Padre Island National Seashore 

Geologic Resources Inventory Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2010/246 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
THIS PAGE: 

Padre Island National Seashore is 

home to a variety of sea turtles. The 

Kemp’s ridley sea turtle (pictured) is 

the smallest. Radio transmitters help 

researchers track turtle populations.

ON THE COVER: 

Dunes on Padre Island National Seashore. The park was established to 

preserve one of the last undeveloped 

barrier islands in the United States.

National Park Service photographs by

Rebecca Beavers (NPS Geologic 

Resources Division). 
Padre Island National Seashore 

Geologic Resources Inventory Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2010/246 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

September 2010 

U.S. Department of the Interior 

National Park Service 

Natural Resource Program Center 

Ft. Collins, Colorado
The National Park Service, Natural Resource Program Center publishes a range of reports that 

address natural resource topics of interest and applicability to a broad audience in the National 

Park Service and others in natural resource management, including scientists, conservation and 

environmental constituencies, and the public. 

The Natural Resource Report Series is used to disseminate high-priority, current natural 

resource management information with managerial application. The series targets a general, 

diverse audience, and may contain NPS policy considerations or address sensitive issues of 

management applicability. 

All manuscripts in the series receive the appropriate level of peer review to ensure that the 

information is scientifically credible, technically accurate, appropriately written for the 

intended audience, and designed and published in a professional manner. This report received 

informal peer review by subject-matter experts who were not directly involved in the 

collection, analysis, or reporting of the data. 

Views, statements, findings, conclusions, recommendations, and data in this report do not 

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Interior. Mention of trade names or commercial products does not constitute endorsement or 

recommendation for use by the U. S. Government. 

Printed copies of this report are produced in a limited quantity and they are only available as 

long as the supply lasts. This report is available from the Geologic Resources Inventory website 

(http://www.nature.nps.gov/geology/inventory/gre_publications.cfm) and the Natural 

Resource Publications Management website (http://www.nature.nps.gov/publications/

NRPM). 

Please cite this publication as: 

KellerLynn, K. 2010. Padre Island National Seashore: geologic resources inventory report. 

Natural Resource Report NPS/NRPC/GRD/NRR—2010/246. National Park Service, Ft. 

Collins, Colorado. 

NPS 613/105619, September 2010 

ii NPS Geologic Resources Division 
PAIS Geologic Resources Inventory Report iii

Contents

List of Figures................................................................................................................ iv

List of Tables ................................................................................................................. iv

Executive Summary ....................................................................................................... v

Acknowledgements...................................................................................................... vi

Credits........................................................................................................................................................................ vi

Introduction ................................................................................................................... 1

Purpose of the Geologic Resources Inventory ...............................................................................................................1

Park Setting .................................................................................................................................................................1

Padre Island History......................................................................................................................................................2

Geologic Issues .............................................................................................................. 5

Coastal Vulnerability ....................................................................................................................................................5

Blowing Sand and Dune Development .........................................................................................................................6

Hurricanes ...................................................................................................................................................................7

Benthic Habitat Mapping .............................................................................................................................................7

Paleontological Resources ............................................................................................................................................8

Bird Island Basin...........................................................................................................................................................9

Vehicle Impacts............................................................................................................................................................9

Dredging ...................................................................................................................................................................11

Oil and Gas Operations..............................................................................................................................................12

Shoreline Debris.........................................................................................................................................................13

Grazing .....................................................................................................................................................................14

Wind Energy..............................................................................................................................................................14

Geologic Features and Processes................................................................................ 19

Wind Activity .............................................................................................................................................................19

Wave and Current Activity..........................................................................................................................................19

Tidal Activity ..............................................................................................................................................................19

Barrier Island System..................................................................................................................................................20

Lagoon System ..........................................................................................................................................................23

Evaporite, Fulgurite, and Oolite ..................................................................................................................................23

Shells.........................................................................................................................................................................24

Map Unit Properties .................................................................................................... 29

Geologic History .......................................................................................................... 33

Glossary........................................................................................................................ 37

Literature Cited............................................................................................................ 39

Additional References ................................................................................................. 45

Appendix A: Overview of Digital Geologic Data....................................................... 46

Attachment 1: Geologic Resources Inventory Products CD
iv NPS Geologic Resources Division 

List of Figures

Figure 1. Map of Padre Island National Seashore..............................................................................................................3

Figure 2. Coastal vulnerability index ...............................................................................................................................15

Figure 3. Blowing sand ..................................................................................................................................................16

Figure 4. Vehicle impacts...............................................................................................................................................17

Figure 5. Oil and gas operations.....................................................................................................................................17

Figure 6. Grasslands of Padre Island National Seashore...................................................................................................18

Figure 7. Geomorphic provinces at Padre Island National Seashore.................................................................................25

Figure 8. Beach and fore-island dune ridge ....................................................................................................................25

Figure 9. Fore-island dune ridge.....................................................................................................................................26

Figure 10. Blowout........................................................................................................................................................26

Figure 11. Vegetated blow out dune..............................................................................................................................27

Figure 12. Sand rose and fulgurite .................................................................................................................................27

Figure 13. Seashells of Padre Island................................................................................................................................28

Figure 14. Geologic timescale ........................................................................................................................................35

Figure 15. Barrier island morphology..............................................................................................................................36

List of Tables

Table 1. Land cover in Padre Island National Seashore......................................................................................................2

Table 2. Hurricanes making landfall near Padre Island (1900–2010)................................................................................16

Table 3. Requirements for renourishment at Bird Island Basin.........................................................................................16

Table 4. Transgressive and regressive characteristics of shorelines...................................................................................34
PAIS Geologic Resources Inventory v

Executive Summary

This report accompanies the digital geologic map data for Padre Island National Seashore

in Texas, produced by the Geologic Resources Division in collaboration with its partners. It

contains information relevant to resource management and scientific research. This 

document incorporates preexisting geologic information and does not include new data or 

additional fieldwork.

In 1962 Congress established a national seashore on 

Padre Island to save a part of the United States shoreline 

from development. Bounded by the Gulf of Mexico to 

the east and Laguna Madre to the west, Padre Island 

National Seashore preserves a barrier island–lagoon 

system, including the nearshore zone; sandy and shelly 

beaches; active and stabilized dunes; barrier flats with 

ponds, marshes, and wind-deflation troughs; wind-tidal 

flats; grassflats; sand and shell islands; and a hypersaline 

estuary. Padre Island National Seashore’s enabling 

legislation intends for the seashore’s resources to be 

enjoyed by the public, and more than 642,000 visitors 

were drawn to its beaches and estuary in 2009.

The dominant shoreline process at the national seashore 

is wind, which creates waves that drive currents, which in 

turn transport sediment. The wind shapes both the foreisland dunes and the back-island landscapes. Converging 

currents around latitude 27°N influence barrier-island 

development, determine the character of the beaches. 

Astronomical tides (produced by the gravitational pull of 

the moon and sun) are minor compared to

meteorological (wind-induced) tides, which have a much 

greater effect on the geomorphic processes and 

landscapes of Padre Island National Seashore.

Geologic issues that may require management attention 

include the following:

• Coastal vulnerability. Based on six variables—

geomorphology, shoreline change (erosion/accretion), 

regional coastal slope, relative sea-level change, mean 

significant wave height, and mean tidal range—

investigators determined that 17% of the Gulf of 

Mexico shoreline at Padre Island National Seashore 

has very high vulnerability to future sea-level rise; 28% 

has high vulnerability; 29% has moderate vulnerability; 

and 26% has low vulnerability. The most vulnerable 

segment of the shoreline is in the southern part of the 

national seashore.

• Blowing sand. Blowing sand is ubiquitous at Padre 

Island National Seashore. Bulldozers continually

scrape windblown sand off paved parking lots. 

Tongues of sand creep relentlessly onto park roads. 

Windblown sand repeatedly engulfs the patrol cabin at 

milepost 42. However, the area of greatest 

management concern regarding blowing sand is at the 

Malaquite Beach area, which has a long history of 

anthropogenic impacts, including removal of the foreisland dunes.

• Hurricanes. Hurricanes periodically make landfall in 

the vicinity of Padre Island National Seashore. 

Hurricanes cause severe erosion, particularly in the 

southern part of the national seashore, where storm 

washover channels are numerous. Washover channels 

eroded during past hurricanes are likely to be 

reactivated during future storms. These storms also 

cause deposition on the lagoon side of the island, 

creating important substrate for seagrass, marshes, and 

sand flats.

• Benthic habitat mapping. Padre Island National 

Seashore contains 42 km2 (16 mi2

) of marine resources. 

This offshore region is known only generally because 

the benthic environment has not been mapped. 

Mapping of this area would clarify the land-water 

interface; relationships of marine features to terrestrial 

ones; bathymetric features, such as navigation hazards; 

seafloor features that influence waves, currents, and 

storm surge; the areal extent and health of marine 

resources; and sediment characterization and quality 

as habitat.

• Bird Island Basin. Bird Island Basin encompasses

486 ha (1,200 ac) of barrier-island and lagoonal 

environments. As a result of its high visitor use and the 

variety and density of recreational activities in the area, 

all the geologic processes requiring management 

attention in Bird Island Basin have a human 

component. These processes are (1) shoreline erosion, 

(2) infilling/sedimentation of the boat basin, (3) 

erosion via runoff, and (4) reduction in sand supply

and renourishment. 

• Vehicle impacts. Most of the national seashore’s 

beaches are open to motor vehicles and motorcycles. 

Various studies have investigated recreational impacts 

and found that beach driving inhibits the development 

of coppice dunes on the back beach, tending to make 

the beach zone wider while also inhibiting accretion 

and decreasing the overall width of the barrier island. 

Beach driving changes the water-holding capacity of 

beach sediments, increases shoreline erosion and 

instability during storms, eliminates the natural 

rebuilding capacity of the foredune system between 

storms, and changes dune forms and creates blowouts 

by destroying vegetation that stabilizes dunes. 

• Dredging. In the vicinity of Padre Island National 

Seashore, one artificial pass (Mansfield Channel) and 

one artificial waterway (Gulf Intracoastal Waterway) 

require maintenance dredging to remain open to boat 

traffic. Yarborough Pass is a “failed” channel through 

the island. The effects of dredging and disposal of 

dredge material are primary management concerns, 

particularly because they affect Laguna Madre’s 
vi NPS Geologic Resources Division 

biological productivity, grassflats, and bird 

populations. 

• Oil and gas operations. Oil and gas activity currently 

consists of 8 pipelines, covering a total of 108 km

(67 mi); 14 active wells (13 gas, 1 water); 5 permitted 

wells; and 172 ha (425 ac) of surface disturbance. 

Although oil and gas activities in the Gulf of Mexico 

can impact park resources, the greatest threats are 

from accidental leaks and spills of hydrocarbons from 

pipelines and producing wells within the national 

seashore.

• Shoreline debris. Geologic processes (i.e., converging 

currents) drive deposition of natural (i.e., sand and 

shell) and unnatural (shoreline trash) objects onto the 

shores of Padre Island National Seashore. In an 

attempt to clean the beaches of trash, many coastal 

communities spend millions of dollars each year and 

conduct volunteer clean-ups. Such measures, 

however, resolve the problem only temporarily, and 

trash usually reappears on the shoreline within a short 

time. A 10-year (1989–1998) study at Padre Island 

National Seashore established protocols and methods 

and compiled a baseline inventory of shoreline debris. 

Current park managers could build on this study in 

order to identify point sources and develop outreach 

efforts to reduce the amount of trash that washes 

ashore. 

• Grazing. Padre Island’s coastal prairies, absence of 

chaparral, and natural boundaries for containing cattle 

made it an ideal location for cattle operations for 166 

years (1805–1971). The primary disturbances resulting 

from cattle grazing are changes in vegetation, 

destabilization of eolian features, and increased 

sedimentation into Laguna Madre. The transition 

from grazing to non-grazing conditions has impacted 

the “beach” at Bird Island Basin, which now requires 

nourishment to be maintained. 

