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Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 1 of 6
CGS Cgs History
California Geological Survey - Geologic Maps of California
History of Geologic Maps of California
Introduction
The First Attempts
Preliminary Mineralogical and Geological Map of the State of California - 1891
Geological Map of the State of California - 1916
Geological Map of California - 1938
Geological Atlas of California - 1958-1969
Fault Map of California - 1975 and Geologic Map of California - 1977
Fault Activity Map of California and Adjacent Areas - 1994
NEW Geologic Map of California and Fault Activity Map of California - 2010
References
HISTORY OF GEOLOGIC MAPS OF CALIFORNIA
Modified from Charles W. Jennings (1985) California Division of Mines and Geology Bulletin 201
INTRODUCTION
More than 100 years have passed since the preparation in 1891 of the 1:750,000 scale Preliminary Mineralogical and
Geological Map of the State of California. The 2010 editions of the Geologic Map of California and the Fault Activity Map
of California are the latest in the series of statewide geologic maps published by the State, and represent a step forward in
the mapping and understanding of California's diverse geology.
THE FIRST ATTEMPTS
Geologic mapping in California began about 180 years ago. The first geologic mapping in the State, done by Lieutenant
Edward Belcher, a British naval officer, was a remarkably accurate geologic map of the Port of San Francisco. Although
Belcher did the surveying for the map in 1826, it was not published until 1839.
Landmarks in the publication of early geologic maps of the entire state begin with the
hand-tinted geologic map of California made by W.P. Blake in 1853 and published in
Volume V of the War Department's "Report of Explorations in California for Railroad
Routes" (Blake, 1857). Utilizing nine geologic units, this 41 X 56 cm (16 x 22 inch)
map was the first published geologic map that specifically and exclusively pertained
to California. This map was followed by the first color-lithographed map of the State,
made in 1867 and published four years later in Paris as part of a report of a French
scientific mission to Mexico and the "ancient Mexican possessions of the north"
(Guillemin-Tarayre, 1871). The geology is portrayed in a most impressive manner by
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 2 of 6
ten geologic units, and the geologic interpretation is much improved over Blake's
map.
The second color-lithographed geologic map of
California was prepared by another Frenchman,
Jules Marcou (1883), and published in the
"Bulletin of the Geological Society of France."
The nine geologic units shown were largely
based on Marcou's observations while working
with the Pacific Railroad Survey in 1854 and
the Wheeler Survey West of the 100th Meridian
in 1875, both Federal surveys. The map was accompanied by a report on the
geology of California.
These early, page-size geologic maps of the State were superseded in 1891 by the
first relatively large-scale statewide geologic map.
PRELIMINARY MINERALOGICAL AND
GEOLOGICAL MAP OF THE STATE OF CALIFORNIA -
1891
The Preliminary Mineralogical and Geological Map of the State of California, at a
scale of 1:750,000 (12 miles equals one inch), was prepared and published in four
sections by the California State Mining Bureau. Beginning with this map, the
responsibility for preparing and publishing succeeding editions of relatively largescale
geologic maps of California has remained with the State. Only eight geologic
units were depicted on the 1891 map; however, their general relations are better
shown than on all the previous geologic maps. Special emphasis was given to
mineral resources. Such units as auriferous gravel, auriferous slate, and limestone
are portrayed, and the locations of known mineral deposits are shown. The map was
issued by the State Mineralogist, William Ireland, but the map compiler is not
credited on the map. However, the 10th Annual Report of the State Mineralogist
(Ireland, 1890, p. 21) states that the topographical and other work on the Preliminary
Geological and Mineralogical Map was "being executed by Mr. Julius Henkenius, who
received aid in the geological and mineralogical locatings from the Field Assistants."
GEOLOGICAL MAP OF THE STATE OF CALIFORNIA - 1916
Twenty-five years after the 1891 Preliminary Mineralogical and Geological Map of the
State of California, another 1:750,000 scale state geologic map was published by the
California State Mining Bureau. This map was prepared by J. P. Smith, Professor of
Paleontology at Stanford University, and was accompanied by a brief bulletin
describing the geology (Smith, 1916).
