California Rocks and Minerals - California Geography

A 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 Appendix Figure A-1 Granitic rocks near the top of Cas-
detail how California’s geologic history is revealed by the tle Crags in northern California are being weathered along a
distribution of so many kinds of rocks in the state. It is series of vertical joints and fractures. The high silica/quartz
provided for those with an appetite for more details about content of such granites makes them a little lighter in color
the evidence California’s geologic history has left behind. and weight and also makes them resistant to erosion.

It would be impossible to find one storage room or the Basin and Range and Mojave. They make up a large
laboratory with the variety and quality of mineral and portion of the basement of the Coast Ranges, and they
rock samples found in California. Figure 2-6 in text illus- are even buried by more recent volcanic material in
trates the rock cycle that produces this variety. northeastern California and by recent alluvial deposits in
many California valleys. This “Sierra Nevada batholith” is
The importance of the distribution of these minerals composed of more than 100 different types of plutons,
and rocks ranges far beyond geologic curiosity. We have most referred to as granitic rock. Light-colored speckles
already followed the clues they provide us about Califor- that usually dominate this more felsic (high in silica)
nia’s tumultuous geologic history. In addition, these rock granitic rock are quartz and feldspar; darker speckles are
types and structures often control local and regional usually less common and include flat flakes of biotite
geomorphology—the California landscapes which evolve mica or other more mafic (higher in iron, magnesium,
from them. The specific chemistry of soils produced when etc.) minerals such as amphibole or pyroxene. Specific
different rocks are weathered plays a significant role in the names for these salt-and-pepper rocks (granite, quartz-
distribution of plants and animals in California. Certain monzonite, granodiorite, quartz diorite, etc.) refer to the
types of rocks are associated with specific geologic haz- specific chemistries of each.
ards, especially when we build on them. Ores and gems
are also found in and extracted from particular rock bod- Recall that these rocks formed when an eastward
ies, and some are quarried for building materials. Even if dipping ocean plate was being subducted below the con-
you are not a rock hound, you must appreciate the signifi- tinent, mostly during the Mesozoic Era and the Nevadan
cant roles rock distributions play not only in the land-
scapes 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 his-
tory, which described the geologic events that produced
California’s rocks. This discussion focuses on a search
for some of the most common, interesting, and impor-
tant rocks, minerals, and earth resources in California.

Plutonic Rocks in California:
Lifted and Exposed

Intrusive igneous rocks are found throughout California;
they dominate the basement rocks, and they are fre-
quently exposed in the Klamath, Sierra Nevada, Trans-
verse, and Peninsular Ranges and in the mountains of

1

2 Appendix California Rocks and Minerals

orogeny. Though some of this activity began even be- and exposed after overlying rocks were eroded away.
fore the Mesozoic Era and some of it continued into the About 60 percent of the surface rocks in the central and
Cenozoic Era, 100 million years will serve as a conve- southern Sierra Nevada are made of these mostly granitic
nient, average age for these rocks. Millions of years plutons.
passed as enormous magma chambers formed below
California and gradually cooled and crystallized to pro- Lighter-colored dikes, sills, and veins in the batholith
duce these rocks with large crystals. Darker plutons are represent more felsic (higher silica) magmas that were
composed of more ocean plate and mantle material from the last to crystallize. The joints and fractures in the ear-
below; lighter granites formed more frequently when lier-crystallized plutons served as conduits through
large volumes of higher silica continental crust were in- which these hot magmas, which were still melted and
corporated into the melt. Many of the once buried often mixed with water, were squeezed and finally
batholiths have since been pushed and faulted upward 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 Mesozoic Era granitic plutons, similar to Sierra Nevada and Peninsular Ranges. Faulting has
Andreas Fault in Salinian Block moved them up next to the Franciscan melange (the other Coast Ranges basement).

Montara Mountain (Santa Cruz Quartz diorites are strikingly similar to Farallon Islands.
Mountains)

Transverse Ranges Granitic plutons similar to other California mountains especially near and above San Gabriel
and San Bernardino Mountain resorts.

