Geologic Map of the San Bernardino North 7.5' quadrangle, San Bernardino County, California

This data set maps and describes the geology of the San Bernardino North 7.5' quadrangle,
San Bernardino County, California. Created using Environmental Systems Research
Institute's ARC/INFO software, the data base consists of the following items: (1) a map
coverage containing geologic contacts and units, (2) attribute tables for geologic units
(polygons), contacts (arcs), and site-specific data (points). In addition, the data set
includes the following graphic and text products: (1) A PostScript graphic plot-file
containing the geologic map, topography, cultural data, a Correlation of Map Units (CMU)
diagram, a Description of Map Units (DMU), an index map, a regional geologic and
structure map, and a key for point and line symbols; (2) PDF files of this Readme
(including the metadata file as an appendix), Description of Map Units (DMU), and the
graphic produced by the PostScript plot file.
The geologic map covers a part of the southwestern San Bernardino Mountains and the
northwestern San Bernardino basin. Granitic and metamorphic rocks underlie most of the
mountain area, and a complex array of Quaternary deposits fill the basin. These two
areas are separated by strands of the seismically active San Andreas Fault. Bedrock
units in the San Bernardino Mountains are dominated by large Cretaceous and Jurassic
granitic bodies, ranging in composition from monzogranite to monzodiorite, and include
lesser Triassic monzonite. The younger of these granitic rocks intrude a complex
assemblage of gneiss, marble, and granitic rock of probable early Mesozoic age; the
relationship between these metemorphic rocks and the Triassic rocks is unknown. Spanning
the Pleistocene in age, large and small alluvial bodies emerge from the San Bernardino
Mountains, and and fill the San Bernardino basin. In the southwestern part of the
quadrangle, Cajon Wash carries sediments from both the San Bernardino and San Gabriel
Mountains, and Lytle Creek heads in the eastern San Gabriel Mountains. Limited bedrock
areas showing through the Quaternary sediments of the basin consist exclusively of
Mesozoic Pelona Schist locally intruded by Tertairy dikes. Youthful-appearing fault
scarps discontinuously mark the traces of the San Andreas Fault along the southern edge
of the San Bernardino Mountains. Unnamed Tertiary sedimentary rocks are bounded by two
strands of the fault between Badger Canyon and the east edge of the quadrangle. Young
and old high-angle faults cut bedrock units within the San Bernardino Mountains, and the
buried, seismically active San Jacinto Fault traverses the southwestern part of the
quadrangle.
The geologic map database contains original U.S. Geological Survey data generated by
detailed field observation and by interpretation of aerial photographs. This digital
Open-File map superceeds an older analog Open-File map of the quadrangle, and includes
extensive new data on the Quaternary deposits, and revises some fault and bedrock
distribution within the San Bernardino Mountains. The digital map was compiled on a
base-stable cronoflex copy of the San Bernardino North 7.5' topographic base and then
scribed. This scribe guide was used to make a 0.007 mil blackline clear-film, which was
scanned at 1200 DPI by Optronics Specialty Company, Northridge, California; minor
hand-digitized additions were made at the USGS. Lines, points, and polygonswere
subsequently edited at the USGS using standard ARC/INFO commands. Digitizing and editing
artifacts significant enough to display at a scale of 1:24,000 were corrected. Within
the database, geologic contacts are represented as lines (arcs), geologic units as
polygons, and site-specific data as points. Polygon, arc, and point attribute tables
(.pat, .aat, and .pat, respectively) uniquely identify each geologic datum.