• Wind energy. The mainland across from Padre Island 

National Seashore is the site of two wind power 

facilities with a total of 118 wind turbines that each 

generates 2.4 megawatts of electricity. The primary 

concerns regarding the development of wind energy in 

the vicinity of the national seashore are impacts on 

migrating bird and marine species, changes in fishing 

pressure (i.e., bait fish congregate near structures for 

food and shelter, thereby attracting predator species), 

changes in habitat as a result of transmission lines (i.e., 

alterations in magnetic field may affect sea turtle 

migration), and terrestrial impacts of routing a rightof-way across the national seashore to access the 

power grid. 

The glossary contains explanations of many technical 

terms used in this report, including terms used in the 

Map Unit Properties Table. Refer to figure 14 for a 

geologic time scale.

Acknowledgements

The Geologic Resources Inventory (GRI) is one of 12 inventories funded by the National 

Park Service Inventory and Monitoring Program. The GRI is administered by the 

Geologic Resources Division of the Natural Resource Program Center.

The Geologic Resources Division relies heavily on 

partnerships with institutions such as the U.S. Geological 

Survey, Colorado State University, state geologic surveys, 

local museums, and universities in developing GRI 

products.

Special thanks to: Darrell Echols with whom the author 

had numerous phone conversations and e-mail 

communications regarding the geologic issues at Padre 

Island National Seashore. Darrell Echols served as park 

biologist at the national seashore from 1999 to 2003 and 

chief of resource management from 2003 to 2008. He 

currently serves as the deputy superintendent of the 

National Park Service, Outer Banks Group.

Edward Kassman and Pat O’Dell (NPS Geologic 

Resources Division) provided information regarding oil 

and gas operations and regulations. 

Rebecca Beavers (NPS Geologic Resources Division) 

provided many of the photographs in the report.

Credits

Author

Katie KellerLynn (Colorado State University)

Review

Darrell Echols (NPS Outer Banks Group)

Jim Gibeaut (Texas A&M University-Corpus Christi)

Courtney Schupp (Assateague Island National Seashore)

Rebecca Beavers (NPS Geologic Resources Division)

Jason Kenworthy (NPS Geologic Resources Division)

Editing

Diane Lane

Digital Geologic Data Production

Ron Karpilo (Colorado State University)

Stephanie O’Meara (Colorado State University)

Digital Geologic Data Overview Layout Design

Georgia Hybels(NPS Geologic Resources Division)

David Green (Colorado State University)
PAIS Geologic Resources Inventory Report 1

Introduction

The following section briefly describes the National Park Service Geologic Resources 

Inventory and the regional geologic setting of Padre Island National Seashore.

Purpose of the Geologic Resources Inventory 

The Geologic Resources Inventory (GRI) is one of 12 

inventories funded by the National Park Service (NPS) 

Inventory and Monitoring Program. The GRI, 

administered by the Geologic Resources Division of the 

Natural Resource Program Center, is designed to 

provide and enhance baseline information available to 

park managers. The GRI team relies heavily on 

partnerships with institutions such as the U.S. Geological 

Survey, Colorado State University, state geologic surveys, 

local museums, and universities in developing GRI 

products.

The goals of the GRI are to increase understanding of the 

geologic processes at work in parks and to provide sound 

geologic information for use in park decision making. 

Sound park stewardship requires an understanding of 

the natural resources and their role in the ecosystem. 

Park ecosystems are fundamentally shaped by geology. 

The compilation and use of natural resource information 

by park managers is called for in section 204 of the 

National Parks Omnibus Management Act of 1998 and in 

NPS-75, Natural Resources Inventory and Monitoring 

Guideline. 

To realize these goals, the GRI team is systematically 

conducting a scoping meeting for each of the 270 

identified natural area parks and providing a parkspecific digital geologic map and geologic report. These 

products support the stewardship of park resources and 

are designed for nongeoscientists. Scoping meetings 

bring together park staff and geologic experts to review 

available geologic maps and discuss specific geologic 

issues, features, and processes. 

The GRI mapping team converts the geologic maps 

identified for park use at the scoping meeting into digital 

geologic data in accordance with their Geographic 

Information Systems (GIS) Data Model. These digital 

data sets bring an interactive dimension to traditional 

paper maps. The digital data sets provide geologic data 

for use in park GIS and facilitate the incorporation of 

geologic considerations into a wide range of resource 

management applications. The newest maps contain 

interactive help files. This geologic report assists park 

managers in the use of the map and provides an overview 

of park geology and geologic resource management 

issues.

For additional information regarding the content of this 

report and current GRI contact information please refer 

to the Geologic Resources Inventory web site 

(http://www.nature.nps.gov/geology/inventory/).

Park Setting

Padre Island National Seashore is relatively undeveloped 

because of its remote location, National Park System 

designation, and lack of permanent roads. The major 

population center in the vicinity of the national seashore 

is Corpus Christi to the north, in Nueces County (fig. 1). 

Port Mansfield is near the southern boundary of the 

seashore, across the Mansfield Channel in Willacy 

County, and the towns of Laguna Vista, Laguna Heights, 

Port Isabel, and South Padre Island are to the south of 

the national seashore in Cameron County. The lack of 

development on the mainland adjacent to the national 

seashore in Kleberg and Kenedy counties is largely a 

result of the conservation of 18,287 ha (45,187 ac) in the 

Laguna Atascosa Unit of Laguna Atascosa National 

Wildlife Refuge, as well as undeveloped state lands and 

large, privately owned ranches, such as the King and 

Kenedy. 

Long, narrow Padre Island National Seashore covers 

52,745 ha (130,332 ac) and separates Laguna Madre from 

the Gulf of Mexico. Of the national seashore’s four sides, 

three are in water (fig. 1). The eastern boundary runs for 

105 km (65 mi) in the Gulf of Mexico. The western 

boundary is located in Laguna Madre, the world’s largest 

hypersaline estuary. Marking the southern boundary is 

Mansfield Channel, which cuts through the island and 

estuary to Port Mansfield, Texas. North Beach marks the 

terrestrial northern boundary.

The northern part of the national seashore is generally 

higher and wider than the southern part. The northern 

part consists of broad beaches; large, stabilized 

foredunes; grasslands; and large, shifting back-island 

dunes (table 1). The average depth of Laguna Madre in 

this area is about 1.2 m (4 ft). The Gulf Intracoastal 

Waterway—a segment of the Intracoastal Waterway that 

runs between New York and Brownsville, Texas—

traverses the entire length of Laguna Madre and marks a 

part of the seashore’s western boundary in the northern 

part of the national seashore. Dredged material from the 

waterway forms spoil banks varying from 0.4 to 2.0 ha

(1 to 5 ac) in area. Two natural islands—North Bird and 

South Bird—are situated at the northern end of the 

national seashore within Laguna Madre. Both the islands 

and spoil mounds are significant bird rookeries.

South of Malaquite Visitor Center, the convergence of 

longshore currents from the north and south changes the 

island’s character. Here, steeper topography and an 

abundance of shells are typical of the beaches whose 

names reflect the setting (i.e., Little Shell Beach and Big 

Shell Beach). High foredunes, grasslands, and extensive 

barrier flats also characterize this segment of shoreline. 
2 NPS Geologic Resources Division 

In the southern half of the national seashore, south of Big 

Shell Beach, the beaches are flat, the foredunes are few, 

and the vegetation is sparse. After storms, temporary 

passes (washover channels) allow Laguna Madre to drain 

into the Gulf of Mexico. When these passes close, ponds 

that are left behind support fish for many months before 

drying up. These ponds also attract wintering birds.

Padre Island History

Throughout its history, Padre Island has been known by 

several names: “la Isla Blanca” (White Island), “Isla de los 

Malaguitas” (Island of the Malaquites, a band of the 

Karankawa people who were nomadic huntergatherers), and most recently, “Padre Island” (named for 

Padre Nicolas Ballí, a Spanish priest and the first 

permanent settler in 1804). Four nations have owned 

Padre Island. The first was Spain, which owned the 

island from its entry into the New World until the 

Mexican Revolution of 1820. Following the revolution, 

Mexico owned Padre Island from 1821 until 1836, when 

the newly formed Republic of Texas claimed the area 

between the Nueces River and the Rio Grande. Padre 

Island was under ownership by the Republic of Texas 

until its territory was acquired by the United States 

following the United States–Mexican War. 

Padre Island’s history has revolved around ranching. 

Padre Ballí introduced cattle to the island, and for three 

decades Ballí and his nephew Juan José used Padre Island 

for cattle grazing, which continued under various hands 

until 1971, when the National Park Service removed 

cattle from Padre Island National Seashore. At the end of 

the United States–Mexican War in 1848, Americans 

replaced Mexicans as the island’s ranchers. By the time 

of the Civil War (1861–1865), the island supported a 

small community of cattle ranchers. In 1876, Patrick 

Dunn, an Irishman, moved to Padre Island and 

developed a large cattle ranch, and ultimately acquired 

nearly all of Padre Island. His family enterprise 

continued until 1971. 

The most prominent and lasting exceptions to ranching 

as the sole enterprise on Padre Island have been the 

development of the tourism industry (including the

development of the town of South Padre Island and the 

national seashore) beginning in the early 1920s, and the 

exploration and development of the island’s oil and gas 

reserves, which began in the 1950s (National Park 

Service 2009).

The age of the automobile and the construction of a 

causeway brought tourism to the island (Callaway 1972). 

Col. Sam Robertson, an early South Texas developer 

who had dreams of establishing tourist facilities on the 

island, purchased the land from Patrick Dunn in 1926, 

although Dunn retained the mineral and grazing rights. 

Robertson opened the causeway to the public on July 4, 

1927; the causeway ran from Flour Bluff, east of Corpus 

Christi, to Padre Island. Tourists expecting a leisurely 

drive were rudely awakened by timber with troughs set 

to accommodate the wheels of a Model T Ford; low 

guard rails provided little protection for a vehicle that 

might bounce out of its trough (Callaway 1972). The 

wooden supports of this causeway are still visible across 

the flats of Laguna Madre. Robertson also operated two 

ferry crossings—one at Port Aransas and the other at 

Port Isabel. Ultimately, the Great Depression broke 

Robertson, and his dreams of land investment, 

development, and tourism were never fully realized 

(Sheire 1971).

In the 1930s, plans for a state park on the island were 

proposed (National Park Service 1973), but World

War II curtailed ideas of development. “The hopes of 

many enterprising businessmen lay dormant until 1950” 

(Callaway 1972, p. 30), when concern that the nation’s 

shoreline would become entirely developed focused 

attention on Padre Island’s outstanding natural features. 

In 1954 the National Park Service recommended 

establishment of three national seashores; one of these 

was Padre Island. The congressional approval necessary 

for the transfer of state lands and the acquisition of 

private lands took 9 years (Callaway 1972) but was 

achieved in 1962 with the passage of Public Law 87-712.

The limitation of $5 million in the original bill made it 

necessary to return to Congress for separate bills (Public 

Law 90-594 and 91-42) to authorize additional funds for 

acquisition. Padre Island National Seashore opened to 

the public in 1963.

Table 1. Land cover in Padre Island National Seashore

Classification type Areal extent Percentage of 

Hectares Acres national seashore

Beach/sand 1,319 3,259 2.5

Dunes (foredunes and back-island dunes) 2,480 6,127 4.7

Emergent vegetation 7,800 19,273 14.8

Grassland 5,434 13,427 10.3

Gulf of Mexico 5,170 12,775 9.8

Inland waters (ephemeral and permanent freshwater ponds) 949 2,346 1.8

Laguna Madre 12,345 30,503 23.4

Rookery islands 106 261 0.2

Sparse vegetation 2,459 6,075 4.6

Unconsolidated shore 2,638 6,518 5.0

Urban (developed areas) 158 391 0.3

Washover channels 482 1,192 0.9

Wind-tidal flats 11,448 28,287 21.7

Total 52,788 130,4341 100.0

1 Total acreage is 130,434; federal acreage is 130,355; nonfederal acreage is 79. Data from National Park Service (2000).
PAIS Geologic Resources Inventory Report 3 Figure 1. Maps of Padre Island National Seashore. National Park Service graphics.

PAIS Geologic Resources Inventory Report 5

Geologic Issues

The Geologic Resources Division held a Geologic Resources Inventory scoping session for

Padre Island National Seashore on May 12–14, 2003, to discuss geologic resources, 

address the status of geologic mapping, and assess resource management issues and 

needs. This section synthesizes the scoping results, in particular those issues that may 

require attention from resource managers. Contact the Geologic Resources Division for 

technical assistance.

