Notes to users:

This README-SH.TXT file is meant to give the user a quick overview of
the files used to create the Geologic Map of the Conterminous
United States.  For more complete and detailed information please
refer to the included metadata file    usgeomet8.doc or usgeomet8.txt

If you have any questions about these coverages please contact
Chuck Bush USGS Denver (303)236-4723 or
Nancy Shock USGS Denver (303)236-7684

Coverages:

calderl.shp
Outlines of major calderas and impact structures.
        Outlines of major calderas and impact structures.  A caldera is a large,        generally circular depression at the summit of a volcano formed when
        magma is withdrawn or erupted from a shallow underground magma
        reservoir.  An impact structure is a crater formed by the impact of a
        meteoroid, asteroid, or comet with the Earth.

faultgl.shp*
Fault lines with line decorations. 
        A fault is a fracture or fracture zone in the Earth's crust along which
        one side moves with respect to the other.  This data set includes both
        the faults and the line decorations.  

faultl.shp**
Fault lines without line decorations.
        A fault is a fracture or fracture zone in the Earth's crust along which
        one side moves with respect to the other.

geolgyl.shp
geogyp.shp
Geologic units as polygons.
        Rock and material that lies at or near the land surface, but not
        surficial materials such as soils, alluvium, and glacial deposits.  The
        units are defined by sedimentary, volcanic, plutonic, or metamorphic
        rock types and by their geologic age.  The Text attribute is included to
        provide a method for generating the appropriate map symbols for display
        and publication purposes.

geotxtl.shp*
Graphic representation of geologic unit text.
        Text labels for the geology coverage, represented as lines.  Standard
        geologic symbols are included.

glacagl.shp*
Glacial limit lines, with line decorations.
        A line showing the southern limit of Late Wisconsin and pre-Late
        Wisconsin glaciation in the United States.  For display purposes, tics
        are included along the line for line decorations.

glacal.shp**
Glacial limit lines, without line decorations.
        A line showing the southern limit of Late Wisconsin and pre-Late
        Wisconsin glaciation in the United States.

impactx.shp
Impact structure locations, as points.
        The center of an impact structure.  An impact structure is a crater
        formed by the impact of a meteoroid, asteroid, or comet with the Earth.

metfacp.shp
Areas and facies types of metamorphism, as polygons.
        Metamorphism is the alteration of texture or composition in a rock by
        pressure, heat, or water.

      * these files are not 'true' Arc/INFO coverages; they are graphic
        representations of symbols and text used on the geologic map.   These
        files should be used to produce appropriately symbolized graphics.
        The decorations and text only appear correctly in Lambert Azimuthal
        Equal Area projection.
        
      ** these files are the same as the files marked with * but do not
         include the line decorations.  They are the 'true' fault and glacial
         limit line Arc/INFO coverages.  These files should be used for data
         analysis.

      Any use of trade, product, or firm names is for descriptive purposes only
      and does not imply endorsement by the U.S. Government

      Although this README-SH.TXT file is intended to document the data set in 
      nonproprietary form, some Arc/INFO-specific terminology is included 
      for clarity and expediency.