Geologic Mapping in the western United States

The Face of Our Land
Shaded Relief Map of the United States

What Kind of Map IS This !?
(It is not an aerial photo, not a radar image, and not a spacecraft picture!)

The concept demonstrated here, hill shading, shows the shape of the land by intensity of the Sun's shadows. This concept is not new. Used by Leonardo da Vinci centuries ago, it was adapted to the computer because manual (artistic) executions of it can economically show only small areas accurately and in detail.

This map is a digital shaded-relief image. It is made up of 12,000,000 tiny squares-each one less than 1/4 of a millimeter across. The gray tone of each square is a brightness value computed from a mathematical relation between ground slope and direction and the position of the observer and a simulated Sun. Light and dark tones show steep areas; intermediate tones are gentle terrain. The map was computed from a checkerboard array of terrain heights (a digital elevation model) spaced 1/2-mile apart and taken from topographic maps.

WHAT'S SPECIAL ABOUT THIS MAP ?

Surface features contain clues to understanding the Earth. Geologists search for the clues and decode them to learn about natural hazards and resources. Many kinds of maps are used to study surface features, but this one has unique properties:

• It combines fine detail with wide coverage
• It shows terrain accurately and in its true complexity, two properties often lost in sketches and diagrams of large areas
• The view is continuous-limited only by size of the digital data set-unlike relief models, aerial photos, and radar images
• The map is free of distortion and the vegetation and cultural features that mask topographic form on satellite images
• Positions of Sun and viewer can be varied to get a different perspective on a landform or to enhance terrain features

WHAT CAN WE SEE ON THIS MAP ?

This is the most detailed and accurate view of terrain for the entire U.S.yet put together. It resembles traditional perspective drawings, but contains much more information. This digital portrait faithfully reflects the geologic work of tectonism, ice-cap glaciation, river activity, volcanism, and other land-shaping events and processes.

SOME LARGE FEATURES [This section refers to a two-page map accompanying the map]

Two tectonic regimes are the static eastern United States, a passive continental margin, and the active plate margin of the west. The two contrast vividly in surface roughness, texture, structural and coastline patterns, and freshness of relief.

Topography north of the Ice Age (Pleistocene) limit (No. 5 on small map) is smooth and lacks the strong river-carved texture of unglaciated terrain to the south. Rough islands of the unglaciated Driftless area in Wisconsin (No. 4) stand out sharply from surrounding glaciated ground.

PHYSIOGRAPHIC DIVISIONS

The major physical regions of the U.S. (small map) were recognized partly on the basis of important contrasts in topography. Particularly distinctive are:

• the Valley and Ridge province of the Appalachian Mountains (No. 1), controlled by rock hardness and fold structures
• the Basin and Range province (No. 21)-a complex region of fault-block structures formed by stretching of a thin crust
• the west-tilted Sierra Nevada fault block (No. 2), including Mt. Whitney, highest peak in the lower 48 states
• eroded domed strata of the Black Hills (No. 3) surround the granite core in which the Mt. Rushmore faces were carved
• the sharp boundary of the Fall Line (No. 15) divides different rocks of the Piedmont and Coastal Plain provinces.

The complexity and variety of our nation's lands are shown on the popular map "Landforms of the Conterminous United States - A Digital Shaded-Relief Portrayal, by G.P. Thelin and R.J. Pike." The map is accompanied by a pamphlet explaining how image processing and computer graphics have automated the art of landscape portrayal. It is available from the U.S. Geological Survey (USGS Branch of Information Services, Box 25286, Denver CO, 80225 ; order Miscellaneous Investigations Series Map I-2206 (1991). Pricing information available at http://mapping.usgs.gov/esic/prices/maps.html).

.Download the full dataset in tiff image format (11.8 MB).

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