During scoping, participants did not prioritize the 

geologic issues affecting resource management at Padre 

Island National Seashore. They are organized here into 

two main groups: natural issues (Coastal Vulerability, 

Blowing Sand and Dune Development, Hurricanes, 

Benthic Habitat Mapping, and Paleontological 

Resources) and anthropogenic issues (Bird Island Basin, 

Vehicle Impacts, Dredging, Oil and Gas Operations, 

Shoreline Debris, Grazing, and Wind Energy).

Coastal Vulnerability

On the basis of six variables—geomorphology, shoreline 

change (erosion/accretion), regional coastal slope, 

relative sea-level change, mean significant wave height, 

and mean tidal range—investigators determined the 

coastal vulnerability of Padre Island National Seashore to 

sea-level rise (fig. 2) (Pendleton et al. 2004). Sea-level rise 

projections vary widely depending on location, future 

emissions scenarios, and modeling technique. Globally, 

at least 0.18 m to 0.59 m (7 in. to 2 ft) of sea level rise is 

projected by 2100 (Meehl et al. 2007; National Park 

Service 2010a). Some projections summarized by Karl et 

al. (2009) suggest much larger increases in sea level. For 

example, Rahmstorf (2007) projected global sea levels to 

rise between 0.5 and 1.4 m (1.6 and 4.6 ft) above the 1990 

level by 2100. For South Texas, a 37-year-long tide-gauge 

record shows that relative sea level has risen at a rate of 

4.6 mm/yr (0.18 in/yr) at Rockport (since 1948), 2.05 

mm/yr (0.08 in/yr) at Port Mansfield (since 1963), and 

3.44 mm/yr (0.14 in/yr) at South Padre Island (since 

1958) (Zervas 2001). This record inherently includes 

both worldwide changes in sea level (eustatic) and 

regional changes in sea level (isostatic and tectonic 

adjustments of the land surface). The relative rise in sea 

level at Port Isabel is the lowest for the entire Texas coast 

(Morton and Holmes 2009). The rate of relative sea-level 

rise in Laguna Madre is 3.4 mm/year (0.14 in/year), 

which is nearly comparable to the average global eustatic 

rate of 3.1 mm/year (0.12 in/year) between 1993 and 2003 

(Parry et al. 2007). 

Human activity, particularly through the emission of 

greenhouse gases, is very likely (more than 90% certain) 

contributing to global warming (IPCC 2007) and thus 

accelerating the rate of climate change and global sea 

level rise. Karl et al. (2009) summarized climate change 

impacts for the Southeast and coastal regions of the 

United States. Along with increases in air and ocean 

surface temperatures and sea-level rise, hurricane 

intensity is expected to increase. Maximum storm surges 

are also projected to increase. These impacts will be 

particularly acute on vulnerable barrier islands. For more 

information regarding climate change, effects 

throughout the National Park Service, and the NPS 

response, visit the NPS Climate Change Response 

Program website: http://www.nature.nps.gov/

climatechange/index.cfm.

At Padre Island National Seashore, beach composition, 

dune height, and washover channels are key geomorphic 

features affecting coastal vulnerability. These features 

determine the relative resistance to erosion. For 

example, beaches having large dunes erode more slowly 

than beaches having low dunes or no dunes (Morton 

2003). Furthermore, lithologic composition and 

hardness (e.g., loose sand vs. compacted shell) influence 

the erodibility of beaches (Morton 2003). 

Investigators determined that 17% of the Gulf of Mexico 

shoreline at Padre Island National Seashore has very 

high vulnerability to future sea-level rise, 28% has high 

vulnerability, 29% has moderate vulnerability, and 26% 

has low vulnerability (Pendleton et al. 2004). Vulnerable 

sections of coastlines are more susceptible to shoreline 

erosion and land loss, saltwater intrusion into aquifers, 

inundation of wetlands and estuaries, loss of cultural and 

historic resources, and damage to infrastructure 

(Pendleton et al. 2004). In areas where beaches and 

wetlands migrate inland to survive elevated sea levels and 

increased storm surge, land managers must consider 

protection and retreat strategies for vulnerable coastal 

resources (Beavers 2005).

On the basis of morphology, investigators divided Padre 

Island National Seashore into three sections: northern, 

central, and southern (Pendleton et al. 2004). This 

tripartite division highlights the significance of the six 

coastal-vulnerability variables and facilitates discussion 

and planning. The area in the national seashore most 

vulnerable to future sea-level rise is the southernmost 

segment (fig. 2). This segment is a “low profile barrier” 

(Morton et al. 2004) consisting of sandy beaches and a 

discontinuous ridgeline of unstable dunes. Numerous 

overwash channels cut across the island in this segment 

of the seashore. With respect to geomorphology, the low 

dunes and washover channels influenced the ranking; 

this part of the seashore also has a history of high 

shoreline erosion.

The gently sloping, sandy Malaquite Beach serves as an 

example of the northern segment of shoreline. Although 

the dunes in this segment of the national seashore are 
6 NPS Geologic Resources Division 

relatively stable and continuous, they are of only 

moderate size. In addition, vehicular traffic suppresses 

dune formation and results in significant shoreline 

erosion. Hence, investigators ranked this part of Padre 

Island National Seashore as highly vulnerable to future 

sea-level rise.

Central Padre Island National Seashore has steep 

beaches consisting mostly of shells. Big Shell Beach 

exemplifies this central segment. Referred to as a “high 

profile barrier” by Morton et al. (2004), the central 

segment has nearly continuous foredunes as much as 15 

m (50 ft) high, many of which are stabilized by 

vegetation. This segment of shoreline ranked either 

moderate (below latitude 27°N [approximately milepost

30]) or low (above latitude 27°N) in shoreline 

vulnerability because of its high dune ridgeline and 

history of shoreline accretion (see “Geologic History”). 

Rates of shoreline change in the various segments of the 

national seashore range from greater than 2 m (0.6 ft) of 

erosion per year to 2 m (0.6 ft) of accretion per year, with 

the greatest accretion occurring at the midpoint of Padre 

Island National Seashore.

Bush and Young (2009) have suggested 7 “vital signs” 

and methods for monitoring coastal features and 

processes: shoreline change; coastal dune morphology; 

coastal vegetation cover; topography/elevation; 

composition of beach material; wetland 

position/acreage, and coastal wetland accretion.

Blowing Sand and Dune Development

Blowing sand is ubiquitous at Padre Island National 

Seashore. Blowing sand can require many hours of 

maintenance. Since the national seashore’s 

establishment, facility managers have employed a variety 

of techniques to mitigate blowing sand, for example, 

planting vegetation and then erecting fences and posting 

signs to protect the plants, posting signs to discourage 

traffic on existing vegetation, and spreading oyster shells 

over barren areas or covering them with hay made from 

native grasses (Weise and White 1980). Today, 

bulldozers continuously scrape windblown sand off the 

paved parking lot at Malaquite Beach. Tongues of sand 

blown by onshore winds creep relentlessly onto park 

roads (fig. 3). Eolian sand routinely engulfs the patrol 

cabin at milepost 42 (Darrell Echols, National Park 

Service, telephone communication, May 1, 2009). 

Nevertheless, from a geomorphic perspective, and from 

many angles of a resource management perspective, 

blowing sand is a “good thing” in that blowing sand 

creates dunes, which protect the island and mainland 

from storm surge (see “Hurricanes”). The barrier island 

provides a buffer zone, absorbing the high wind and 

wave energies of tropical storms and thus lessening the 

impact on the mainland. If the fragile dunes of the barrier 

island are damaged by development, then the first line of 

coastal storm defense is weakened (Boylan 1986).

A major impact from development occurred in 1969 with 

the placement of the Malaquite facilities within the foreisland dune ridge. Contractors removed the foredunes to 

provide space for the buildings and create an ocean view 

for visitors. The Malaquite pavilion had continuous 

structural problems, and Hurricane Allen weakened it in 

1980. Damage fully manifested itself several years after 

this hurricane, when park staff reported patches of 

falling concrete. Saltwater that had infiltrated the 

building during the storm had finally reached the steel 

supports and led to flaking concrete. By January 1986 the 

Malaquite Beach pavilion could no longer withstand 

daily use without being a safety hazard (Jones 1999). 

At this same time, park planners began recognizing the 

necessity of “maintaining the island’s defense” and 

designed new facilities at Malaquite Beach to conform to 

the island’s natural processes, rather than trying to adjust 

them to fit human needs (Jones 1999). Opened in July 

1989, the visitor center at Malaquite Beach was built 

behind the foredune ridge. However, part of the landing 

and the boardwalk were situated within the 

“nonexistent” foredune ridgeline. As the ridgeline has 

begun to reestablish itself, blowing sand continually 

covers the boardwalk, which leads to the beach. Park 

employees have added sand fencing and vegetation to 

help direct sand away from the structure and stabilize the 

dune, but sand continues to fill in around the boardwalk 

and up towards the landing. As stated in a 2001 NPS 

memo, “the dune line is functioning normally so we can 

expect the sand to continue to cover the walkway as long 

as the dune is unvegetated” (Ken McMullen, Padre 

Island National Seashore, memorandum, May 2001).

Short-term solutions to protect the existing walkway at 

the Malaquite Visitor Center include (1) raising the 

height of the existing sand fences to screen more sand, 

(2) vegetating the dune to stabilize blowing sand, and (3) 

extending the sand fence approximately 9 m (30 ft) to 

reduce the amount of sand that is blown around the 

walkway (Ken McMullen, Padre Island National 

Seashore, memorandum, May 2001). For similar 

situations, Conti (2007) proposed vertical barriers made 

of plywood, silt fencing, or geotextiles attached to the 

handrails of elevated walkways as temporary solutions. 

Other simple solutions, though time consuming, are 

shoveling sand off the structure or simply continuing to 

walk over the deposited sand. 

The long-term solution is to relocate the present 

walkway (Ken McMullen, Padre Island National 

Seashore, memorandum, May 2001). Conti (2007) 

suggests the following construction methods, which may 

be applicable to Padre Island:

• Construct walkway, including terminus, above the 

surface of the surrounding dunes.

• Construct walkway with at least a 1:1 height-to-width 

ratio. This promotes growth of vegetation by reducing 

shading.

• Space deck boards at least 1.3 cm (0.5 in) between 

adjacent boards to allow rain, sunlight, and deposited 

sand to reach the underlying surface. 

• Orient the walkway terminus away (leeward) from the 

prevailing wind to reduce sand accumulation. 
PAIS Geologic Resources Inventory Report 7

• Construct walkway away from sources of loose, 

unstable sand such as piles resulting from beach 

maintenance.

Hurricanes

Padre Island has had a history of hurricane events

(table 2). In the Gulf of Mexico, a major hurricane causes 

land losses and substantial property damage about every 

10 years (Hayes 1967; Nummedal et al. 1980; Morton and 

Paine 1985). Making landfall at Port Lavaca, Hurricane 

Carla in 1961 is remembered for its storm surge. In 1967, 

Hurricane Beulah spawned torrential rainfall and more 

than 100 tornadoes. Hurricane Celia in 1970 was 

characterized by extreme wind velocities (Brown et al. 

1976, 1977, 1980). Hurricane Allen, one of the greatest 

hurricanes of the 20th century, stalled and lost strength 

as it approached the Texas coast in August 1980. 

However, the hurricane’s impact was severe along the 

national seashore, where storm surge heights were 

approximately 3 m (10 ft) above mean sea level. The 

myriad effects of Hurricane Allen on Padre Island closely 

followed the pattern of previous hurricanes (Weise and 

White 1980). Hurricane Allen reoccupied more than 60 

washover channels, destroying segments of the main 

road that runs the length of South Padre Island (Morton 

et al. 2004).