Peninsular Range—San Jacinto Southern California batholith dated at 70–120 mya. Slightly less felsic than northern
Mountains from Palm Springs California batholiths, but weather to produce landscapes similar to Sierra Nevada.
Tram and above Idyllwild; Numerous pegmatite dikes noticeably cut through to produce gem sites. Upper San Luis
Santa Rosa and Laguna Mountains 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 Papoose Flat pluton (75–81 mya) squeezed through, thinned, and metamorphosed
of Big Pine 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 Vol-
canic boulders are scattered
across the relatively young Sut-
ter Buttes that erupt from the
Sacramento Valley. Pasty erup-
tions of andesite and rhyolite
left these rocks to be weath-
ered into darker colors.

into the melt and emplaced within the vein. Some of nia. Weathering processes are accelerated in the surface
these intrusions are responsible for changing the human cracks between these rock masses, a topic covered in
history and economy of California. detail within Chapter 3.

Today’s vertical and horizontal joints and fractures Volcanic Rocks: California’s
in these plutonic rocks are sometimes a result of com- Geologic History Flares Up
pressional and/or extensional tectonic forces that have
stressed the buried rocks over millions of years. Other It may surprise some Californians that active and poten-
joints and fractures were formed more recently as over- tial volcanic hot spots are scattered about in at least four
lying rocks were eroded away (unloaded), releasing different areas of the state today. They include the majes-
pressure and allowing expansion of the rock masses. tic volcanoes and lava plateaus of the Cascades and
This exfoliation breaks the rocks at the surface into northeastern California, the area above the Napa Valley
onion-like sheets and sometimes produces great out- near Clear Lake (including the geysers), the Basin and
standing domes. You might observe some or all of these Range east of the Sierra Nevada, and south of the Salton
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 mil-
lions of years later, they were lifted up in the
Sierra Nevada and are now exposed to
weathering here at May Lake in Yosemite.

4 Appendix California Rocks and Minerals

Sea in the Imperial Valley. Each of these areas has also A few of these ancient hot spots are near areas where
experienced some modern volcanic activity. In this sec- volcanic activity happens to have continued to today.
tion, however, we will examine only the distribution of Where there is not a clear line, we will yield to Chapter
the volcanic rocks that have been left behind by past vol- 3’s section on modern volcanic processes and landforms.
canic activity. Volcanic activity in human history will be To manage this organizational problem, we ask the ques-
examined in Chapter 3. tion: Where are some of the more common and impor-

WHERE TO FIND ANCIENT
VOLCANIC ROCKS AND THEIR LANDSCAPES

Table Mountain north A more resistant ancient lava flow (Lovejoy basalt) stands out in the foothills above weaker rocks that
of Oroville 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
Sierra Nevada Foothills andesite and rhyolite. Probably an extension of the southern Cascades, they are very young. (The
east of Sacramento; surrounding late Pliocene Epoch Tehama Formation was warped by these eruptions.) They are
Sonora included here because they are isolated and detached geographically from the Cascades.

San Luis Obispo; Morro After erosion denuded the ancient Sierra Nevada to lower hills about 20–25 mya, great deposits of
Bay 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
Pinnacles National it to reach gold deposits. After some erosion of that surface, extensive volcanic mudflows (including large
Monument east of volcanic cobbles) and basalts covered it. This is known as the Mehrten Formation (4–20 mya). Latite
Salinas (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.
Transverse Ranges;
Santa Monica Along Hwy. 1 from San Luis Obispo to Morro Bay is the chain of fourteen resistant throats of
Mountains 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
Mojave Desert; eastern between rhyolite and andesite) and was once extensively quarried to make breakwater and fill. Refer to
Mojave Chapter 2’s section on Cenozoic Era geologic history for the origins of these volcanoes.

Barstow to Amboy; On the Monument’s west side, overlooking Soledad and the Salinas Valley, are Miocene Erhyolitic
Ludlow 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.

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.

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.

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 5

tant volcanic rocks that reflect California’s geologic his- rocks in California is also a discussion of some of the old-
tory? See the adjacent box for answers. est geologic history of California.