The map legend lists 21 geologic units. Although Professor Smith's bulletin clearly
explains that certain areas of California were still unmapped, his map, unlike the
earlier 1891 edition, shows the entire area of the state covered by colors
representing geologic units with delineated contacts. This, unfortunately, leaves the
map-user without any clue as to what is known and what has merely been projected.
The absence of geologic faults on this map is also somewhat puzzling. Although
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 3 of 6
faults were by this time widely recognized and mapped—as, for example, in the atlas
accompanying the "State Earthquake Investigation Commission" report on the
disastrous 1906 San Francisco earthquake— not even the San Andreas fault is
shown on the 1916 geologic map of California.
The 41-page bulletin, "The Geologic Formations of California," which accompanies
the 1916 Geological Map of California, consists of an expanded legend for the reconnaissance geologic map; a
description of the geologic record of California; a description of the "rock-forming agencies of California" wherein the
formation of various rock types are briefly discussed; a listing of the sources of data for the geologic map; and, lastly, a
listing of the formations included in each geologic unit shown on the map.
GEOLOGIC MAP OF CALIFORNIA - 1938
Twenty-two years later, another milestone in California geologic maps appeared in
the form of a 1:500,000 scale map published in six sections by the California Division
of Mines. This map was prepared by Olaf P. Jenkins, Chief Geologist of the Division
of Mines, and represented nine years of careful geological research.
Much larger in scale than any preceding geologic maps of the state, the 1938 map
shows much more detail than the earlier maps. The geologic boundaries of the 81
units depicted were drawn with greater precision than before. Care was taken to
follow the source data faithfully. In areas where no geologic maps were available, or
where previous maps were too general in nature or at a scale much smaller than the
base map, the area was purposely left blank. This portrayal of the geology of the
state showed that about 25 percent of the state was unmapped. The largest
unmapped areas at that time were in the Klamath Mountains, the northern Coast
Ranges, the southern Sierra Nevada, and the desert areas of southeastern
California. For the first time, faults were shown on an official geologic map of
California.
GEOLOGIC ATLAS OF CALIFORNIA - 1958-1969
The ground work for the Geologic Atlas of California began in 1951, after the popular 1938 edition went out of print. Great
demand prompted OIaf P. Jenkins to set up a program for preparing a new edition of the state map that would incorporate
the large amount of new geologic data collected since the earlier map was compiled (Jahns, 1961). The first map to be
completed, the Death Valley Sheet, was published in 1958. At the outset of the project, the difficulties associated with
compiling a geologic map of the entire state, with all of its geologic diversity and complexity, were for the most part
recognized, but it was hoped that the abundance of new data on hand would make overcoming these difficulties fairly
simple. Actually, the inconsistent nature of these new data made the task more difficult. The source maps for some areas
of the state were excellent; in other areas they were very poor, incomplete, or simply nonexistent. Frequently, welldescribed
areas were adjacent to poorly understood, incompletely mapped, or totally unmapped areas. Often, too, there
was no continuity between maps for adjacent areas because of differences in geologic interpretation. Thus, the
compilation could easily have resulted in a patchwork of data. Fortunately, perhaps, the scale of the atlas made it possible
to ignore a multitude of discrepancies. Numerous problems, however, had to be resolved in the field, and almost all blank
areas in the state were filled in by reconnaissance mapping by the California Division of Mines and Geology.
Nevertheless, in a few areas of complex geology, or where particularly detailed work was surrounded by less detailed
mapping, white areas were left around the more detailed area to preserve as much information as possible. Blank areas
marked on the atlas sheets as "unmapped" or "incomplete" amount to a very small percentage of some map sheets and
are absent entirely from others.
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 4 of 6
FAULT MAP OF CALIFORNIA - 1975 and GEOLOGIC MAP OF CALIFORNIA -
1977
Even before the 1:250,000 scale Geologic Atlas of California was completed, it was
recognized that a smaller-scale map of the state, one that would present an overview
of the geology of the entire state, was highly desirable. It had become apparent that
the individual atlas sheets, as useful as they were for field and office purposes, were
not satisfactory for evaluation of statewide geologic and structural trends. Therefore,
late in 1965, plans were made for a smaller scale map of California (Jennings, 1965).
