the north in the Transverse and Coast Ranges. The rocks in these basins were
weathered, eroded, and transported from mountain
ranges and deposited and lithified in basins that exist to
this day.
Sedimentary Formations in the Transverse Ranges.
In the Transverse Ranges, the Ventura Basin has more
than 17,700 m (58,000 feet) of deposits from the Cretaceous Period to today. These include what may be the
thickest Pliocene Epoch deposits on Earth. This is all part
of a structural syncline that also includes the Soledad
Basin, which has filled with nonmarine sediments.
Some of the most interesting sedimentary rock formations in the Transverse Ranges have been radically
lifted up and exposed by recent tectonic activity along
and near the San Andreas Fault Zone. For instance, it
might appear that the spectacularly dipping sandstones
and conglomerates at three locations in and adjacent to
the San Gabriel Mountains are similar: Vasquez Rocks
along Hwy. 14 in the San Gabriels, Devil’s Punchbowl
above Pearblossom, and Mormon Rocks in Cajon Pass. It
turns out that the Devil’s Punchbowl and Mormon Rocks
are both composed of Miocene Epoch deposits of the
Punchbowl Formation. Embedded land vertebrate fossils
are found at both sites; these two strikingly similar landforms may have been separated by sliding action along
the San Andreas Fault Zone. The Vasquez Formation is
also nonmarine, but dates back to the Oligocene Epoch.
Interestingly, rock outcrops of the Diligencia Formation
in the Orocopia Mountains of the Colorado Desert on
the other side (east) of the San Andreas Fault match the
Vasquez in type and age.
There are so many other beautiful exposures of sedimentary rocks in the Transverse Ranges; this variety of
complexly folded and faulted formations makes examination of even the most important ones an impossible
task in this book. One example is the drive inland
through the Malibu Canyon gorge from Hwy. 1 (Pacific
Coast Highway) in the Santa Monica Mountains. You will
see (in order) more than 15 million years of earth history
from the Oligocene to the Miocene Epochs in exposures
of nonmarine pink and purple Sespe, marine Vaqueros,
Topanga, Conejo volcanics, and Modelo Shale (related to
the Monterey) Formations within 15 minutes as you drive
inland. The folded and faulted rock layers are now dipping at steep angles. Another example is in the Santa Ynez
10
Appendix California Rocks and Minerals
Mountains north of Santa Barbara. Numerous rock formations, from the Jurassic and Cretaceous Periods’ Franciscan melange to the late Pleistocene Epoch Santa Barbara
Formation, have been contorted and lifted upward.
Sedimentary Rocks in Exposed Landscapes
of Transmontane California
Basin and Range. In the Basin and Range, there are
many other interesting sedimentary rock formations in
addition to the oldest ones, which we have previously
examined. Besides the folded and faulted marine Cambrian Period limestones and dolomites exposed in Titus
Canyon in Death Valley and surrounding areas, you will
find the Oligocene Epoch Titus Canyon beds, which are
also related to the Artist’s Drive Formation. Fossils of the
great, horse-like herbivore Titanothere, rhinoceros, and
camel suggest that a lush savanna existed here during
the Oligocene Epoch. Fossils found in Pliocene Epoch
rocks at Furnace Creek include leaves, diatoms, and
animal footprints, while in the Coso Mountains, late
Pliocene Epoch mammal fossils include dogs, peccaries,
camel, horses, and mastodons. The area apparently became drier (except for occasional wet periods during the
Pleistocene Epoch Ice Ages) as the Sierra Nevada was
lifted higher to the west, blocking moisture from the Pacific, as it does today.
Of course, the sedimentary rock record in the Basin
and Range is far more extensive. (Some of these exposures were reviewed in the section on geologic history in
Chapter 2.) You will find more details in specific geology
publications.
More recently, in the Basin and Range and into the
Mojave Desert since the Pleistocene Epoch, numerous
inland basins have been filling with silt and clay during
wetter glacial periods and filling with salts during drier
periods such as today’s. Some of these relatively young
sediments, such as the Funeral Formation in Death Valley, have already been folded and faulted.