Metamorphic Rocks Reveal Most of the metamorphic rocks in California were
Geologic History formed from the late Paleozoic through the Mesozoic
Eras and into the early Tertiary Period by two different
As their name implies, metamorphic rocks are older processes. First, rocks caught between the two plate
rocks changed by geologic processes that usually re- boundaries of a subduction zone were squeezed and
quire millions of years. Since most metamorphic rocks metamorphosed by intense pressure. Second, other rocks
form in high pressure and/or heat environments deep were changed, but not totally melted, by intense heat
below the surface, it takes millions of years of uplift and during contact metamorphism. These contact zones and
erosion of overlying rocks to expose them. Therefore, a their metamorphic rocks formed when deep magmas
discussion of the current distribution of metamorphic generated by subduction rose toward the surface. Excep-
tions to these generalities include some of the oldest

WHERE TO FIND CALIFORNIA’S METAMORPHIC ROCKS

The Oldest Rocks The oldest rocks include gneisses and Franciscan when deep magma from the upper mantle with
schists dated at 1.72–1.82 billion years old and are basement high magnesium content flowed into faults and cracks and
rocks of a continental crust with slightly younger granitic in- crystallized. These intrusives can be found on the San Fran-
trusions. They are scattered about the Transverse Ranges, cisco Peninsula because all of the hills of San Francisco are
northwest Mojave, and the Basin and Range. (See the Precam- made of Franciscan rock. Mount Diablo, east of Walnut Creek
brian Era geologic history section in Chapter 2.) (between the Bay Area and Central Valley), is also made of Fran-
ciscan rocks and is surrounded by serpentinite, California’s
The Klamaths Metamorphic rocks of the Klamaths were official state rock.
also crushed and molded onto the North American continent.
They are related to the Franciscan melange, but numerous Western Sierra Nevada Various metamorphic rocks are
plate collisions must have been responsible for their forma- found in the Foothill Metamorphic Belt in the western Sierra
tion because their ages range from mid-Paleozoic Era through Nevada, east of Sacramento and west of the Melones thrust
the Mesozoic Era, especially during the Nevadan orogeny. fault. They likely formed when colliding plates crushed island
The South Fork Mountain Schists (blueschists formed about arc material against the continent during the Jurassic Period,
120 mya) are examples. They indicate that the Coast Ranges at the beginning of the Nevadan orogeny.
were even attached to the Klamaths during part of the Meso-
zoic Era. Sheared Metamorphic Rocks from the Transverse
Ranges, South The Pelona-Orocopia schists are found scat-
Coast Ranges The many rocks of the Franciscan Com- tered from the Transverse Ranges down to the Chocolate
plex (or melange) can be found throughout the Coast Ranges. Mountains east of the Imperial Valley. They formed when a va-
These sedimentary and metamorphic rocks formed when sea riety of rocks from a deep ocean basin or trench were thrust
floor materials were dragged east from the western Pacific, together during the late Mesozoic Era. The related Garlock
then caught and crushed against the continental plate during and Rand schists may have been broken off and dragged away
the Jurassic and Cretaceous Periods. The rocks were carried from the other side of strike-slip faults to their present loca-
deep into the subduction zone, but they were pressed and tions in the northwest Mojave; these rocks have been the
squeezed up toward the surface, like toothpaste. The blue- focus of recent studies of the tremendous displacement that
green schists exposed in the north and south Coast Ranges has occurred along the San Andreas Fault Zone during the past
contain glaucophane, a mineral which forms when mafic ma- several million years.
rine rocks are exposed to high pressures, not extreme temper-
atures. Such minerals as jadeite are common in the green peb- Contact Metamorphism Examples of rocks metamor-
bles that have weathered off these melanges and washed up phosed millions of years ago by intruding igneous plutons are
on central California beaches. throughout California. The Crestmore limestone quarries in
western Riverside County, where there are more than 140
Many other rocks (such as the common graywacke and contact metamorphic minerals, is just one example. You will
chert) in the Franciscan Complex, or melange, show how it got also find them scattered in road cuts along Hwy. 243 from Ban-
the name melange, which means a mixture of different rocks. ning up to Idyllwild. These are just a few examples of the ex-
Some sedimentary sections of the melange are many thousands tensive metamorphic outcrops in the Peninsular Ranges.
of feet thick. Conspicuous redcherts in the slopes of Marin These events and their rocks are reviewed in other sections
County overlooking San Francisco are examples. California’s within this appendix and Chapter 2 where plutonic intrusions
famous waxy/greasy, greenish serpentinites also formed in the and earth resources are discussed.

