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Padre Island National Seashore: A Guide to the Geology, Natural Environments, and History of a Texas Barrier Island



Geologic environments shown on plate I were mapped on color infrared stereoscopic aerial photographs (taken in May 1975) provided by the General Land Office of Texas. Most of the map units were adopted from studies by L. F. Brown, Jr., and J. H. McGowen for the Environmental Geologic Atlas of the Texas Coastal Zone (Brown and others, 1976, 1977). The map base was constructed from orthophotographic maps made in 1975 and provided by the General Land Office and from U.S. Geological Survey topographic maps issued in the years from 1951 to 1969.

The environments initially were mapped on the photographs at a scale of 1:24,000 (approximately 2.5 inches on the photographs equal 1 mile on the ground) and printed on plate I at a scale of 1:48,000 (1.3 inches per mile). At this scale, the map offers considerable detail without being too large and awkward to handle conveniently. The long and narrow National Seashore was divided into three sections to permit printing at a scale of 1:48,000.


The location map at the bottom right of the Geology and Natural Environments Map (pl. I) shows the position of Padre Island National Seashore along the Texas coast. The three map sections are labeled North Section, Central Section, and South Section and are oriented roughly parallel on the map sheet for efficient use of space. Because of this arrangement, each section has a different orientation with respect to north. North arrows are shown in the upper right corner of each section. Other means of determining directions are reference to latitude and longitude and Universal Transverse Mercator coordinate systems.

The three map sections can be mentally pieced together to reconstruct the entire National Seashore. The North and Central Sections can be joined at Match-line A, and the Central and South Sections can be joined at Match-line B. A small index map in the upper right corner of the North Section shows the entire National Seashore as a composite of the three map sections and indicates the positions of the matchlines. Note that the map boundaries of the three sections do not coincide with those of the National Seashore, but include some areas outside the limits of the Seashore. For example, the western boundary of all map sections is the Intracoastal Waterway, whereas in certain areas the Seashore boundary (dot/dash line) actually lies much farther east of the Waterway.


Three coordinate systems are shown on the Geology and Natural Environments Map. The standard latitude and longitude system is indicated by black tick marks, and the Universal Transverse Mercator (UTM) coordinate system is shown by blue tick marks. The third system, which uses letter-number coordinates, is a location grid designed specifically for this map.

Latitude and Longitude

Longitude lines can be projected across the map from values printed at the margins of each map section. Longitude lines run north-south and mark positions east or west of the Prime Meridian at Greenwich, England. For example, the longitude line of 97°25' in the Central Section may be followed from one tick mark to another from north to south (right to left) across the map. This line is 97 degrees and 25 minutes west of the Prime Meridian. In this region, 1 degree of longitude equals about 62 miles (1 minute equals 1.03 miles).

Latitude lines run east-west and mark positions north or south of the Equator. For example, the 27°25' line passing near the Malaquite Beach development is 27 degrees and 25 minutes north of the Equator. One degree of latitude equals 69 miles, and 1 minute equals 1.15 miles.

Universal Transverse Mercator System

The Universal Transverse Mercator (UTM) coordinate system is probably unfamiliar to most readers. The spacing between grid marks of this system represents a distance of 5,000 meters (16,400 feet). Coordinate values given at the margins of the map sections refer to the distance in meters from a central meridian of longitude and from the Equator. For a detailed explanation of the UTM system see The Universal Grid Systems, published by the Departments of the Army and the Air Force (1951).

Location Grid System

To aid in locating specific features, a letter-number grid system similar to those used on many street and highway maps has been superimposed on the Geology and Natural Environments Map. The solid black grid lines form a system of squares on the map. Vertical columns of squares are labeled "A" through "Y" (except "I") at the top and bottom of the map sheet; the horizontal rows of squares are numbered 1 through 21 along both sides of the map sheet. Thus, each square can be designated by a unique set of coordinates, such as N-4 or K-11. Do not confuse map coordinates, which are hyphenated (M-4, L-3), with map unit symbols, which are not hyphenated (M4, L3).

Map coordinates are given for location of photographs in the legend as well as for many of the natural and man-made features discussed in this guidebook. Specific features may be located by the intersection of the proper vertical column and horizontal row. For example, the Malaquite Beach development, shown as unit M4 in the map legend, is located at map coordinates Q-4. Follow the vertical column under Q at the top of the map downward to the row of squares designated 4. The square at the intersection of column Q and row 4 is the location of the development. Although the vertical grid lines are not printed between map sections, lettered columns apply to all three map sections.


Fractional and graphic scales are shown at the bottom of the Geology and Natural Environments Map. The fractional, or ratio, scale is 1:48,000. This means that one unit on the map equals 48,000 of the same units in the area mapped. For example, 1 inch on the map represents 48,000 inches (or approximately 0.76 mile) on the ground.

The graphic, or bar, scales are more useful than the ratio scale for determining distances. Distances can be quickly estimated or actually measured with a simple, easily constructed paper ruler with the same scale as the bar scale. Graphic scales for three different units—miles, feet, and kilometers— are printed on the map.


Significant characteristics of the 23 map units, or environments, are briefly described in the map legend. These 23 units include 11 barrier island units (B1 through B11), 8 lagoon units (L1 through L8), and 4 man-made or man-modified units (M1 through M4). At the left of each unit description is a box showing the unique map color and letter-number symbol of the unit. Colors selected for most of the map units are similar to the dominant natural colors of the environments: greens are used for vegetated environments, blues for water units, and tans and yellows for barren sand units.

A color photograph illustrates each environment, or map unit. Locations of the photographs are indicated by means of the letter-number grid system explained above. These photographs, on which significant features are labeled, should help the map reader visualize the environment described.

Photographs were selected to be representative of the mapped environments described in the legend. Many variations occur in each geologic environment, however, and the reader should consult the appropriate section of this guidebook for a discussion of these variations and for additional photographs and illustrations.


Map unit symbols (for example, B4 or L7) are printed on the map to aid in identifying the environmental units. Other symbols used on the map, including those for roads, facilities, dune orientations, water-level lines, and beach and navigation markers, are explained in the legend.

Positions and numbers of the navigation markers plotted on plate I for waters along the intracoastal Waterway and Mansfield Channel were derived from nautical charts published by the National Oceanic and Atmospheric Administration (NOAA). Lights and light beacons are designated by numbers in quotation marks, as well as symbols. Because the Geology and Natural Environments Map does not show all markers or any depths, the nautical charts are needed for navigation.

Many of the symbols given on the map are accompanied by labels, giving more specific information about features such as cattle line camps. In addition, most of the other significant natural and man-made features are labeled. These features include specific Seashore elements, such as North Bird Island, as well as general areas, such as the Land-Cut Area.

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Last Updated: 28-Mar-2007