After consideration of various scales, 1:750,000 was chosen because this scale is
sufficiently large to show a significant amount of geologic information and also is
consistent with the first two official geologic maps of California published by the State
in 1891 and 1916. At the beginning of the project, it was decided that a multipurpose
map of the state would be most desirable. In addition to the geology, the map would
emphasize recently active faults, recent volcanic rocks and volcanoes, thermal
springs, offshore structures, and major fold axes. Therefore, all these data were
plotted on the work sheets; but it became evident that, for publication purposes, it
would be much more effective to separate some of this information and make two
maps rather than one. Pursuing this concept, it was planned to present a series of
maps at the same 1:750,000 scale illustrating various geologic and geophysical
parameters that can conveniently be studied individually or in relation to one another.
Thus, the first map in this series was the Fault Map of California with Locations of
Volcanoes, Thermal Springs and Thermal Wells, Geologic Data Map No. 1 (1975). This was followed by The Geologic
Map of California, Geologic Data Map No. 2 (1977).
FAULT ACTIVITY MAP OF CALIFORNIA AND ADJACENT AREAS - 1994
New data concerning the age and activity of California faults necessitated a revision to the 1975 Fault Map of California. In
1990 Charles W. Jennings began compilation of new data and classification of Quaternary faults into a five age
subdivisions (historic, Holocene, late Quaternary, undivided Quaternary, pre-Quaternary. This map took advantage of the
extensive mapping of active faults for the Alquist-Priolo Earthquake Fault Zoning act. Age designations were also
enhanced by the extensive work for Alquist-Priolo zoning: faults must be shown to have Holocene surface displacement to
be included in “A-P” zones. Any fault evaluated and included in such a zone can be shown on the Fault Activity Map as
Historic or Holocene. In addition, a more extensive explanatory text was keyed to numbers for many faults to allow
geologists to look up the source of the original fault mapping and reason for the age classification.
GEOLOGIC MAP OF CALIFORNIA AND FAULT ACTIVITY MAP OF
CALIFORNIA - 2010
New editions of the Geologic Map of California and the Fault Activity Map of
California have been prepared in recognition of the California Geological
Survey’s 150th Anniversary. Both are all-digital products built on the original
compilations of C.W. Jennings published in 1977 and 1994. The digital
version of the Jennings (1977) geologic map was released in 2000 (Saucedo
and others, 2000) and serves as the base digital geologic data layer for the
new geologic map with additions and modifications described below. The
new editions remove, as much as possible, distortions in the previous digital
versions that were inherited from the composite base map of the original
printed versions.
The 2010 Fault Activity Map of
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 5 of 6
California presents a much
more detailed depiction of
faults in California than
previous versions. Faults
active in the Quaternary (2.6
Ma) were digitized from their
original sources in order to
preserve as much of the
original detail as possible.
Similar to the 1994 Fault
Activity Map, the new map is
accompanied by an extensive
listing of the sources of fault maps and age determinations. In the 2010 Fault
Activity Map, the faults are depicted in as much detail as possible at the
scale of the map. The 2010 Geologic Map of California includes
modifications to the fault traces for consistency with the 2010 edition of the
Fault Activity Map of California. Due to the complexity of the onshore
geologic data depicted on the Geologic Map of California, it was not
technically feasible to incorporate the detailed fault data in its entirety. A
comparison of the datasets was made and numerous fault traces were
replaced with simplified fault traces based on the new fault map. In addition,
several faults that have been mapped since the compilation of the original
geologic map were also identified and added, and traces no longer believed
to be faults were removed. In general, the Quaternary faults shown in the onshore region of this map are intended to be a
simplified representation of the faults depicted on the Fault Activity Map. Although the modification of faults in some areas
necessitated the modification of bedrock geology adjacent to those faults, the distribution of bedrock geologic units
compiled by Jennings (1977) has otherwise not been modified.
