USGS Geologic Investigations Series I-2720, Pamphlet
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A Tapestry of Time and Terrain Pamphlet to accompany Geologic Investigations Series I–2720 U.S. Department of the Interior U.S. Geological Survey This page left intentionally blank A Tapestry of Time and Terrain By José F. Vigil, Richard J. Pike, and David G. Howell Pamphlet to accompany Geologic Investigations Series I–2720 U.S. Department of the Interior Bruce Babbitt, Secretary U.S. Geological Survey Charles G. Groat, Director Any use of trade, product, or firm names in this publica- tion is for descriptive purposes only and does not imply endorsement by the U.S. Government. United States Government Printing Office: 2000 Reprinted with minor corrections: 2008 For additional copies please contact: USGS Information Services Box 25286 Denver, CO 80225 For more information about the USGS and its products: Telephone: 1–888–ASK–USGS World Wide Web: http://www.usgs.gov/ Text edited by Jane Ciener Layout and design by Stephen L. Scott Manuscript approved for publication, February 24, 2000 2 Introduction are given in Thelin and Pike (1991). Systematic descriptions of the terrain features shown on this tapestry, as well as the Through computer processing and enhancement, we have geology on which they developed, are available in Thornbury brought together two existing images of the lower 48 states of (1965), Hunt (1974), and other references on geomorphology, the United States (U.S.) into a single digital tapestry. Woven the science of surface processes and their resulting landscapes into the fabric of this new map are data from previous U.S. (Graf, 1987; Bloom, 1997; Easterbrook, 1998). Geological Survey (USGS) maps that depict the topography and geology of the United States in separate formats. The Landscape features contain many of the clues needed to resulting composite is the most detailed and accurate portrait understand the Earth and the agents that have shaped it (Pike, of the U.S. land surface and the ages of its underlying rock 1991). Finding and decoding these clues enables scientists to formations yet displayed in the same image. The new map learn more about natural hazards, the events of Earth history, resembles traditional 3-D perspective drawings of landscapes and resources of land, water, energy, and minerals. Many with the addition of a fourth dimension, geologic time, which methods of landscape portrayal are used to view surface fea- is shown in color. This union of topographic texture with the tures, but shaded-relief imaging by computer is unique in the patterns defined by units of geologic time creates a visual syn- following respects: thesis that has escaped most prior attempts to combine shaded • It provides fine-scale detail over a wide area, a combination relief with a second characteristic shown by color, commonly not possible in other techniques of illustration; height above sea level (already implicit in the shaded relief). In mutually enhancing the landscape and its underlying tem- • It shows terrain accurately and in its true complexity, two poral structure, this digital tapestry outlines the geologic story properties commonly lost in sketches and diagrams of large of continental collision and break-up, mountain-building, river areas; erosion and deposition, ice-cap glaciation, volcanism, and • Unlike mosaics of aerial photos and radar images, the other events and processes that have shaped the region over view—limited only by extent of the digital data set—is the last 2.6 billion years. The map and the many features de- continuous across the country; scribed in this pamphlet are online at http://tapestry.usgs.gov. • Relief shading is free of distortion and the vegetation and The terrain cultural features that conceal topographic form on satellite images. [Most terms that may be unfamiliar to a general reader- ship are italicized where first mentioned] One of this map’s two components is a digital shaded- Geologic time relief image that shows the shape of the land surface by varia- The second component of this tapestry, color, represents tions in brightness. The degree of light and dark artificially geologic time and is simplified from thegeologic map of King mimics the intensity of the Sun’s light on different types of and Beikman (1974a). Rocks contain information essential to topography. This technique, called chiaroscuro, dates back an intelligent understanding of the Earth and its long natural some 400 years to sketches of the Tuscan landscape by Leon- history. Geologists determine the location, geographic extent, ardo da Vinci. Because manual (artistic) portrayals of terrain age, and physical and chemical characteristics of rocks and can practicably show only small areas both accurately and in unconsolidated (loose) materials. To express these attributes detail (Raisz, 1931), the technique has been automated by the geologists have developed a unique