6 Appendix California Rocks and Minerals

Appendix Figure A-4 Cretaceous Hornbrook sedimentary more detailed descriptions of California’s sedimentary
rock formations are exposed in numerous locations near I-5 and rocks are designed for more serious earth scientists—
the Oregon border. Younger, resistant lava flows typical of the rock hounds who have set a goal to further extend their
Cascades are seen looming over these formations higher in the understanding of the distribution of rocks in California.
distance. Human geographers and others who have had enough of
rocks and geologic history may want to skip this material.
rocks in California, which were formed by similar pro-
cesses much earlier in the Precambrian. See the adjacent First, we will locate some of the oldest sedimentary
box for some metamorphic rock details. rocks. Then, we will skip around California to find the
most interesting and important sedimentary rock forma-
Sedimentary Rocks and Their Fossils tions. We will investigate landforms these rocks have
Reveal Geologic History helped shape and the clues they have left about Califor-
nia’s geologic history. This section also serves as an ex-
Sediments and sedimentary rocks make up the majority cellent source for planning field trips.
of the surface of the earth’s crust and that is also true for
California. Sedimentary rock formations provide some of California’s Oldest Sedimentary Rocks:
the most dramatic and beautiful landscapes in California. Showing Connections
In addition, they and the materials that were deposited Some of the oldest sedimentary rocks in California are
and lithified with them also provide the most detailed the 800 million–1.2 billion-year-old Pahrump Group
accounts of ancient California environments and the rocks of the Basin and Range. These conglomerates,
processes that must have changed them. They often pro- sandstones, siltstones, shales, limestones, and dolomites
vide us with energy and mineral resources. Also, be- of the Crystal Spring, Beck Spring, and Kingston Peak
cause new sediments continue to accumulate on top of Formations (from oldest to youngest, they are known as
their older kin in nearly every California basin and val- the Pahrump Group) are thousands of meters thick.
ley, the most productive farmland and almost all of the They contain some very primitive single-celled algae fos-
towns and cities of California are located on sediment sils and stromatolites deposited in shoreline waters. De-
and sedimentary rock. posited on those rocks are the Noonday Dolomite, John-
nie Formation, and Stirling Quartzite that mark the end
In the following section, we will examine some of of the Precambrian (see Figure 2-1). Thousands of me-
the more interesting and important sedimentary rock for- ters of Cambrian Period rocks were deposited on top of
mations in California. This is an opportunity to differenti- them. These rocks and their fossils (especially found in
ate between two very different landscapes in California. the White-Inyo Range) were deposited when the first
There are the relatively flat, sedimentary basins where multicellular animals, followed by the invertebrates
deposition of sediments continues and where most of (hard-shelled animals), appeared. Cambrian Period lime-
the farms and people are located. Standing in contrast stones and dolomites are also found in the mountains
are the higher relief, mountainous areas where older east of Death Valley (such as in Titus Canyon) and are
sedimentary rocks have been tectonically uplifted and scattered in other Basin and Range and Mojave Desert
are exposed to differential weathering and erosion. locations.

Focus on the Distribution of The roof pendants of the Sierra Nevada (examined
Sedimentary Rocks: An In-Depth Section in a previous section of Chapter 2) probably include the
same sequence of rocks as found in the Basin and
By now, you should have a general working knowledge Range, but they were more extensively metamorphosed
of California’s geologic history and rocks. The following by the Nevadan orogeny. Additionally, the marine depo-
sitional 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 de-
posited 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 sedimen-
tary rocks increases with each younger geologic period.