Jennings (1977) simplified the depiction of Quaternary geologic units on the original map. As he put it: “various surficial
deposits of Quaternary age are lumped into the unit ‘Q’.” Since Jennings’ work, subdivisions of these deposits have been
found to have very different potential for liquefaction and for amplification of seismic shaking. Relative age of Quaternary
alluvial fan deposits have also been found to correlate with potential for flooding. Since these units are important for
evaluation of geologic hazards, the 2010 update of the geologic map includes a subdivision of Jennings “Q” into younger
alluvium “Q” and older alluvium “Qoa”. In general, younger alluvium was deposited in Holocene time and represents the
modern deposition in flood plains and on alluvial fans. Older alluvium is generally of Pleistocene age and represents
depositional systems that are not currently active.
References
Blake, W.P., 1857, Geological report, in Williamson, R.S., Report of explorations in California for railroad routes (from
Mississippi River to the Pacific Ocean) 1853 [Senate Ex. Doc. no. 78], v. 5, pt. 2, 310p.
Guillemin-Tarayre, Edmond, 1871, Description des anciennes possessions mexicaines du nord: Paris, France, Mission
Sci. au Mexique et dans I'Amerique Centrale, Geologie, pt. 2, 216 p.
Ireland, William, Jr., 1890, Report of the State Mineralogist: California State Mining Bureau tenth annual report, 983 p.
Jahns, R.H., 1961, Geologic Map of California, Olaf P. Jenkins edition: Economic Geology, v. 56, no. 6, p. 1154-1156.
Jennings, C.W., 1965, 1:750,000 geologic map project: Memo to lan Campbell, Chief California Division Mines and
Geology, December 30, 1965, 8 p.
Jennings, C.W., 1975, Fault map of California with location of volcanoes, thermal springs, and thermal wells:California
Division of Mines and Geology Geologic Data Map No. 1, scale 1:750,000.
Jennings, C.W., 1977, Geologic Map of California:California Division of Mines and Geology Geologic Data Map No. 2,
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 6 of 6
scale 1:750,000.
Marcou, Jules, 1883, Note sur la geologie de la Californie: Soc. Geol. France Bull., 3d ser., v. 11, p. 407-435.
Smith, J.P., 1916, The geologic formations of California with reconnaissance geologic map: California State Mining Bureau
Bulletin No. 72, 47 p.
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 1 of 6
CGS Cgs History
California Geological Survey - Geologic Maps of California
History of Geologic Maps of California
Introduction
The First Attempts
Preliminary Mineralogical and Geological Map of the State of California - 1891
Geological Map of the State of California - 1916
Geological Map of California - 1938
Geological Atlas of California - 1958-1969
Fault Map of California - 1975 and Geologic Map of California - 1977
Fault Activity Map of California and Adjacent Areas - 1994
NEW Geologic Map of California and Fault Activity Map of California - 2010
References
HISTORY OF GEOLOGIC MAPS OF CALIFORNIA
Modified from Charles W. Jennings (1985) California Division of Mines and Geology Bulletin 201
INTRODUCTION
More than 100 years have passed since the preparation in 1891 of the 1:750,000 scale Preliminary Mineralogical and
Geological Map of the State of California. The 2010 editions of the Geologic Map of California and the Fault Activity Map
of California are the latest in the series of statewide geologic maps published by the State, and represent a step forward in
the mapping and understanding of California's diverse geology.
THE FIRST ATTEMPTS
Geologic mapping in California began about 180 years ago. The first geologic mapping in the State, done by Lieutenant
Edward Belcher, a British naval officer, was a remarkably accurate geologic map of the Port of San Francisco. Although
Belcher did the surveying for the map in 1826, it was not published until 1839.
Landmarks in the publication of early geologic maps of the entire state begin with the
hand-tinted geologic map of California made by W.P. Blake in 1853 and published in
Volume V of the War Department's "Report of Explorations in California for Railroad
Routes" (Blake, 1857). Utilizing nine geologic units, this 41 X 56 cm (16 x 22 inch)
map was the first published geologic map that specifically and exclusively pertained
to California. This map was followed by the first color-lithographed map of the State,
made in 1867 and published four years later in Paris as part of a report of a French
scientific mission to Mexico and the "ancient Mexican possessions of the north"
(Guillemin-Tarayre, 1871). The geology is portrayed in a most impressive manner by
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 2 of 6
ten geologic units, and the geologic interpretation is much improved over Blake's
map.