method of portrayal, the computer to cover large regions. The resulting digital shaded- geologic map, which has the following properties: relief map of the lower 48 states (Thelin and Pike, 1991) suc- • It recognizes similarities and differences among materials ceeded the unique, if less accurate, hand-drawn landform map that comprise the Earth’s crust and classifies them by type by master cartographer Erwin Raisz (1939). of rock or surficial deposit; The shaded-relief image of U.S. hills and dales is made up of some 12,000,000 minute squares—each one about 1/100 • It ascribes Earth materials to a specific environment or of an inch (0.25 mm) on a side. The gray tone of each square mode of origin—for example, volcano, river deposit, wind- is a brightness value computed from a mathematical rela- blown dune, limestone reef, or alteration at depth by heat or tion that includes steepness of the local ground, the direction pressure; that its slope faces, and the positions of the observer and a simulated Sun. Light and dark tones show steeper areas; • It identifies rock formations of distinctive materials and ages intermediate tones indicate gentler terrain. No actual shadows that are the three-dimensional building blocks of the Earth’s are included. The angle of surface slope and its sun-facing crust; it further shows the relative position of one formation direction were computed from a digital elevation model, or to another at the Earth’s surface; DEM, a square-grid array of terrain heights spaced about • It arranges rock formations of different ages into a time 1/2-mile (800 m) apart on the ground. These elevations were sequence from which the geologic history of the planet can derived by semi-automated means from the height contours be deciphered. (at 50-, 100-, or 200-foot intervals) of older 1:250,000-scale USGS topographic maps. Further information about the The original 161 colors and line patterns used by King shaded-relief technique, with details of the map’s construction, and Beikman (1974a,b) to portray the age, location, and extent 3 of materials that make up the lower 48 states are here reduced The geologic story of the U.S. told by the new map to 52 colors that show just the geologic ages of rocks and begins with tectonic plates, large segments of the Earth’s outer surficial deposits at the Earth’s surface. Combined colors and rigid layers that move about, or “drift”, on the plastic material patterns were retained only for a few altered (metamorphic) beneath. The thinner oceanic plates are composed largely of rocks of uncertain age. Lines representing faults (for example, denser (basaltic) rock; the thicker continental plates are rich the San Andreas in California) were omitted. The map is a in lighter (granitic) rocks. All the continental U.S. except its necessary compromise in that it emphasizes time intervals far west coast lies within the North American Plate. Continu- defined by bedrock rather than surficial materials. In areas -af ous creation and recycling of plates—through the spreading of fected by continental glaciation, for example, the ages shown fresh sea-floor formed at oceanic ridges,subduction (descent are of underlying rocks, not the veneer of glacial (surficial) and consequent melting) of one plate beneath another, and deposits. other broad-scale processes of plate tectonics—builds the Geologists subdivide time variously by eras (Precambrian planet’s largest features, its continents, and then redistributes to Cenozoic), periods (Cambrian to Quaternary), and epochs— and eventually reduces or destroys them. At least twice, all only Pleistocene (early Quaternary) and Holocene (late Qua- of Earth’s continental plates are believed to have collided and ternary) are used here (see column on map). These units are remained connected, forming a single landmass. The older quite uneven in elapsed time, the older intervals generally being of these proposed supercontinents, Rodinia (1.3 billion years of much longer duration. Rocks in the U.S. range in age from ago), was followed much later by Pangaea (200-300 million early Precambrian (2.6 billion years ago) to Holocene, which years ago). Rocks and geologic structures in the lower 48 includes the present. The orderly sequence of Earth materials, states still bear traces of both events. Ice-cap glaciation during from oldest to youngest, is represented by an equally well- the Pleistocene Epoch (about 1.8 million to 10,000 years ago), ordered sequence of “prismatic” colors (based on the rainbow). which created much of the topographic detail in the tapestry, is To improve the color balance of the tapestry, the Holocene is the most recent major chapter in the story. represented by two hues, light gray in the East and beige in the These and many other contending processes and events West. have given rise to the myriad of physical features portrayed The King and Beikman geologic map of the U.S. was in the new digital map.