Recent storm events include Hurricane Dolly (July 23, 

2008). This storm made landfall on Padre Island and 

caused extensive damage to the town of South Padre 

Island. In July 2010, the island received heavy rains from 

Hurricane Alex, but the storm left the island generally 

unscathed (Barajas 2010). The most-recent, major 

hurricane to hit Padre Island National Seashore was 

Hurricane Bret in August 1999. Originating off the coast 

of Nicaragua, Hurricane Bret peaked in intensity as a 

category 4 storm (on the Saffir-Simpson hurricane 

intensity scale). As it made its turn towards Texas, the 

storm began encountering much cooler sea-surface 

temperatures near the coast; the weakening system made 

landfall in sparsely populated Kenedy County, striking 

the national seashore in the area from milepost 32.5 

south to the milepost 56.8 (National Park Service 2000) 

(see fig. 1). Winds reached 225 km/h (140 mph). The 

hurricane also produced six coastal tornadoes inland as 

far as Burnet County (Roth 2004). The storm surge cut 12 

washover channels into Padre Island; three were major. 

Most hurricanes hitting the Texas coast originate in the 

Caribbean Sea or Gulf of Mexico (Weise and White 

1980). Hurricanes entering or originating in the Gulf of 

Mexico normally follow a northward or westward path. 

Because a hurricane spirals in a counterclockwise 

direction in the Northern Hemisphere, it develops a 

larger storm surge on its right side as it approaches the 

coastline. As such, flooding will be greater in low-lying 

areas to the right of the hurricane as viewed from the 

approaching eye of the storm (Weise and White 1980).

As a hurricane approaches mainland Texas along the 

central Gulf of Mexico coast, barrier islands are the first 

land feature in the path of the surge. Where a welldeveloped, fore-island dune ridge exists (e.g., along Big 

Shell Beach), a barrier island provides a line of defense 

for the back-island and mainland; the barrier helps to 

block the surge and dissipate large amounts of wave and 

current energy (Weise and White 1980). However, storm 

surge produced by hurricanes does not always stop at the 

beach and fore-island area. If a category 2 or stronger 

hurricane makes landfall near Padre Island National 

Seashore, storm surge will likely inundate the cuts 

between the dunes. If water levels rise sufficiently high to 

allow wave attack, dune erosion could be severe (U.S. 

Geological Survey 2008). Furthermore, the surge may 

wash across low-lying segments of barrier islands, 

breaching the fore-island dune ridge and scouring

washover channels (Scott et al. 1969) (see “Storm 

Washover Channels and Fans”). Hurricane intensity and 

associated storm surges are projected to increase as 

climate continues to warm (Karl et al. 2009).

In geologic terms, hurricanes accelerate coastal 

processes so that during the hours of storm passage, the 

degree of erosion and deposition in coastal systems 

amounts to what would normally take months or even 

years (Price 1958; Hayes 1967; McGowen et al. 1970; 

McGowen and Scott 1975; Brown et al. 1976, 1977, 1980; 

McGowen et al. 1977; Morton and McGowen 1980).

Hurricanes produce a gently sloping, nearly planar 

hurricane beach (Hayes 1965). When the hurricane has 

passed, normal processes resume and, in time, a normal 

beach profile is restored. Sand eroded from the beach 

during the hurricane is gradually carried back by fair–

weather wave action to rebuild the normal backshore 

and foreshore. A lowered backshore may persist for 

several years between hurricanes because it is outside the 

influence of normal wave actions (Hill and Hunter 1987). 

Longshore drift smoothes irregularities in the Gulf of 

Mexico shoreline and builds bars and beaches across the 

mouths of the hurricane channels (Weise and White 

1980). 

Benthic Habitat Mapping

Padre Island National Seashore contains 42 km2 (16 mi2) 

of marine resources. That is, the national seashore’s 

eastern boundary extends approximately 0.4 km (0.25 

mi) into the Gulf of Mexico. The characteristics of this 

offshore region are only generally known, however, 

because mapping has not been completed. Mapping of 

the area would clarify the land-water interface; marine 

relationships to terrestrial features; bathymetric features, 

such as navigation hazards; seafloor features that 

influence waves, currents, and storm surge; the areal

extent and health of marine resources, including seafloor 

habitat and species; sediment characterization and 

quality as habitat; and the location and assessment of 

shipwrecks, anthropogenic debris, and marine 

archaeological artifacts and sites.

To date, two investigators have compiled preliminary 

data for use in a future benthic habitat map for Padre 

Island National Seashore. In 2001, Dr. Jennifer Rahn 

from Baylor University, an NPS Geoscientist-in-thePark, completed an assessment of GIS data covering 

coastal processes and geomorphology along Padre Island 
8 NPS Geologic Resources Division 

National Seashore, with particular emphasis on the 

effects of U.S. Army Corps of Engineers projects (Lisa 

Norby, Geologic Resources Division, e-mail message, 

August 24, 2009). These data are retained in the Natural 

Resources Division office at Padre Island National 

Seashore. In addition, GeoCorps America researcher 

Lisa Fay completed an assessment of existing geospatial 

data for Padre Island National Seashore in June 2008. 

These data are retained in Geologic Resources Division 

files in Lakewood, Colorado. 

The National Park Service has collaborated with partners 

to produce detailed benthic-habitat classification maps 

for some of the marine areas within and adjacent to 

National Park System units. This is a multidisciplinary 

task that combines geological, biological, oceanographic, 

and chemical components. The intent of such projects is 

to facilitate the management, interpretation, and 

understanding of park resources by providing baseline 

maps, a GIS database, and descriptions of the biological 

and geological resources of marine lands (Gibbs et al. 

2007). In 2007 the U.S. Geological Survey collaborated 

with the National Park Service to create benthic-habitat 

classification maps for Pu‘ukoholā Heiau National 

Historic Site and Kaloko-Honokōhau National 

Historical Park in Hawai‘i, using existing color aerial 

photography, scanning hydrographic operational 

airborne LIDAR (light detection and ranging) survey 

(SHOALS), georeferenced underwater video, and still 

photography (Cochran et al. 2007; Gibbs et al. 2007). 

Collaborators within the National Park Service for such a 

project for Padre Island National Seashore would 

include staff from the national seashore, Geologic 

Resources Division, Gulf Coast Network, and Ocean and 

Coastal Resources Branch of the Water Resources 

Division.

Paleontological Resources

Investigators have not conducted any formal field-based 

paleontological resource surveys for Padre Island 

National Seashore (Kenworthy et al. 2007). However, the 

national seashore’s museum collection contains 13 

paleontological specimens that either park staff or 

visitors found within the national seashore; most 

specimens had washed ashore (Kenworthy et al. 2007). 

Museum specimens include bone fragments of 

mammalian limbs, a mammalian acetabulum (pelvis), 

lower molars of a bison and horse, mammoth teeth or 

tooth plates, possible remains of a giant tortoise, and a 

number of unidentifiable bone fragments. The age of the 

specimens ranges from Pleistocene (2.6 million to 11,700 

years ago) to Holocene (11,700 years ago to present) 

(Kenworthy et al. 2007). 

While the island itself is not a major source of 

paleontological resources, fossils from the offshore 

Pleistocene Seven and One-Half Fathom Reef have 

washed ashore. Although this reef is submerged outside 

the national seashore’s boundaries, it is likely the source 

of most of the seashore’s paleontological resources. The 

reef—interpreted by Thayer et al. (1974) as a late 

Pleistocene intermittent lake deposit—is approximately 

50 m (164 ft) wide and 350 m (1,148 ft) long, having a 

maximum relief of 5.4 m (18 ft). It is located 3.2 km (2 mi) 

offshore, 74 km (46 mi) south of the national seashore’s 

northern entrance in approximately 14 m (46 ft) of water 

(Thayer et al. 1974). These lake sediments were 

submerged as sea level rose during the Holocene Epoch. 

As a result of their relative hardness (well-cemented 

sandy marl), they now form an underwater ridge.

In addition, Lundelius (1972) described the Pleistocene 

Ingleside fauna from a fossil locality near Corpus Christi. 

This locality is one of the largest and most diverse known 

in Texas and contains 42 vertebrate taxa, including 

mammals, birds, reptiles, amphibians, and fish 

(Lundelius 1972). Although this deposit does not occur 

within Padre Island National Seashore, these fauna 

illustrate the diversity of paleontological resources in the 

vicinity of the national seashore. McBride and Honda 

(1994) described exposures of the Ingleside Barrier 

Complex along the western margin of Laguna Madre,

8 km (13 mi) south of Baffin Bay. These exposures—

beach sandstone contemporaneous with the Ingleside 

fauna—contain fossils in the form of a shell coquina.

Miller and Hunter (1979) recovered Pleistocene shells 

from subsurface lagoonal deposits within Padre Island 

National Seashore. Radiocarbon dates of dwarf surf clam 

(Mulinia lateralis) from two wells—approximately 21 m 

(69 ft) below sea level—ranged in age from 29,980 to 

27,380 years before present (Miller and Hunter 1979). 

The deposit contains a fossil shell layer 8–15 m (26–49 ft) 

below modern sea level in the southern part of Padre 

Island and about 19 m (62 ft) below modern sea level in 

the northern reaches. This subsurface layer hosts more 

than 90 species of marine invertebrates and one 

terrestrial snail. 

Spoil mounds associated with dredging of local channels 

may also host Pleistocene fossils, coquina, or serpulid 

(worm) reef material. Although obviously not in situ, the 

piles provide an opportunity to view the subsurface 

sediments and paleontological resources of Padre Island 

National Seashore’s Laguna Madre (Kenworthy et al. 

2007).

Continued discovery of fossils on the shoreline of Padre 

Island is very probable (Kenworthy et al. 2007). Although 

modern, unoccupied shells can be collected within Padre 

Island National Seashore, collecting paleontological 

resources of any type is not permissible. This dichotomy 

can cause some confusion among the visiting public. 

Hence, Kenworthy et al. (2007) recommended continued 

interpretation and education because distinguishing 

between modern and fossil shells can be difficult. Shell 

collecting may bring newly discovered paleontological 

specimens to light, and education may decrease illegal

(though typically unintentional) collecting. In addition, 

Kenworthy et al. (2007) recommended continuing to 

maintain the museum collection, increasing staff 

awareness of material that may wash ashore, retaining 

potentially identifiable material to be included in the 

collection, and consulting local experts for identification.

The 2009 Paleontological Resources Preservation Act 

directs the Secretaries of Interior and Agriculture to 
PAIS Geologic Resources Inventory Report 9

implement comprehensive, science-based resource 

management programs for fossils, which are nonrenewable. The NPS and other federal land managing 

agencies are developing joint regulations associated with 

the Act.

Bird Island Basin

In 2000, 71,700 people visited Bird Island Basin (Withers 

et al. 2004) because of its “fantastic opportunities for 

windsurfing, kayaking, boating, and fishing” (National 

Park Service 2010b). Additionally, camping sites, both 

recreational vehicle and tent, are available in the area. 

Human use has modified the landscape and affected the 

geologic processes operating at Bird Island Basin. 

Processes requiring management attention are shoreline 

erosion, infilling and sedimentation of the boat basin, 

erosion via runoff, and reduction in sand supply. Withers 

et al. (2004) suggested alternatives to mitigate adverse 

effects on geologic processes while accommodating a 

variety of recreational activities. Since 2004, park 

managers have ameliorated many of the geologic and 

visitor-use problems by upgrading the infrastructure at 

Bird Island Basin. However, two issues still require 

attention: restoring the shoreline adjacent to the boat 

ramp and fully opening the overwash area from the 

channel into the mudflat west of Bird Island Basin 

(Darrell Echols, National Park Service, e-mail message, 

July 6, 2009). 

Shoreline Erosion

Investigators have attributed some shoreline erosion to 

vehicular traffic and parking on the shore, at least locally 

(National Park Service 1995; Withers et al. 2004; SmithEngle et al. 2006). The area is being compressed by the 

weight of the vehicles and eroded away by the wave 

action in the lagoon (National Park Service 1994).

Sedimentation

The boat basin, which did not exist before 1964, is an 

artificial depression created via dredging. An isolated 

island just south of the basin and a small mound situated 

along the boat channel slightly west of the isolated island 

were built up by the dredged material. By 1969 a windtidal flat had prograded sufficiently as a result of 

increased eolian transport from drought and grazing to 

tie the small spoil mound to Padre Island; it remains 

connected today (Smith-Engle et al. 2006). 

Runoff

During storms, runoff from the elevated road causes 

erosion (i.e., gullying). Investigators suggested using 

either some kind of reinforcement, such as a geotube, 

along the slope edge or physically adjusting the slope to 

direct most runoff back toward the wetland landward of 

the road (Smith-Engle et al. 2006). 