A Brief Field Guide for Rock Hounds and Natural History Detectives 7

Returning to our standard method of reviewing rock for- ceous Period Hornbrook conglomerates, seen near I-5
mations, we will now discuss some of the most interest- from Hornbrook to the Oregon border. In contrast, plant
ing sedimentary rocks from many different ages. We fossils in the conglomerates, sandstones, and shales of
start a sweep around the state with the Great Central the Weaverville Formation (found near Weaverville)
Valley region. show it to be nonmarine, deposited on a swampy flood
plain during the Oligocene Epoch. A variety of Miocene
Sedimentary Rocks Found Throughout Epoch Wimer Formation loose clastic rocks are seen in
Northern and Central California the extreme northwestern Klamaths.
Central Valley. Thousands of feet of sediments have
been accumulating since the Cretaceous Period in the Structurally downwarped basins may be smaller on
downwarped basin we call the Central Valley. The layers the northwest coast compared to the expansive coastal
generally dip down away from the Sierra Nevada and valleys farther south, but some have accumulated similar
Coast Ranges toward the valley floor, so we see the older rock formations of the same ages. The Eel River Basin is
rocks only on the edges of the valley and in well samples. one of the largest. It extends from Cape Mendocino to
Starting with the marine Cretaceous sediments, overlying Eureka and is filled with sedimentary rocks deposited
rocks become more frequently nonmarine into the Qua- during the last few million years. Pliocene Epoch marine
ternary Period. Some produce oil and gas; other rocks are sedimentary mudstones and other rocks of the Wildcat
tapped for groundwater. The sedimentary rock layers are Group include formations with names such as the Eel
more than 7,600 m (25,000 feet) thick in the Sacramento River, Rio Dell, and St. George. One excellent exposure
Valley. These rock layers dip toward the valley on its of the Eel River Formation has been uplifted and is seen
western edge where the Coast Ranges rise above it and at Scotia Bluff along the Eel River.
have weathered into long, parallel “hogback” ridges.
Cascades and Modoc Plateau. Since the Cascades
It is clear that sediments have washed down from and Modoc Plateau are dominated by volcanic activity
the surrounding steep mountain ranges (especially from and rocks, they provide relatively few opportunities to
the east) into the Central Valley since the Mesozoic Era. study sedimentary rocks. However, there are some
The most recent unconsolidated Holocene Epoch sedi- basins that have been filling with sediment that includes
ments cover today’s valley floor. lake deposits and weathered and eroded volcanic mate-
rial. At Lava Beds National Monument, fossils of a
The Kettleman Hills (south of Coalinga) represent a mastodon and prehistoric camel were found in the lava
stretched anticlinal dome of Central Valley sedimentary tubes. Will the recent sedimentary deposits be buried by
rock folded up above the San Joaquin Valley floor. future ash and cinder deposits or lava flows as some of
Rocks of this young structural feature produce not only their ancestors were?
oil and gas, but many different kinds of Tertiary Period
fossils. Here, fossils from a shallow marine environment Sierra Nevada. In Chapter 2 we examined the meta-
are found in the younger Pliocene Epoch Etchegoin and morphosed, mostly Paleozoic Era roof pendants of the
San Joaquin Formations. Below, in the late Miocene Sierra Nevada, which lie on top of the granitic rocks, es-
Epoch Santa Margarita Formation, Ostrea titan giant oys- pecially in higher terrain. They represent the sedimen-
ter fossils up to 15 cm (6 inches) long are found. The still tary rocks that formed when a depositional environment
deeper and older Miocene Epoch Temblor Formation, ruled in the Sierra, but they were later squeezed and
which is also exposed in the nearby Coast Ranges, yields faulted on a plate boundary. Most of them have been
Desmostylus sea cow and Merychippus horse fossils. eroded away. In the northern Sierra Nevada, rock forma-
tions of the Silurian Shoo Fly complex include the cherts
Northwest California. In the Trinity Forest, lime- and limestones of an old ocean crust with accumulated
stones between Red Bluff and Humboldt Redwoods material along a plate boundary.
State Park on Hwy. 36 and near Hall City caves contain
some of the youngest Permian Period animal fossils in As expected, there are few deposits left in the Sierra
North America; they must have been dragged east to this Nevada from the mountain-building episode called the
location on an ocean plate because they match fossils in Nevadan orogeny. Today, scattered remnants of sedi-
the western Pacific. ments accumulated after the Nevadan orogeny and its
erosion are more common. Marine sediments and sand-
In the Klamaths, Cretaceous Period marine deposits stones with fossils from the late Cretaceous Period are
were accumulated when the area was slightly below sea found on top of basement rocks along the current bound-
level just after the Nevadan orogeny. Marine fossils in- ary between the Central Valley and the Sierra Nevada. In
clude ammonites and pelecypods in the Great Valley Se- some northern Sierra Nevada foothills, the younger
quence that were deposited before the Klamaths were Eocene Epoch Ione Formation consists of sand, clay, and
uplifted. Conglomerates of these formations are ex- coal beds that have been mined to produce glass, bricks,
posed south of Weaverville and in the eastern Klamaths. wax, and fuel.
Shallow-water marine fossils are found in the late Creta-