Early Pleistocene Epoch fossils of horses, camel,
deer, pronghorns, hog-like tapirs, and rabbits have been
recovered along the shores of ancient Lake Manix in the
Mojave. Many different kinds of salts (including chlorides, carbonates, potassium, and borates) are being
mined from the saline deposits in these mostly dry lake
beds in today’s Basin and Range and Mojave.
Mojave Desert Sedimentary Formations. Moving
toward the Mojave, in Red Rock Canyon, the lighter
sandstone layers and the darker lava flows often found
above them are commonly oxidized red. The nonmarine
sedimentary rocks of this Miocene Epoch Ricardo Group
contain fossils of petrified wood, rodents, primitive
horses, camel, antelope, mastodons, rhinoceros, and
saber-toothed cats that lived in a higher-rainfall savanna
perhaps when lower hills to the west allowed moist
ocean air masses to penetrate farther inland.
Farther east, in Rainbow Basin north of Barstow, are
the colorful Miocene Epoch sedimentary rocks of the
Barstow Formation. The fossil pollen, rodent, tortoise,
dog, cat, rabbit, pronghorn, horse, camel, rhinoceros,
and saber-toothed cat suggest that the Miocene was wetter with summer rains.
Continuing east, across the Mojave, in the Providence Mountains, are the limestones of the Permian Bird
Spring Formation. Mitchell Caverns were still active during wetter periods as late as the Pleistocene Epoch,
when water combined with limestone to produce carbonic acid that dissolved the rock and formed the caverns. Today, Mitchell Caverns are dry, but are open to
tour. Geologists recognize the rock record near Mountain Pass to the north to be far more extensive than at
popular Mitchell Caverns.
Nearby, brachiopods and trilobites are found in the
much older Cambrian Period Latham shale. This formation is made of red, green, and gray mudstones. For
many years, fossil collectors have converged on the Marble Mountains east of Amboy, where the relatively accessible and remarkable crustacean trilobite, Fremontia fremonti, index fossils of the Paleozoic Era, are up to 20 cm
(8 inches) long.
Sedimentary Formations in the Southern Deserts.
Farther southeast, toward the Colorado Desert, fossil
wood has been found in Cretaceous Period sediments of
the McCoy and Palen mountains.
The Colorado Desert contains thick accumulations
of mostly younger Cenozoic Era rocks. The oldest include Eocene Epoch marine deposits in the Orocopia
Mountains (east of the Coachella Valley) and nonmarine
rocks in the Palo Verde Mountains (southwest of
Blythe). Slightly younger Oligocene Epoch continental
deposits of the Diligencia Formation are also found in
the Orocopias.
Post-Oligocene Epoch sedimentary rocks are common and are many thousands of meters thick. They are
exposed in the western Imperial Valley (including those
in Anza Borrego State Park). They also lie below the surface of the valley into Mexico as well as below the
Coachella Valley. The majority of these sedimentary
rocks are nonmarine, except when the sea encroached
in the Miocene Epoch, depositing the brown oyster
beds of the Split Mountain Formation and the Pliocene
Imperial Formation. After the sea retreated (probably
blocked by the building Colorado River delta), the
Pliocene Epoch Canebrake Conglomerate and the finergrained Palm Springs Formation were deposited. The
Palm Springs Formation (with some petrified wood) can
be seen west of the Salton Sea, where it is folded and
being eroded. Still younger Lake Cahuilla deposits
(since the Pleistocene Epoch) were laid flat on the surface of the older rocks and contain marine mollusks.
These oyster beds were probably deposited in the high
A Brief Field Guide for Rock Hounds and Natural History Detectives
salinity of the late Pleistocene Epoch lake, rather than in
ocean water.
Basement rocks of the Valecito and Fish Creek
Mountains on the western edge of the Imperial Valley resemble those of the Peninsular Range to the west. However, overlying sedimentary rocks resemble those of the
Colorado Desert to the east. Consequently, following deposition of the Anza Formation during a dry period in
Laguna Rocks by Patty Kellner
11
the Miocene Epoch, many of the younger deposits are
somewhat similar to those in the Imperial Valley.
Though we have come to the end of our more thorough survey of ancient rocks and the stories they tell, we
have only started here what could be volumes and lifetimes of research on California’s earth history. It is time
to move ahead to examine 21st Century processes and
landscapes.