Altered hydrologic conditions between Laguna Madre 

and the wind-tidal flat behind the shoreline require 

geologic engineering solutions. Creation of a raised 

beach profile west of the road would likely block the 

exchange of water, unless low points are incorporated at 

selected locations (Joel Wagner, Water Resources 

Division, written comment on Smith-Engle et al. 2006, 

June 5, 2006).

Beach Renourishment

Reduction in sand supply (and corresponding 

elimination of the beach) is a byproduct of the natural 

recovery of the island from overgrazing (see “Grazing”). 

The beach is an artifact of prior land use and is not viable 

without artificial maintenance (Rebecca Beavers, 

Geologic Resources Division, written comment on 

Smith-Engle et al. 2006, June 5, 2006). NPS management 

policies provide justification to renourish the beach at 

Bird Island Basin with the purpose of maintaining an 

altered shoreline in order to protect present 

development in the short run and focus high-density 

visitor use there, thus minimizing visitor impacts 

elsewhere in the short run. See section 4.8.1.1 of 

Management Policies 2006 (National Park Service 2006a). 

Over the long term, it may be advisable for the park to 

seek a waiver of the NPS policy to allow shoreline 

processes to continue without interference. Factors 

supporting a waiver request include “the park’s enabling 

statute which mentions ‘public recreation’ in the first 

sentence, the size of this shoreline at the time of the 

park’s establishment, the high level of public use at this 

area, consistency with park planning documents and 

objectives, and the park’s overall commitment to 

protection of natural processes” (Julia Brunner, Geologic 

Resources Division, written comment on Smith-Engle et 

al. 2006, June 5, 2006). 

Four geologic factors would require consideration 

before renourishment commenced: (1) sediment (sand) 

size, (2) volume of sediment needed, (3) source of 

sediment, and (4) location/placement of sediment 

(Smith-Engle et al. 2006, 2007). Using fine-grained sand 

would mimic the existing geomorphic environment and 

provide the substrate to which users are accustomed. 

Using medium-grained sand would require less volume 

to achieve the same lifespan (table 3); however, the 

source of medium-grained sand is not local, so cost of 

materials and transport is a factor. In addition, mediumgrained sand is “nonnative,” with the potential for 

introducing invasive species in the imported sand 

(Courtney Schupp, National Park Service, written 

communication, September 30, 2009).

According to Smith-Engle et al. (2006), the optimal 

placement of sand is between the peninsula to the north, 

which was created by dredge material from the boat 

basin, and the entrance road to the south in the area of 

heaviest visitor use.

Vehicle Impacts

At the end of World War II, Patrick Dunn (see 

“History”) purchased an army surplus truck, which he 

converted to a “bobtail” (Sheire 1971). This short, 

straight, four-wheel-drive (4WD) vehicle could move 

easily across the beaches of Padre Island. It changed the 

style of the cattle roundup, cutting the time necessary to 

gather the herd for market and reducing weight loss of 
10 NPS Geologic Resources Division 

the cattle by eliminating the month-long cattle drive up 

the island. 

After 1971, cattle left the island, but 4WD vehicles used 

for other purposes stayed. The national seashore came 

into existence when personal ownership of 4WD 

vehicles was not widespread and exploring the beach in a 

“dune buggy” was considered a unique experience and 

rare treat (National Park Service 1973). Driving on 

beaches has become a common occurrence, and 101 km 

(63 mi) of the national seashore’s beaches are open to 

vehicle traffic (fig. 4). Only 7.2 km (4.5 mi) of beach are 

off limits to vehicular traffic (i.e., the area between the 

North Beach and South Beach access roads, including 

Malaquite Beach). Beaches in the park are subject to 

Texas highway laws and regulations. All vehicles must be 

street legal and licensed. A high priority data need for 

Padre Island National Seashore is information about 

impacts of beach driving on park resources (Darrell 

Echols, Padre Island National Seashore, GRD technical 

assistance update, March 30, 2007). 

Primarily in the 1970s and 1980s, various studies at Padre 

Island investigated this issue and found that beach 

driving 

• Inhibits shoreline accretion and decreases the island’s 

overall width (Behrens et al. 1976; Weise and White 

1980).

• Increases shoreline erosion and instability during 

storms (Baccus and Horton 1979; Weise and White 

1980).

• Eliminates the natural rebuilding capacity of the 

foredune system between storms (Behrens et al. 1975).

• Changes dune forms, creating plateaulike foredunes 

and lower elevations (McAtee 1975; McAtee and 

Drawe 1980).

• Inhibits formation of “embryo dunes” (coppice dunes) 

in front of the foredune ridge, tending to make the 

beach zone wider (McAtee 1975; McAtee and Drawe 

1980).

• Creates blowouts by destroying vegetation that 

stabilizes dunes (Baccus and Horton 1979; Blum and 

Jones 1985).

• Reduces vegetation height, percentage cover, and the 

number of plant species present (McAtee 1975; 

McAtee and Drawe 1980).

• Changes species density and richness of vegetation and 

promotes the invasion of exotic species (Behrens et al. 

1976; Baccus and Horton 1982; Lonard et al. 1999).

• Affects fauna (Baccus et al. 1977), including the 

endangered Kemp’s ridley turtle (Lepidochelys kempii) 

(Echols and Kassman 2004).

• Changes the distribution of invertebrates such as ghost 

crabs (Ocypode quadrata) (Teerling 1970).

• Impacts benthic invertebrates and their habitat 

(Wicksten et al. 1987), which are an important food 

source for endangered piping plover (Charadrius 

melodus) (Dansby et al. 2008).

• Changes microenvironmental parameters (McAtee 

and Drawe 1981). As the intensity of human activity 

increased, elevation, especially of areas near the beach, 

decreased. Near the ground surface, average wind 

velocities, evaporation, atmospheric salinity, and 

wind-carried sand particles increased, as did soil 

salinity, soil pH, average soil temperature and range in 

temperature, soil bulk density, and soil moisture (with 

increasing depth) (McAtee and Drawe 1981).

The primary geomorphic processes affected by vehicular 

(and pedestrian) traffic are wind erosion, transport, and 

deposition. The mean annual wind speed at Padre Island 

National Seashore is 19 km/h (12 mph) (Hill and Hunter 

1976). Where exposed beaches are crossed by winds that 

exceed 14 km/h (9 mph), sand is moved off the beach 

(Godfrey 1976). Wind velocities decrease near the 

ground surface, so sand lifted into the air is exposed to 

greater wind action. Vehicle wheels and foot traffic lift 

sand above the surface, exposing it to greater wind 

velocities and carrying it greater distances than under 

normal wind stress (Baccus and Horton 1982). 

Vegetation helps baffle the wind higher above the 

ground surface, entrapping more sand. However, 

vehicles and foot traffic crush and dislodge beach 

vegetation, destroying this capacity of plants to capture 

sand. As a result, areas with vehicular traffic have much 

smoother, flatter, and broader backbeach areas than

those without vehicular traffic (Weise and White 1980). 

Such grading of beaches is detrimental to embryo 

(coppice) dune formation, as well as to pioneering and 

colonizing plant species (Baccus and Horton 1982). 

Park managers began to anticipate the need to restrict 

access of 4WD vehicles to beaches and roads, prohibiting 

driving cross-country and over dunes. The 1973 master 

plan for the national seashore states, “Vehicle traffic may 

build to such an extent in the future that management 

will prohibit vehicles completely or permit them only on 

a reservation basis” (National Park Service 1973, p. 19) 

Padre Island National Seashore does not yet have a 

management plan for this popular activity. However, 

managers have established regulations that allow driving 

along the beach and within designated 4WD routes (i.e., 

Yarborough Pass and adjacent to Mansfield Channel) 

(Darrell Echols, National Park Service, e-mail message, 

July 6, 2009).

Considerations in the development of a management 

plan include the sensitivity of various environments; that 

is, some zones may be totally off limits, while others 

allow for increasing amounts of pedestrian or vehicular 

traffic. The digital geologic map may be useful in 

identifying these areas (see “Map Unit Properties 

Table”). For instance, damage to wind-tidal flats (i.e., 

map units L1, L2, and L3) may be irreparable. Tire tracks 

and footprints left more than 20 years ago are still 

present in some parts of wind-tidal flats (National Park 

Service 2006b). Sensitive areas also include marshes 

(units B4a, B4b, B4c, and B4d), foredunes (unit B3), 

coppice dunes (unit B2), other active dunes (unit B10), 

stabilized dunes (unit B5a), and vegetated barrier flats 

(unit B4) (Drawe and Ortega 1996; Withers 1996). 
PAIS Geologic Resources Inventory Report 11

Although past studies have demonstrated the detrimental 

effects of vehicular traffic on the geomorphic, biologic, 

and microclimatologic processes and features in the 

national seashore (see above), park employees have 

identified specific questions about vehicular impacts that 

need scientific answers (Darrell Echols, Padre Island 

National Seashore, GRD technical assistance update, 

March 30, 2007):

• At what level of activity (i.e., number of vehicles) does 

ecological decline begin?

• What specific factors affect decline (e.g., vehicle 

weight or the number of passes within the same area)?

• What is the effect of tire ruts (puddles and channels) 

on the susceptibility of the system to erosion?

Managers want to ensure that any decisions regulating 

vehicle use are legally defensible and based on scientific 

data.

Dredging

Dredging is excavating in coastal environments, 

generally at least partly underwater. The process creates 

channels that make shallow waterways navigable or cuts 

passes through islands to connect water bodies. 

Dredging creates spoils (excess material), which are 

either disposed of in open water to the side of the 

channel or conveyed to a location away from the 

dredged area, for example, to a beach for renourishment. 

The environs of Padre Island National Seashore have 

been subject to dredging in many places. 

Gulf Intracoastal Waterway

The Gulf Intracoastal Waterway, an artificial channel, 

extends the length of Laguna Madre, connecting Corpus 

Christi, Port Mansfield, Port Isabel, Port of Harlingen, 

and Brownsville. The waterway is 3.6 m (12 ft) deep and 

38 m (125 ft) wide, cutting through the 209-km-

(130-mi-) long lagoon. The U.S. Army Corps of 

Engineers constructed the channel in 1949 and maintains 

it through periodic dredging. Construction of the 

channel through the wind-tidal flats altered 

hydrodynamics by reducing both the frequency and 

duration of flooding (Rusnak 1960). Before dredging, 

strong or persistent winds from either the south or north 

would force water onto the flats, where it would remain 

until either gradually draining back into the permanently 

flooded part of Laguna Madre or evaporating. However, 

since its construction, the Gulf Intracoastal Waterway 

has provided an alternate route for the wind-driven 

water, and some water flows through the dredged 

channel rather than onto the flats (Breuer 1957; Rusnak 

1960). Moreover, water that is driven onto the flats 

probably remains a shorter time because it can drain into 

the waterway rather than having to flow back across the 

flats (Courtney Schupp, National Park Service, written 

communication, September 29, 2009).

In addition, oil and gas operators dredged several 

shallow channels across Laguna Madre from the Gulf 

Intracoastal Waterway to access hydrocarbon well sites 

on the wind-tidal flats. Although these channels are not 

currently maintained by the operators, they are still 

present on the landscape. These channels are conduits 

for water flowing onto or off the flats. Field evidence 

indicates that the channels act primarily as drains: water 

draining off the flats erodes branching channels at the 

heads of the channels and locally lowers elevations of the 

surrounding flats (Morton and Holmes 2009).

As a result of the original construction and periodic 

maintenance of the Gulf Intracoastal Waterway, the U.S. 

Army Corps of Engineers has repeatedly disposed of 

dredge material (i.e., silt mixed with mud) in the open 

waters of Laguna Madre. Over the years, the dredge 

spoils have created banks stretching the length of the 

waterway. When surveyed in the 1970s, 109 spoil islands 

lined the waterway; an additional 40 were located along 

privately owned channels (Chaney et al. 1978). 