8 Appendix California Rocks and Minerals

Sedimentary Rocks in the Coast Ranges Appendix Figure A-5 Travertine has accumulated over the
Our search for sedimentary rocks continues in the Coast rocks at Nojoqui Falls in Santa Barbara County and plants are try-
Ranges. Here, the geology is so complex and the rock ing to grow over it. The deposits of calcium and magnesium car-
formations (dating from the Mesozoic Era to the Pleis- bonate leave this cliff looking like solid limestone, though the
tocene Epoch) are so diverse, we can only pick out a few underlying rock is not.
examples. For more detail, refer back to the section in
Chapter 2 on California geologic history, read articles in at Drakes Beach (Point Reyes) and sandstones at Moss
other publications, or take field trips to the Coast Ranges Beach. Additional fossils at other locations, such as near
region. Año Nuevo State Beach, also confirm that the coastline is
being lifted, exposing rocks once deposited below the
The Coast Ranges are dominated by mostly Ceno- sea.
zoic Era sedimentary rocks deposited on two types of
basement, one of which is the previously examined Sedimentary Formations in Southern
Franciscan melange. Mixed with the metamorphic rocks California Coastal Mountains and Valleys
of the melange are graywackes. They were deposited in In the Peninsular Ranges. You will notice late Creta-
chaotic currents and mudflows within deep marine ceous Period mainly marine sedimentary rocks in the
basins below steep volcanic highlands, all near the sub- Peninsular Ranges from the Santa Ana Mountains and
duction zone that existed during the Mesozoic Era. Thick scattered south into Baja. Among these, the mostly non-
deposits are exposed in numerous Coast Range locations marine Trabuco Conglomerate is common in the west-
along with the red cherts which contain single-celled ern Santa Ana Mountains. In contrast, marine sandstones
deep sea fossils (radiolarian) and deep sea limestones. and shales of the Rosario Formation contain huge am-
monites (flat, coiled mollusk fossils from the Mesozoic
Along the margin between the Coast Ranges and the Era). Exposures of the Rosario sandstones can be found
Great Central Valley are exposures of late Jurassic to late along the coast at La Jolla Cove, Point Loma, and points
Cretaceous Periods marine conglomerate, sandstone, south into Mexico.
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 ex-
tensively 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 north-
east 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 Penin-
sular 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 re-
treated for the last time toward the beginning of the
Pleistocene Epoch. More recent deposits continue to ac-
cumulate 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

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 ter-
races rise in the background.