The second color-lithographed geologic map of
California was prepared by another Frenchman,
Jules Marcou (1883), and published in the
"Bulletin of the Geological Society of France."
The nine geologic units shown were largely
based on Marcou's observations while working
with the Pacific Railroad Survey in 1854 and
the Wheeler Survey West of the 100th Meridian
in 1875, both Federal surveys. The map was accompanied by a report on the
geology of California.
These early, page-size geologic maps of the State were superseded in 1891 by the
first relatively large-scale statewide geologic map.
PRELIMINARY MINERALOGICAL AND
GEOLOGICAL MAP OF THE STATE OF CALIFORNIA -
1891
The Preliminary Mineralogical and Geological Map of the State of California, at a
scale of 1:750,000 (12 miles equals one inch), was prepared and published in four
sections by the California State Mining Bureau. Beginning with this map, the
responsibility for preparing and publishing succeeding editions of relatively largescale
geologic maps of California has remained with the State. Only eight geologic
units were depicted on the 1891 map; however, their general relations are better
shown than on all the previous geologic maps. Special emphasis was given to
mineral resources. Such units as auriferous gravel, auriferous slate, and limestone
are portrayed, and the locations of known mineral deposits are shown. The map was
issued by the State Mineralogist, William Ireland, but the map compiler is not
credited on the map. However, the 10th Annual Report of the State Mineralogist
(Ireland, 1890, p. 21) states that the topographical and other work on the Preliminary
Geological and Mineralogical Map was "being executed by Mr. Julius Henkenius, who
received aid in the geological and mineralogical locatings from the Field Assistants."
GEOLOGICAL MAP OF THE STATE OF CALIFORNIA - 1916
Twenty-five years after the 1891 Preliminary Mineralogical and Geological Map of the
State of California, another 1:750,000 scale state geologic map was published by the
California State Mining Bureau. This map was prepared by J. P. Smith, Professor of
Paleontology at Stanford University, and was accompanied by a brief bulletin
describing the geology (Smith, 1916).
The map legend lists 21 geologic units. Although Professor Smith's bulletin clearly
explains that certain areas of California were still unmapped, his map, unlike the
earlier 1891 edition, shows the entire area of the state covered by colors
representing geologic units with delineated contacts. This, unfortunately, leaves the
map-user without any clue as to what is known and what has merely been projected.
The absence of geologic faults on this map is also somewhat puzzling. Although
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 3 of 6
faults were by this time widely recognized and mapped—as, for example, in the atlas
accompanying the "State Earthquake Investigation Commission" report on the
disastrous 1906 San Francisco earthquake— not even the San Andreas fault is
shown on the 1916 geologic map of California.
The 41-page bulletin, "The Geologic Formations of California," which accompanies
the 1916 Geological Map of California, consists of an expanded legend for the reconnaissance geologic map; a
description of the geologic record of California; a description of the "rock-forming agencies of California" wherein the
formation of various rock types are briefly discussed; a listing of the sources of data for the geologic map; and, lastly, a
listing of the formations included in each geologic unit shown on the map.
GEOLOGIC MAP OF CALIFORNIA - 1938
Twenty-two years later, another milestone in California geologic maps appeared in
the form of a 1:500,000 scale map published in six sections by the California Division
of Mines. This map was prepared by Olaf P. Jenkins, Chief Geologist of the Division
of Mines, and represented nine years of careful geological research.
Much larger in scale than any preceding geologic maps of the state, the 1938 map
shows much more detail than the earlier maps. The geologic boundaries of the 81
units depicted were drawn with greater precision than before. Care was taken to
follow the source data faithfully. In areas where no geologic maps were available, or
where previous maps were too general in nature or at a scale much smaller than the
base map, the area was purposely left blank. This portrayal of the geology of the
state showed that about 25 percent of the state was unmapped. The largest
unmapped areas at that time were in the Klamath Mountains, the northern Coast
Ranges, the southern Sierra Nevada, and the desert areas of southeastern
California. For the first time, faults were shown on an official geologic map of
California.