Currently, the U.S. Army Corps of Engineers uses 61 

“placement areas” (i.e., spoil islands) along the main 

channel (U.S. Army Corps of Engineers 2003). Multiple 

islands make up each placement area, ranging from one 

to four islands (Darrell Echols, National Park Service, email message, July 6, 2009). Ten of these placement areas 

are within the boundaries of Padre Island National 

Seashore (Darrell Echols, National Park Service, e-mail 

message, December 31, 2003). The spoil mounds (see 

“Map Unit Properties Table,” units M1, M2, and M3) 

have become habitat for 25 species of colonial nesting 

waterbirds, including American white pelican (Pelecanus 

onocrotalus), great blue heron (Ardea herodias), reddish 

egret (Egretta rufescens), interior least tern (Sterna 

antillarum athalassos), sooty terns (Onychoprion 

fuscatus), and laughing gulls (Leucophaeus atricilla). 

In the 1990s, the effects of dredging within Laguna 

Madre became a concern for conservation groups and 

sport-fishing enthusiasts (e.g., National Audubon Society 

and the Lower Laguna Madre Foundation) in the region. 

The groups’ primary concern was that maintenance 

dredging in such a shallow lagoon was destroying 

seagrass beds, which are critical to the lagoon’s waterbird 

habitat, fisheries, and overall biological productivity. The 

National Park Service was concerned about the effects of 

turbidity and contaminants from the dredge sediments. 

In 1994, the National Audubon Society and its partners 

filed suit against the U.S. Army Corps of Engineers. The 

court ordered preparation of an environmental impact 

statement (EIS) and that all maintenance dredging stop 

(save for emergency dredging) in the interim. In 

response, the U.S. Army Corps of Engineers formed an 

interagency coordinating team (ICT) to review the 

effects of dredge disposal on Laguna Madre. Between 

1993 and 2003, researchers completed 35 studies as part 

of the Laguna Madre Project, including the effects of 

dredge deposits on seagrass beds, wind-wave 

resuspension and circulation of sediment and dredge 

material, the effects of dredge material on piping 

(Charadrius melodus) and snowy (C. alexandrinus) 

plovers, temporal and spatial effects of the disposing of 

dredge material in open water on habitat of fishery and 

forage organisms, sediment budget analysis, benthic 

macroinfaunal analysis of areas where dredged material 

is placed, analysis of water and sediment quality, and 
12 NPS Geologic Resources Division 

tissue chemistry analysis (U.S. Army Corps of Engineers 

2005). 

The U.S. Army Corps of Engineers, along with the 

National Marine Fisheries Service, U.S. Fish and Wildlife 

Service (serving as the lead for the U.S. Department of 

the Interior), U.S. Environmental Protection Agency, 

Texas Parks and Wildlife Department, Texas General 

Land Office, Texas Water Development Board, Texas 

Department of Transportation, and Texas Natural 

Resource Conservation Commission (now the Texas 

Commission on Environmental Quality), formed the 

interagency coordinating team. The U.S. Coast Guard, 

National Park Service (Padre Island National Seashore), 

and Coastal Bend Bays and Estuaries Program served in 

secondary, advisory roles. 

The ICT process resulted in the release of a final EIS (i.e., 

a 50-year dredge disposal plan). Some felt that more 

creative solutions could have resulted from the process 

(Texas Center for Policy Studies 2001). Nevertheless, the 

final EIS did address some of the National Park Service’s 

concerns. The plan included the use of training levees to 

contain the dredge material for a short period of time. 

Once the sediment had settled out, the levees would be 

removed. This would help nesting shorebirds continue 

to use the islands because they could see the water and 

be aware of predators if the levees were removed (Darrell 

Echols, National Park Service, e-mail message, July 6, 

2009). However, the National Park Service remained 

concerned that the dredging intervals suggested in the 

plan could impact seagrass beds in areas of open 

disposal; some research based on the estimated timelines 

for revegetation indicated that seagrass beds could be 

impaired. The U.S. Army Corps of Engineers addressed 

these concerns by stating that it would monitor the beds 

after disposal (Darrell Echols, National Park Service, email message, July 6, 2009).

Mansfield Channel

Two artificial passes, Mansfield Channel and Brazos 

Santiago Pass, connect Laguna Madre to the Gulf of 

Mexico. Brazos Santiago Pass is south of Padre Island 

National Seashore in Cameron County. Mansfield 

Channel cuts through Padre Island and marks the 

southern extent of Padre Island National Seashore

(fig. 1). The channel provides access to the Gulf 

Intracoastal Waterway and the fishing village at Port 

Mansfield. In September 1957 the Willacy County 

Navigation District completed the channel, but storms 

destroyed the pass and associated jetties in November of 

that same year. The U.S. Army Corps of Engineers 

completed the present channel and jetties in 1962 

(Leatherwood 2008). Most recently, the Corps dredged 

the Mansfield Channel in April 2009. The dredge 

material was used to renourish beaches eroded during 

Hurricanes Dolly and Ike (2008) on the north side of the 

channel at the southernmost point of Padre Island 

National Seashore (Christensen 2009).

Besides facilitating boating access between Laguna 

Madre and the Gulf of Mexico, a long-standing 

justification for connecting the lagoon with the Gulf of 

Mexico has been to reduce or moderate high salinities in 

the lagoon, an action thought to nurture more abundant 

fish and shrimp populations, such as redfish/red drum 

(Sciaenops ocellatus), brown shrimp (Farfantepenaeus 

aztecus), flounder (Paralichthys lethostigma and

P. abligutta), and speckled trout/spotted seatrout 

(Centropomus nebluosus), thereby enhancing the sport 

and commercial fishing economy of Port Mansfield 

(Leatherwood 2008). However, given high rates of 

longshore drift of sediments, blowing sand, low tidal 

range, and no freshwater inflow, dredging the channel is 

a continuous process with high maintenance costs 

(Tunnell et al. 2002). Along the coast of Texas, generally 

only one natural pass per lagoon or bay system can be 

maintained by natural processes. Additional passes (i.e., 

artificial channels) reduce the tidal exchange through 

existing ones, necessitating increased dredging to 

maintain them (Brown et al. 1976, 1977, 1980). In 

addition, artificial channels alter the natural circulation 

pattern, allow greater tidal exchange between the Gulf of 

Mexico and Laguna Madre, and subject protected 

lagoonal waters to greater effects from storm surges 

(Morton 1994; Tunnell et al. 2002).

Yarborough Pass

Yarborough Pass, at milepost 15 between Little Shell and 

Big Shell beaches, is an example of a failed channel 

dredged repeatedly through Padre Island. In 1941, the 

Texas Game and Fish Commission excavated this 

channel in an attempt to provide an exchange between 

the Gulf of Mexico and Laguna Madre to reduce the 

high salinities in the lagoon’s north-central section. The 

channel remained open for five months (Breuer 1957). 

Other unsuccessful attempts to create a channel were 

made in 1942, 1944, and finally 1952. Today, Yarborough 

is often flooded and filled with exceptionally deep and 

soft sand. It serves as one of two four-wheel-drive 

(4WD) areas running perpendicular to the island; the 

other is along Mansfield Channel.

Oil and Gas Operations

On March 23, 1951, Sun Oil Company (now Sunoco), 

headquartered in Philadelphia, Pennsylvania, drilled the 

first gas well within the current boundaries of Padre 

Island National Seashore. Since then other companies 

have explored for, and produced oil and gas from, the 

hydrocarbon reservoirs beneath Padre Island (fig. 5). 

The source rocks are the Oligocene (33 million to 28 

million years old) Vicksburg and Frio formations (deltaic 

sandstone) and the lower and middle Miocene (23 

million to 11 million years old) Fleming Formation 

(coastal-plain and continental-shelf clay and sandstone). 

In 1999, the U.S. Geological Survey estimated the total 

mean amount of undiscovered oil and gas within Padre 

Island National Seashore to be 52.67 million m3 (65.74 

billion ft3) of gas and 40,776 m3 (1.44 million ft3) of 

natural gas liquids (Norby 2008). 

Oil and gas operations are potential threats to park 

resources through accidental leaks and spills of 

hydrocarbons from pipelines and producing wells, 

groundwater contamination resulting from the injection 
PAIS Geologic Resources Inventory Report 13

of produced water into deep formations, and external oil 

spills (Withers et al. 2004).

Authorized operators must follow regulations in 36 Code 

of Federal Regulations (CFR), Part 9, Subpart B (“9B 

regulations”), which require each oil and gas operator to 

develop a plan of operation that outlines the specific 

location, process, and protection measures that it will 

employ. The National Park Service evaluates the 

submitted plan and determines whether operations will 

affect park visitors or resources and, if so, how to 

eliminate, minimize, or mitigate impacts.

Park policy requires that operators use best available

technology. The National Park Service reviews and 

updates established mitigation measures, evaluates past 

practices, applies current research findings, incorporates 

public comment, and coordinates with partners to 

preserve and protect park resources. These actions help 

provide an enjoyable visitor experience and protect park 

natural and cultural resources while allowing 

congressionally mandated access to nonfederal oil and 

gas resources (National Park Service 2006d).

The National Park Service has developed many 

mitigation measures to minimize or eliminate the impacts 

on park resources and visitor experiences. Mitigation 

measures include the following:

• Limiting the maximum speed limit of oil and gas 

vehicles to 24 km/h (15 mph) and private vehicles to 40 

km/h (25 mph) throughout the national seashore.

• Limiting the maximum number of trucks that can be in 

the national seashore each day.

• Prohibiting operation of oil and gas equipment along 

the beach at night.

• Requiring all oil and gas equipment to convoy as a 

group escorted by an NPS-trained observer of the 

endangered Kemp’s ridley turtle (Lepidochelys kempii).

• Establishing a 152-m (500-ft) buffer around permanent 

freshwater ponds.

Shoreline Debris

Without modern lumberyards in Corpus Christi to 

frequent, Padre Ballí, Patrick Dunn, and others who built 

structures on Padre Island in historical times turned to 

the island itself for building materials. According to 

Sheire (1971), the Gulf of Mexico currents delivered a 

constant supply of hard and softwood boards of all 

lengths and widths for huts, kitchens, fences, and corrals. 

Furthermore, the sea provided furniture:

Barrels with the tops removed and filled with sand became 

small stoves. Stools and chairs came in and found their way 

to the camps. Indeed, it is said Dunn furnished his 

headquarters home with furniture from the Nicaragua, an 

unlucky Mexican ship that stranded on Padre in 1912 or 

1913. Tar barrels floated in and were melted to close holes. 

Other barrels collected rain water. Ropes of all sizes were 

used for a variety of purposes (Sheire 1971, p. 47).

The seashore’s 1973 master plan highlights “fascinating” 

and “unusual” debris that drifts onto the beaches, which 

visitors delighted in finding. In the 1990s the tone 

towards the island’s generosity of offering up useful and 

interesting objects changed when sofas, refrigerators, 

televisions, chairs, buoys, and all types of plastics began 

to appear (National Park Service 1994). Mixed with 

benign items are ones potentially dangerous to humans 

and wildlife: fishing line, six-pack holders, nets, plastic 

bags, and hard plastics have killed turtles, birds, and 

other marine life (National Park Service 1994). Also, 

washed up on the beach are drums of toxic waste, 

medical supplies, military ordnance, broken glass, and 

sharp metal objects (National Park Service 1994). 

The primary reason for trash accumulation on Padre 

Island is the convergence of currents along the Gulf of 

Mexico shoreline; thereby, the driver is geologic. The 

Loop Current enters the Gulf of Mexico through the 

Yucatan Channel and exits through the Florida Straits, 

moving north-northeast. Gyre Currents begin moving 

southward in October. These currents trap and later 

deposit trash onto the South Texas coastline, primarily 

around latitude 27°N. Significant for barrier-island 

development, this segment of the barrier island is the 

recipient of a continuous supply of sand and shell eroded 

from adjacent coastal sectors and transported by waves 

and longshore currents (Morton 1994). However, these 

currents also carry trash. Where currents collide, 

prevailing southeasterly winds push the debris onto 

Padre Island beaches. 

The amount of trash on the beach is directly related to 

the strength, direction, and duration of the wind and the 

height of incoming tides, and, of course, human factors, 

such as the amount of trash that has been dumped into 

the Gulf of Mexico. Winds and tides also affect the 

movement of trash across the beach, from the shoreline 

to the backbeach and dunes. Old trash accumulates and 

becomes buried in the sand of the backbeach zone, for 

example in coppice dunes. Subsequent storms and high 

tides, combined with offshore winds, uncover and 

transport trash from the backbeach into the forebeach 

zone. Depending on the wind speed, direction, and 

duration, trash can be blown back into the Gulf of 

Mexico and subsequently float with the currents until 

redeposited onto the shoreline with the incoming high 

tides (Miller and Jones 2003). 