Sedimentary rocks deposited after the Cretaceous Some of the most interesting sedimentary rock for-
Period in the Peninsular Ranges are widespread. Early mations in the Transverse Ranges have been radically
Tertiary Period rocks include the Paleocene Epoch non- lifted up and exposed by recent tectonic activity along
marine Silverado Formation in the northern Santa Ana and near the San Andreas Fault Zone. For instance, it
Mountains, where coal, clay, and mixed silicates used in might appear that the spectacularly dipping sandstones
glass making have been mined. Eocene Epoch deposits and conglomerates at three locations in and adjacent to
of rounded pebbles at the mouth of the ancient Poway the San Gabriel Mountains are similar: Vasquez Rocks
River became the Poway Formation in San Diego (exam- along Hwy. 14 in the San Gabriels, Devil’s Punchbowl
ined in the Cenozoic Era discussion in Chapter 2). The above Pearblossom, and Mormon Rocks in Cajon Pass. It
Miocene San Onofre breccia, with its chunks of broken turns out that the Devil’s Punchbowl and Mormon Rocks
schist, is found in spots along the southern coast. are both composed of Miocene Epoch deposits of the
Punchbowl Formation. Embedded land vertebrate fossils
More extensive are the rocks formed since the later are found at both sites; these two strikingly similar land-
Tertiary Period in the Peninsular Ranges. They include forms may have been separated by sliding action along
the nonmarine conglomerate, sandstone, and siltstone the San Andreas Fault Zone. The Vasquez Formation is
of the Pliocene Epoch Mount Eden and San Timoteo also nonmarine, but dates back to the Oligocene Epoch.
Canyon Formations. Abundant and impressive marine Interestingly, rock outcrops of the Diligencia Formation
fossils are easily recognized in the Pliocene Epoch San in the Orocopia Mountains of the Colorado Desert on
Diego Formation north of Mission Bay (see Fig. 2-1). the other side (east) of the San Andreas Fault match the
Some of the thickest sedimentary deposits are found in Vasquez in type and age.
the Imperial and Coachella Valleys, Elsinore trough,
Perris Plain, Los Angeles Basin, and offshore basins, as There are so many other beautiful exposures of sedi-
well as in the coastal basins to the north in the Trans- mentary rocks in the Transverse Ranges; this variety of
verse and Coast Ranges. The rocks in these basins were complexly folded and faulted formations makes exami-
weathered, eroded, and transported from mountain nation of even the most important ones an impossible
ranges and deposited and lithified in basins that exist to task in this book. One example is the drive inland
this day. through the Malibu Canyon gorge from Hwy. 1 (Pacific
Coast Highway) in the Santa Monica Mountains. You will
Sedimentary Formations in the Transverse Ranges. see (in order) more than 15 million years of earth history
In the Transverse Ranges, the Ventura Basin has more from the Oligocene to the Miocene Epochs in exposures
than 17,700 m (58,000 feet) of deposits from the Creta- of nonmarine pink and purple Sespe, marine Vaqueros,
ceous Period to today. These include what may be the Topanga, Conejo volcanics, and Modelo Shale (related to
thickest Pliocene Epoch deposits on Earth. This is all part the Monterey) Formations within 15 minutes as you drive
of a structural syncline that also includes the Soledad inland. The folded and faulted rock layers are now dip-
Basin, which has filled with nonmarine sediments. ping at steep angles. Another example is in the Santa Ynez

10 Appendix California Rocks and Minerals

Mountains north of Santa Barbara. Numerous rock forma- Farther east, in Rainbow Basin north of Barstow, are
tions, from the Jurassic and Cretaceous Periods’ Francis- the colorful Miocene Epoch sedimentary rocks of the
can melange to the late Pleistocene Epoch Santa Barbara Barstow Formation. The fossil pollen, rodent, tortoise,
Formation, have been contorted and lifted upward. dog, cat, rabbit, pronghorn, horse, camel, rhinoceros,
and saber-toothed cat suggest that the Miocene was wet-
Sedimentary Rocks in Exposed Landscapes ter with summer rains.
of Transmontane California
Basin and Range. In the Basin and Range, there are Continuing east, across the Mojave, in the Provi-
many other interesting sedimentary rock formations in dence Mountains, are the limestones of the Permian Bird
addition to the oldest ones, which we have previously Spring Formation. Mitchell Caverns were still active dur-
examined. Besides the folded and faulted marine Cam- ing wetter periods as late as the Pleistocene Epoch,
brian Period limestones and dolomites exposed in Titus when water combined with limestone to produce car-
Canyon in Death Valley and surrounding areas, you will bonic acid that dissolved the rock and formed the cav-
find the Oligocene Epoch Titus Canyon beds, which are erns. Today, Mitchell Caverns are dry, but are open to
also related to the Artist’s Drive Formation. Fossils of the tour. Geologists recognize the rock record near Moun-
great, horse-like herbivore Titanothere, rhinoceros, and tain Pass to the north to be far more extensive than at
camel suggest that a lush savanna existed here during popular Mitchell Caverns.
the Oligocene Epoch. Fossils found in Pliocene Epoch
rocks at Furnace Creek include leaves, diatoms, and Nearby, brachiopods and trilobites are found in the
animal footprints, while in the Coso Mountains, late much older Cambrian Period Latham shale. This forma-
Pliocene Epoch mammal fossils include dogs, peccaries, tion is made of red, green, and gray mudstones. For
camel, horses, and mastodons. The area apparently be- many years, fossil collectors have converged on the Mar-
came drier (except for occasional wet periods during the ble Mountains east of Amboy, where the relatively acces-
Pleistocene Epoch Ice Ages) as the Sierra Nevada was sible and remarkable crustacean trilobite, Fremontia fre-
lifted higher to the west, blocking moisture from the Pa- monti, index fossils of the Paleozoic Era, are up to 20 cm
cific, as it does today. (8 inches) long.

