Geologic Mapping in the western United States
Geologic Mapping in the western United States
What is a Geologic Map?
Geologic maps: Describe the rocks and
soils at the surface, provide information about what rocks lie at depth,
describe the ages of rocks and soils, and show where features such as earthquake
faults and landslides lie. Geologic maps are made by studying the rocks
and materials exposed at the surface and depicting information about those
rocks on a map.
Colored patches represent
areas underlain by certain geologic units, each color-coded to the age of
the unit. The symbol for each colored blob is coded to the name and age
of the rock unit. A short description of the rock unit is given in the legend
for the map.
Yellow colors typically are reserved for surface geologic
units such as soils, young river deposits, and glacial deposits. On this
map, for example, the yellow colors delineate two kinds of deposits along
a meandering river.
Other colors represent older rock units deposited millions
of years ago and since faulted and distorted by earthquakes, and eroded
to the modern landscape.
Special line symbols indicate faults. One is shown by
the strainght heavy line cutting across geologic units on the left side
of themap. These and the thinner lines for other contacts between geologic
units are solid, dashed, or dotted depending on how completely the contacts
are exposed.
Symbols give the strike and dip of layers
in rock, two measurments that describe their orientation in space.
Symbols show the shapes of folded rock layers.
The geology is printed on topographic maps to show the
landforms as well as roads, cities and rivers (the topography doesn't show
on this example).
Geologic mapping is much more than a description of the rocks and materials. It involves testing models for the evolution of rocks by laboratory and numerical methods, and incorporates related fields of:
Program Objectives
To create geologic maps and digital geologic data bases to solve resource, land use, hazards, and environmental problems and to develop geographic information system (GIS)-based methods for assessing risks, for reducing economic losses from natural hazards, and for making optimal land-use decisions. To build a digital geologic-map data base for the Nation to guide public policy.
Rationale
Geologic maps are uniquely suited to solving problems involving Earth resources, hazards, and environments. In particular, digital geologic maps are interactive electronic documents that put the Nation's earth science issues into geospatial frameworks. They capture the size, the shape, the depth, and the physical and chemical contexts of earth materials and they blend data display with the results of interpretive research. These are actually four-dimensional data systems, and it is the fourth dimension of time, that is crucial to assessing natural hazards and environmental or socio-economic risk. To read a geologic map is to understand not only where earth resources and characteristics are located, but also how and when these earth features formed. Were they produced last year in a fiery hurricane of volcanic ash, a hundred years ago in a major flood, or ten million years ago from a fertile sea or estuary? Should we plan for another earthquake, eruption, or flood in the same area next year, or not in our lifetimes. Is the water beneath the surface part of a regional aquifer or is it controlled by a local fracture zone? How does subsurface distribution of porous and impermeable rock affect the flow of water, the potential for contamination, and the volume available for use? Geologic maps and data bases address all of these questions.
The URL for this page is: <http://geology.wr.usgs.gov/wgmt/aboutmaps.html>
Page Maintained by: David Miller
Last Modified: 11 November 1997