According to Miller and Jones (2003), the most costeffective and long-term solution to the beach-trash 

problem is to identify the sources and eliminate illegal 

dumping. The amount of garbage washing onto beaches 

at Padre Island National Seashore will not be reduced, 

nor will the overall problem in the Gulf of Mexico be 

adequately addressed, unless point sources are 

identified, violators are educated, and regulations such as 

the Marine Plastic Pollution Research and Control Act of 

1988 (MARPOL V) are enforced.

Another geologically related issue concerning shoreline 

trash is the public perception that the oil and gas 

industry, in particular offshore platforms and rigs, is the 

primary culprit in dumping trash into the Gulf of Mexico 

(Ditton and Gramann 1987). However, according to a 

10-year (1989–1998) study of shoreline trash, this is 
14 NPS Geologic Resources Division 

actually not the case (Miller and Jones 2003). 

Investigators of this study concluded that 9.2% of Padre 

Island trash originates with offshore oil and gas 

operations. By contrast, the shrimping industry was the 

primary violator and source of 80.8% of the trash 

collected according to the study. 

Although not the first of its kind, the 10-year study, 

funded by six government agencies and trade groups, is 

the most exhaustive (Hart 2003). From March 1994 to 

February 1996 and March 1997 to February 1998, park 

crews collected trash daily from a 26-km (16-mi) stretch 

of shoreline in Padre Island National Seashore. Crews 

collected 376,396 pieces of trash, not including 

fragments. Researchers focused their analysis on 43 

categories of debris, which they established during 

earlier surveys as the most commonly found along the 

shore. 

Improvements to future studies would include working 

with port authorities and the Coast Guard to identify as 

many point sources as possible, then gathering 

distinctive items for each source, as well as determining 

how to categorize items that are not exclusive to any one 

source (e.g., bleach bottles and aluminum cans). Sources 

may also be terrestrial, such as coastal campsites and 

upland urban debris transported via runoff and flooding.

Grazing

Padre Island’s coastal prairies, absence of chaparral, and 

natural boundaries for containing cattle made it an ideal 

location for cattle operations (National Park Service 

1973) (fig. 6). Hence, for 166 years (1805–1971), Padre 

Island was cattle range:

• 1805–1846: Padre Nicolas Ballí cattle operation.

• 1846–1876: Various Mexican and American cattle 

operations, which provided roasts and steaks as 

military “mess” during the United States–Mexican 

War and Civil War; Richard King and Mifflin Kenedy 

ran cattle operations on the island from 1870 to 1886 

(Callaway 1972).

• 1879–1971: Dunn family cattle operation.

Prior to cattle ranching, Padre Island was “as green as a 

garden”—an observation made by entrepreneur and 

rancher Mifflin Kenedy when he first saw the island 

(Price and Gunter 1942). Padre Island typically had a 4.5-

to 15-m- (15- to 50-ft-) high, densely vegetated, foredune 

ridge behind the beach complex. In addition to grasses, 

vegetation included live oak (Quercus fusiformis) and 

brush (Price 1987). With vegetative cover depleted as a 

result of overgrazing, once-stabilized dunes moved 

across the landscape. Severe droughts also contributed to 

increased eolian material and transport across the windtidal flats and sedimentation into Laguna Madre (Price 

and Gunter 1942), but overgrazing was the main factor 

that resulted in prevailing winds blowing sand westward 

into the estuary (Prouty 1989).

The primary disturbances resulting from cattle grazing 

are changes in vegetation (species composition and 

percentage cover), destabilization of eolian features, and 

increased sedimentation onto the wind-tidal flats and 

into Laguna Madre. Changes in vegetation led to changes 

in fauna (Rabalais 1977).

Although Prouty (1989) noted that the island was 

significantly revegetated during the 1970s and 1980s with 

higher rainfall and the elimination of grazing, various 

investigators have found that even after cattle were 

removed from the island, vegetation has failed to 

advance to its expected climactic stage (Kattner 1973; 

Drawe 1990), that is, mid- or even tall-grass climax 

vegetation dominated by seacoast bluestem (Andropogon

spp. and Schizachyrium spp.), bushy bluestem 

(Andropogon glomeratus), and gulf beach panicum 

(Panicum amarum) (Drawe and Ortega 1996).

Wind Energy

Kenedy Ranch, on the mainland across from Padre 

Island National Seashore, clocks some of the highest 

wind speeds along the entire Gulf of Mexico. Average 

wind speeds are 28.4 km/h (17.7 mph) at the site 

(Babcock & Brown 2008). Kenedy Ranch now hosts a 

wind power facility with 118 wind turbines that each 

generates 2.4 megawatts of electricity. The turbines are 

13 km (8 mi) inland from the Gulf of Mexico and 8 km

(5 mi) west of Laguna Madre (The John G & Marie Stella 

Kenedy Memorial Foundation 2008). 

Wind energy development is addressed in a geologic 

resources inventory for two reasons: (1) eolian features 

and processes are a geologic resource to be protected 

throughout the National Park System, and (2) the NPS 

Geologic Resources Division provides policy and 

technical support to parks contending with energy 

development (both renewable and nonrenewable) within 

and adjacent to their boundaries. While the National 

Park Service supports the use of renewable energy, it 

wants to ensure that the appropriate technologies are 

used in the appropriate locations (McCoy 2009). At 

Padre Island National Seashore, the National Park 

Service has concerns about the impacts of wind power 

facilities on migrating bird and marine species, as well as 

changes in fishing pressure (i.e., bait fish congregate near 

structures for food and shelter, thereby attracting 

predator species), changes in habitat as a result of 

transmission lines (i.e., alterations in magnetic field may 

affect sea turtle migration), and terrestrial impacts of 

routing a right-of-way across the park to access the 

power grid (Darrell Echols, National Park Service, 

written communication, July 6, 2009).

In 2009, the Natural Resource Science and Stewardship 

Directorate (NRSS) and the Natural Resources Advisory 

Group, consisting of park and regional representatives, 

drafted an action plan for energy development. The plan 

is an initial step to increase the National Park Service’s 

capacity to respond more effectively and influence 

outcomes to protect parks. 

Monitoring at the Kenedy Ranch facility could test the 

effectiveness of implemented mitigation measures; 

increase understanding of impacts on avian, terrestrial, 

and marine species; and help determine whether 

particular technologies are preventing avian mortality. 
PAIS Geologic Resources Inventory Report 15

Figure 2. Coastal vulnerability index. Investigators classified 17% of the Gulf of Mexico shoreline at Padre Island National Seashore as 

having very high vulnerability to future sea-level rise, 28% as having high vulnerability, 29% as having moderate vulnerability, and 

26% as having low vulnerability. Image from Pendelton et al. (2004). Used by permission of the author, R. L. Beavers.
16 NPS Geologic Resources Division 

Figure 3. Blowing sand. Although a natural process and necessary for dune formation, mitigation of blowing sand requires many hours 

of maintenance at Padre Island National Seashore, for example along the road to Malaquite Beach shown in the photograph. National 

Park Service photograph.

Table 2. Hurricanes making landfall near Padre Island (1900–2010)

Name Date Landfall Category Wind speed

km/h mph

Dolly July 23, 2008 South Padre Island 1 138 86

Bret August 22, 1999 Padre Island 3 185 115

Gilbert September 17, 1988 South of Brownsville 4 217 135

Allen August 10, 1980 Port Mansfield 3 185 115

Celia August 3, 1970 Corpus Christi 3 201 125

Beulah September 20, 1967 Brownsville 3 217 135

Carla September 11, 1961 Port Lavaca 4 233 145

Number 3 July 25, 1934 Rockport 2 161 100

Number 11 September 4, 1933 Brownsville 3 201 125

Number 2 September 14, 1919 Corpus Christi 4 230 143

Number 6 August 18, 1916 Corpus Christi 4 217 135

Number 2 June 26, 1902 Port Lavaca 1 129 80

Source: Roth (2009). 

Note: Roth (2009) gave unnamed storms a name (e.g., “number 3”) based on its occurrence in a particular year. Category is 1 to 5 on the SaffirSimpson hurricane intensity scale.

Table 3. Requirements for renourishment at Bird Island Basin

Estimated project lifespan, in years Distance offshore Volume

Meters Feet Cubic meters Cubic yards

Fine sand, 2.75φ (locally derived)

5 5.0 16.4 1,802 2,357

10 10.0 32.8 2,827 3,698

20 19.9 65.6 5,483 7,172

Medium sand, 2.0–1.5 φ (quarry derived)

5 5.0 16.4 1,391–1,213 1,820–1,587

10 10.0 32.8 2,440–2,255 3,192–2,949

20 19.9 65.6 4,751–4,636 6,214–6,063

Data from Smith-Engle et al. (2006, 2007).
PAIS Geologic Resources Inventory Report 17

Figure 4. Vehicle impacts. Beach driving is a popular activity at Padre Island National Seashore. Regulations serve to minimize potential 

impacts and disturbance to resources. Note speed limit sign. National Park Service photograph by Rebecca Beavers (NPS Geologic 

Resources Division).

Figure 5. Oil and gas operations. The National Park Service evaluates operations plans from authorized operators and has developed 

regulations and mitigation measures to allow access to nonfederal oil and gas resources beneath the park while minimizing impacts to 

visitors and resources. This well, drilled in 2007, was a short-lived producer and is now plugged. Reclamation is ongoing (Pat O’Dell, NPS 

Geologic Resources Division, personal communication, September 20, 2010). National Park Service photograph courtesy Pat O’Dell. 
18 NPS Geologic Resources Division 

Figure 6. Grasslands of Padre Island National Seashore. Padre Island’s coastal prairies are dominated by bluestems (Andropogon spp. 

and Schizachyrium sp.), sea oats (Uniola paniculata), and Paspalum spp. National Park Service photograph.
PAIS Geologic Resources Inventory Report 19

Geologic Features and Processes

This section describes the most prominent and distinctive geologic features and processes 

in Padre Island National Seashore.

Wind Activity

Wind is perhaps the most important geologic agent at 

Padre Island National Seashore because its effects are 

both direct and indirect (Morton and McGowen 1980). 

Directly, winds move sand through eolian processes. 

Depending on whether overall conditions are wet or dry, 

winds build or move dunes, accrete or erode wind-tidal 

flats, and inflate or deflate swales and depressions 

(Withers et al. 2004). Indirectly, wind stresses generate 

waves, currents, and tides that circulate water in Laguna 

Madre, flood the wind-tidal flats, and control sediment 

transport onto and off of beaches and wind-tidal flats.

In many areas of Padre Island National Seashore, 

overwash does not reach the lagoon side of the island; 

deposition stops on the beach or interdunal areas. 

Hence, eolian transport and deposition are important for 

building the wind-tidal flats and helping the island keep 

pace with rising sea level (see “Coastal Vulnerability”) 

(Courtney Schupp, National Park Service, written 

communication, September 29, 2009). 

Wave and Current Activity

More than 12,000 waves break on the shore daily at 

Padre Island National Seashore (Weise and White 1980). 

Gulf waves breaking along the shoreline are usually less 

than 1 m (3.3 ft) high, but some reach heights of 2 m

(6.6 ft) during fall, winter, and spring storms (Hill and 

Hunter 1976). Waves, large and small, build the island 

lagoonward in both subtle and dramatic ways. Each wave 

lifts sediment, shifting and sorting every grain. Little by 

little, daily wave swash transports sediment to and from 

the shore. More dramatically, storm-surge waves 

transport sediment across the island in washover 

channels and fans. 

The lifting action of waves makes sand available for 

currents to transport sediment parallel to the shoreline, a 

process called longshore drift. Winds blowing into a 

concave shoreline, such as the South Texas coast, 

produce waves that approach a beach at an angle. These 

waves generate currents along the shore, which in turn 

generate longshore drift. The longshore currents of the 

Texas coast include those along the North Texas 

coastline, which move southward, and those along the 

South Texas coastline, which move northward. In the 

vicinity of Padre Island National Seashore, these currents 

transport sediment from two source areas—the Rio 

Grande to the south and the Brazos−Colorado River 

system to the north. 