GEOLOGIC ATLAS OF CALIFORNIA - 1958-1969
The ground work for the Geologic Atlas of California began in 1951, after the popular 1938 edition went out of print. Great
demand prompted OIaf P. Jenkins to set up a program for preparing a new edition of the state map that would incorporate
the large amount of new geologic data collected since the earlier map was compiled (Jahns, 1961). The first map to be
completed, the Death Valley Sheet, was published in 1958. At the outset of the project, the difficulties associated with
compiling a geologic map of the entire state, with all of its geologic diversity and complexity, were for the most part
recognized, but it was hoped that the abundance of new data on hand would make overcoming these difficulties fairly
simple. Actually, the inconsistent nature of these new data made the task more difficult. The source maps for some areas
of the state were excellent; in other areas they were very poor, incomplete, or simply nonexistent. Frequently, welldescribed
areas were adjacent to poorly understood, incompletely mapped, or totally unmapped areas. Often, too, there
was no continuity between maps for adjacent areas because of differences in geologic interpretation. Thus, the
compilation could easily have resulted in a patchwork of data. Fortunately, perhaps, the scale of the atlas made it possible
to ignore a multitude of discrepancies. Numerous problems, however, had to be resolved in the field, and almost all blank
areas in the state were filled in by reconnaissance mapping by the California Division of Mines and Geology.
Nevertheless, in a few areas of complex geology, or where particularly detailed work was surrounded by less detailed
mapping, white areas were left around the more detailed area to preserve as much information as possible. Blank areas
marked on the atlas sheets as "unmapped" or "incomplete" amount to a very small percentage of some map sheets and
are absent entirely from others.
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 4 of 6
FAULT MAP OF CALIFORNIA - 1975 and GEOLOGIC MAP OF CALIFORNIA -
1977
Even before the 1:250,000 scale Geologic Atlas of California was completed, it was
recognized that a smaller-scale map of the state, one that would present an overview
of the geology of the entire state, was highly desirable. It had become apparent that
the individual atlas sheets, as useful as they were for field and office purposes, were
not satisfactory for evaluation of statewide geologic and structural trends. Therefore,
late in 1965, plans were made for a smaller scale map of California (Jennings, 1965).
After consideration of various scales, 1:750,000 was chosen because this scale is
sufficiently large to show a significant amount of geologic information and also is
consistent with the first two official geologic maps of California published by the State
in 1891 and 1916. At the beginning of the project, it was decided that a multipurpose
map of the state would be most desirable. In addition to the geology, the map would
emphasize recently active faults, recent volcanic rocks and volcanoes, thermal
springs, offshore structures, and major fold axes. Therefore, all these data were
plotted on the work sheets; but it became evident that, for publication purposes, it
would be much more effective to separate some of this information and make two
maps rather than one. Pursuing this concept, it was planned to present a series of
maps at the same 1:750,000 scale illustrating various geologic and geophysical
parameters that can conveniently be studied individually or in relation to one another.
Thus, the first map in this series was the Fault Map of California with Locations of
Volcanoes, Thermal Springs and Thermal Wells, Geologic Data Map No. 1 (1975). This was followed by The Geologic
Map of California, Geologic Data Map No. 2 (1977).
FAULT ACTIVITY MAP OF CALIFORNIA AND ADJACENT AREAS - 1994
New data concerning the age and activity of California faults necessitated a revision to the 1975 Fault Map of California. In
1990 Charles W. Jennings began compilation of new data and classification of Quaternary faults into a five age
subdivisions (historic, Holocene, late Quaternary, undivided Quaternary, pre-Quaternary. This map took advantage of the
extensive mapping of active faults for the Alquist-Priolo Earthquake Fault Zoning act. Age designations were also
enhanced by the extensive work for Alquist-Priolo zoning: faults must be shown to have Holocene surface displacement to
be included in “A-P” zones. Any fault evaluated and included in such a zone can be shown on the Fault Activity Map as
Historic or Holocene. In addition, a more extensive explanatory text was keyed to numbers for many faults to allow
geologists to look up the source of the original fault mapping and reason for the age classification.