Of course, the sedimentary rock record in the Basin Sedimentary Formations in the Southern Deserts.
and Range is far more extensive. (Some of these expo- Farther southeast, toward the Colorado Desert, fossil
sures were reviewed in the section on geologic history in wood has been found in Cretaceous Period sediments of
Chapter 2.) You will find more details in specific geology the McCoy and Palen mountains.
publications.
The Colorado Desert contains thick accumulations
More recently, in the Basin and Range and into the of mostly younger Cenozoic Era rocks. The oldest in-
Mojave Desert since the Pleistocene Epoch, numerous clude Eocene Epoch marine deposits in the Orocopia
inland basins have been filling with silt and clay during Mountains (east of the Coachella Valley) and nonmarine
wetter glacial periods and filling with salts during drier rocks in the Palo Verde Mountains (southwest of
periods such as today’s. Some of these relatively young Blythe). Slightly younger Oligocene Epoch continental
sediments, such as the Funeral Formation in Death Val- deposits of the Diligencia Formation are also found in
ley, have already been folded and faulted. the Orocopias.

Early Pleistocene Epoch fossils of horses, camel, Post-Oligocene Epoch sedimentary rocks are com-
deer, pronghorns, hog-like tapirs, and rabbits have been mon and are many thousands of meters thick. They are
recovered along the shores of ancient Lake Manix in the exposed in the western Imperial Valley (including those
Mojave. Many different kinds of salts (including chlo- in Anza Borrego State Park). They also lie below the sur-
rides, carbonates, potassium, and borates) are being face of the valley into Mexico as well as below the
mined from the saline deposits in these mostly dry lake Coachella Valley. The majority of these sedimentary
beds in today’s Basin and Range and Mojave. rocks are nonmarine, except when the sea encroached
in the Miocene Epoch, depositing the brown oyster
Mojave Desert Sedimentary Formations. Moving beds of the Split Mountain Formation and the Pliocene
toward the Mojave, in Red Rock Canyon, the lighter Imperial Formation. After the sea retreated (probably
sandstone layers and the darker lava flows often found blocked by the building Colorado River delta), the
above them are commonly oxidized red. The nonmarine Pliocene Epoch Canebrake Conglomerate and the finer-
sedimentary rocks of this Miocene Epoch Ricardo Group grained Palm Springs Formation were deposited. The
contain fossils of petrified wood, rodents, primitive Palm Springs Formation (with some petrified wood) can
horses, camel, antelope, mastodons, rhinoceros, and be seen west of the Salton Sea, where it is folded and
saber-toothed cats that lived in a higher-rainfall savanna being eroded. Still younger Lake Cahuilla deposits
perhaps when lower hills to the west allowed moist (since the Pleistocene Epoch) were laid flat on the sur-
ocean air masses to penetrate farther inland. face 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 11

salinity of the late Pleistocene Epoch lake, rather than in the Miocene Epoch, many of the younger deposits are
ocean water. somewhat similar to those in the Imperial Valley.

Basement rocks of the Valecito and Fish Creek Though we have come to the end of our more thor-
Mountains on the western edge of the Imperial Valley re- ough survey of ancient rocks and the stories they tell, we
semble those of the Peninsular Range to the west. How- have only started here what could be volumes and life-
ever, overlying sedimentary rocks resemble those of the times of research on California’s earth history. It is time
Colorado Desert to the east. Consequently, following de- to move ahead to examine 21st Century processes and
position of the Anza Formation during a dry period in landscapes.

Laguna Rocks by Patty Kellner