GEOLOGIC MAP OF CALIFORNIA AND FAULT ACTIVITY MAP OF
CALIFORNIA - 2010
New editions of the Geologic Map of California and the Fault Activity Map of
California have been prepared in recognition of the California Geological
Survey’s 150th Anniversary. Both are all-digital products built on the original
compilations of C.W. Jennings published in 1977 and 1994. The digital
version of the Jennings (1977) geologic map was released in 2000 (Saucedo
and others, 2000) and serves as the base digital geologic data layer for the
new geologic map with additions and modifications described below. The
new editions remove, as much as possible, distortions in the previous digital
versions that were inherited from the composite base map of the original
printed versions.
The 2010 Fault Activity Map of
California Geological Survey - Geologic Maps of California 2/15/11 2:26 PM
www.conservation.ca.gov/cgs/cgs_history/Pages/geologicmaps.aspx#1994 Page 5 of 6
California presents a much
more detailed depiction of
faults in California than
previous versions. Faults
active in the Quaternary (2.6
Ma) were digitized from their
original sources in order to
preserve as much of the
original detail as possible.
Similar to the 1994 Fault
Activity Map, the new map is
accompanied by an extensive
listing of the sources of fault maps and age determinations. In the 2010 Fault
Activity Map, the faults are depicted in as much detail as possible at the
scale of the map. The 2010 Geologic Map of California includes
modifications to the fault traces for consistency with the 2010 edition of the
Fault Activity Map of California. Due to the complexity of the onshore
geologic data depicted on the Geologic Map of California, it was not
technically feasible to incorporate the detailed fault data in its entirety. A
comparison of the datasets was made and numerous fault traces were
replaced with simplified fault traces based on the new fault map. In addition,
several faults that have been mapped since the compilation of the original
geologic map were also identified and added, and traces no longer believed
to be faults were removed. In general, the Quaternary faults shown in the onshore region of this map are intended to be a
simplified representation of the faults depicted on the Fault Activity Map. Although the modification of faults in some areas
necessitated the modification of bedrock geology adjacent to those faults, the distribution of bedrock geologic units
compiled by Jennings (1977) has otherwise not been modified.
Jennings (1977) simplified the depiction of Quaternary geologic units on the original map. As he put it: “various surficial
deposits of Quaternary age are lumped into the unit ‘Q’.” Since Jennings’ work, subdivisions of these deposits have been
found to have very different potential for liquefaction and for amplification of seismic shaking. Relative age of Quaternary
alluvial fan deposits have also been found to correlate with potential for flooding. Since these units are important for
evaluation of geologic hazards, the 2010 update of the geologic map includes a subdivision of Jennings “Q” into younger
alluvium “Q” and older alluvium “Qoa”. In general, younger alluvium was deposited in Holocene time and represents the
modern deposition in flood plains and on alluvial fans. Older alluvium is generally of Pleistocene age and represents
depositional systems that are not currently active.
References
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Mississippi River to the Pacific Ocean) 1853 [Senate Ex. Doc. no. 78], v. 5, pt. 2, 310p.
Guillemin-Tarayre, Edmond, 1871, Description des anciennes possessions mexicaines du nord: Paris, France, Mission
Sci. au Mexique et dans I'Amerique Centrale, Geologie, pt. 2, 216 p.
Ireland, William, Jr., 1890, Report of the State Mineralogist: California State Mining Bureau tenth annual report, 983 p.
Jahns, R.H., 1961, Geologic Map of California, Olaf P. Jenkins edition: Economic Geology, v. 56, no. 6, p. 1154-1156.
Jennings, C.W., 1965, 1:750,000 geologic map project: Memo to lan Campbell, Chief California Division Mines and
Geology, December 30, 1965, 8 p.
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Division of Mines and Geology Geologic Data Map No. 1, scale 1:750,000.
Jennings, C.W., 1977, Geologic Map of California:California Division of Mines and Geology Geologic Data Map No. 2,
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scale 1:750,000.
Marcou, Jules, 1883, Note sur la geologie de la Californie: Soc. Geol. France Bull., 3d ser., v. 11, p. 407-435.
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