Quartz is the primary constituent of Padre Island 

sediment, but particular minerals can be diagnostic of 

source areas: Rio Grande sediments (to the south) are 

derived from a variety of rocks characterized by the 

accessory minerals hornblende and pyroxene. To the 

north, sedimentary rocks dominate river basins, and 

garnet, tourmaline, rutile, staurolite, and zircon 

characterize heavy minerals in these sediments. In 

addition, sediments supplied in the Brazos−Colorado 

River drainage system have a considerable amount of 

green hornblende derived from igneous and 

metamorphic rocks in the Central Basin (Llano Uplift) 

of Texas (Bullard 1942; van Andel 1960). 

Rio Grande sediments are carried northward along 

Padre Island by longshore drift and converge in a 

transition zone with Brazos−Colorado River sediments 

carried southward in the same manner. In addition to 

lithologic differences, grain-size distribution on Padre 

Island shows that fine-grained sediment comes from the 

northern source, coarse-grained sediment from the 

southern source, and both fine- and coarse-grained 

sediment from the transition zone (Brezina 2004). This 

zone of transition and convergence is the site of the 

largest accumulations of heavy minerals, shells, and sand 

(see “Shells”).

Tidal Activity

At Padre Island, meteorological tides (i.e., wind-driven 

tides) are much more important than astronomical tides 

(i.e., those produced by the gravitational pull of the 

moon and the sun) (Tunnell 2002a). Commonly, windinduced changes in water level are larger than those 

caused by astronomical tides. For example, an 80-km/h 

(50-mph) wind produced a 46-cm (18-in) drop in lagoon 

water level in an hour (National Park Service 2007). Such 

dramatic wind-induced drops in water level can leave 

boats stranded in Laguna Madre.

Wind stress coupled with changes in barometric pressure 

often raises or lowers water levels on Gulf of Mexico 

beaches as much as 1 m (3.3 ft) compared to the 

predicted astronomical tides (Morton and Speed 1998), 

which average about 40 cm (16 in) (Weise and White 

1980) and range from 45 cm to 60 cm (18 in to 24 in) 

(Morton and McGowen 1980). Depending on the time of 

the month, astronomical tides in the Gulf of Mexico are 

diurnal (vary systematically and repeat themselves once 

daily) or mixed (vary irregularly twice daily). Variations 

in astronomical tides are considerably lower in bays and 

lagoons (Morton and McGowen 1980). Wind tides on 

Laguna Madre are in the range of 0.3 m to 1.2 m (1 ft to

4 ft) (Rusnak 1960). 

These parameters classify the Gulf of Mexico as a 

“microtidal,” low-energy coastline. Such coastlines are 

essentially synonymous with storm-dominated coasts 

because the expended energy and geologic shaping of the 

land accomplished during storms overwhelm daily 

processes (Morton and McGowen 1980). 
20 NPS Geologic Resources Division 

Barrier Island System

A series of distinct geomorphic provinces comprises 

Padre Island (fig. 7). These provinces are the physical 

outcome of wind, wave, and tidal activity; vegetation also 

plays a significant role. The map units presented on the 

digital geologic map for Padre Island National Seashore 

(i.e., Gibeaut and Tremblay 2005) can be grouped into 

these provinces. Although some map units are part of 

more than one geomorphic province (e.g., active dunes), 

generally speaking, starting from the Gulf of Mexico and 

working towards Laguna Madre, the provinces are as 

follows: beach (see “Map Unit Properties Table,” unit 

B1), coppice dunes (unit B2), fore-island dune ridge (unit 

B3), active dunes (unit B10), stabilized dunes (unit B5a), 

vegetated and sparsely vegetated barrier flats (units B4 

and B5), and back-island sand flats (B11). Other sand 

flats (unit B8a) fringe vegetated barrier flats and coppicedune fields in the back-island area. Brackish and fresh 

marshes and ponds (units B4a–d and B9) form in barrier 

flats and storm washover channels (unit B7). In the 

southern part of the national seashore, washover 

channels slice across the island and associated fans (unit 

B7a) are deposited on wind-tidal flats. Transitional 

between the barrier island and lagoon systems are winddeflation flats (unit B8) or troughs.

Nearshore Zone

Often included in classification systems of barrier-island 

morphology is the nearshore zone (Rebecca Beavers, 

Geologic Resources Division, e-mail message, September 

1, 2010). The nearshore zone is represented by the 

“upper shoreface” and “forebeach” of Weise and White 

(1980) (see fig. 6), and as part of the beach (unit B1) on 

the digital geologic map for Padre Island National 

Seashore. According to Gibeaut and Tremblay (2005), 

the beach (B1) unit is bounded to the east by marine 

open waters of the Gulf of Mexico and landward by a 

prominent fore-island dune ridge. The gulfward 

shoreline was mapped using 2000 LIDAR (light detection 

and ranging) data. To complete this classification 

scheme, the “offshore zone” is the open water that lies 

seaward of the nearshore zone.

The nearshore zone is a significant part of the barrierisland environment because it is the site of sediment 

transport, both longshore (parallel to the shoreline) and 

cross shore (perpendicular, onto and off of the 

shoreline). Subjected to the unending onslaught of 

waves, the nearshore zone is divided into “subzones” 

based on wave behavior. The nearshore zone extends 

from where waves begin to break (breaker zone) across 

where most of the wave energy is dissipated (surf zone) 

to where the beach is covered and uncovered repeatedly 

by each surging wave (swash zone) (Pinet 1992). 

Beach

Padre Island National Seashore is traditionally 

subdivided into five named beaches (fig. 1). North Beach 

extends for 1.8 km (1.1 mi) from the national seashore’s 

northern boundary to a row of posts that crosses the 

beach and marks the boundary with Closed (Malaquite) 

Beach. Closed Beach extends for 7.2 km (4.5 mi) to a row 

of pylons that marks its southern boundary. Closed 

Beach incorporates the Malaquite Beach facilities, 

including the visitor center and campground. Closed 

Beach’s southern boundary is the zero point for 

measuring distances on the remaining 96 km (60 mi) of 

beach within Padre Island National Seashore (see fig. 1). 

South Beach extends from Closed Beach’s southern 

boundary approximately 11 km (7 mi) to the beginning 

of Little Shell Beach, which is not marked by any posts or 

pylons. Little Shell Beach extends from the southern 

boundary of Closed Beach approximately 11 km (7 mi) to 

milepost 12. Big Shell Beach extends from approximately 

27 km (17 mi) to 45 km (28 mi) south of Closed Beach. As 

the names imply, the seashells making up Little Shell 

Beach are smaller than the shells found at Big Shell Beach 

(see “Shells”). The beaches south of Big Shell Beach are 

not named and are usually designated only by their 

distances from Closed Beach’s southern boundary.

Padre Island beaches are composed of fine sand, which 

coarsens slightly from north to south (Hayes 1965). The 

primary constituent is the mineral quartz, but the sand 

also contains rock fragments, the mineral feldspar, and 

heavy minerals, such as hornblende, pyroxene, garnet, 

staurolite, rutile, zircon, and tourmaline (Bullard 1942). 

In addition to these terrestrial components, shell and 

shell fragments make up varying percentages of the total 

beach sediment. The highest concentrations of shell (as 

much as 80%) occur where longshore currents converge 

between Big Shell and Little Shell beaches (Weise and 

White 1980).

Geomorphologists often divide the beach province into 

two zones: forebeach (or “foreshore beach”) and 

backbeach (or “backshore beach”) (Weise and White 

1980; see fig. 7). The forebeach slopes gently towards the 

Gulf of Mexico and is subject to the daily swash of waves

(fig. 8). As a result, sedimentary structures in the beach 

environment are primarily seaward-dipping laminations 

produced by wave swash. Commonly, concentrations of 

heavy minerals and shell fragments accentuate the 

laminations (Morton and McGowen 1980). Burrowing 

organisms on the beach tend to destroy primary 

stratigraphic forms, however. The backbeach and upper 

forebeach are dominated by the burrowing ghost crab 

(Ocypode quadrata), while ghost shrimp (Callianassa 

islagrande) characterize the lower forebeach (Hunter et 

al. 1972). Scientists use variations in burrow morphology, 

including spatial density, size, and shape, to designate 

subenvironments of the beach (Brezina 2004).

The apex of the forebeach—the berm crest—divides the 

forebeach from the backbeach. Generally, the backbeach 

is either horizontal or slopes gently landward, creating a 

shallow trough between the berm crest and fore-island 

dune ridge. Rills formed by flood runoff may produce 

scour and fill (sedimentary structures) in the backbeach 

zone, but generally surfaces are smooth and internal 

stratification is faint (Morton and McGowen 1980).

During fair weather, the backshore is inactive with 

respect to wave swash, although the wind continually 

moves sand landward in this zone (Hill and Hunter 

1987). However, because of the dampness of the sand 

due to its low elevation, wind erosion seldom lowers the 
PAIS Geologic Resources Inventory Report 21

level of the backshore by more than a few centimeters. 

Wave swash does reach the backshore several times a 

year, mostly during winter storms, but only hurricanes 

cause severe damage in this zone (Hill and Hunter 1987). 

The sand eroded from the beach during storms is 

deposited in nearshore areas and is gradually carried 

back to the beach by normal wave activity (Hunter et al. 

1972). Higher than normal waves and tides move coarser 

sediments and shell fragments onto the backbeach. 

Storm surge transports coarser material farther landward 

through storm washover channels (i.e., breaches in the 

fore-island dune ridge). After initial storm deposition, a 

lag deposit of coarse shell fragments may form on the 

backbeach in response to eolian removal (“winnowing”) 

of finer sediments. 

Coppice Dunes

As a result of prevailing southeasterly winds, coppice 

dunes form in the backbeach zone and fringe the edges 

of active back-island dune fields. Sediment composing 

coppice dunes is fine beach sand that has blown 

landward and collected around clumps of vegetation and 

the wind shadows behind them (Weise and White 1980). 

Coppice dunes in the backbeach form a narrow band of 

sandy mounds that parallel the beach for miles. In the 

southern part of the national seashore, where the 

foredune ridge is absent or segmented, broader coppice 

dune fields extend landward from the beach (Weise and 

White 1980). Normally, coppice dunes remain small, less 

than 0.9 m (3 ft) high, but they may grow to 1.8 m (6 ft) or 

more if the vegetation that helps to build them remains 

healthy and is not destroyed by storms or human 

activities. Larger coppice dunes may merge and become 

part of the fore-island dune ridge (Weise and White 

1980). 

Fore-Island Dune Ridge

The fore-island dune ridge—also referred to as the 

“foredune ridge” or simply “foredunes”—is immediately 

landward of the backbeach and aligned parallel to the 

shoreline (figs. 8 and 9). These dunes protect the barrier 

flats from the full onslaught of tropical storms. 

Foredune sediments on Padre Island consist of very well 

sorted, fine and very fine sands, which are blown from 

the backbeach and stabilized by vegetation. Climate, soil 

moisture, and soil temperature affect the growth of 

vegetation and, therefore, the stability of foredunes. The 

drier, southern parts of Padre Island have low foredunes. 

By contrast, elevations of large foredunes on Big Shell 

Beach reach 15 m (50 ft) where denser vegetation traps 

sand. Interestingly, foredunes in the area of longshore 

drift convergence (i.e., around latitude 27°N or mile 

marker 30) show a distinct correlation between 

increased or maximum amount of shell fragments on the 

beach and the location of high, continuous foredunes 

(Watson 1968). Low dunes or storm washover channels 

correspond to local minima in shell fragment 

percentages. This positive correlation does not occur 

elsewhere on Padre Island, however (Brezina 2004).

In addition to vegetation, foredune development and 

foredune height are directly related to sediment supply 

(Brezina 2004). Rivers are the primary source of sand for 

building barriers and beaches in the western Gulf of 

Mexico (Morton 1994). According to Hayes (1963), the 

source of sand in beaches and dunes along the southern 

end of Padre Island is the Rio Grande. The Nueces, 

Colorado, and several other rivers are the sources of 

sand to the north. When rainfall and associated fluvial 

activity decreased in the early Holocene, sand supplied 

to the beaches also decreased (see “Geologic History”). 

More recently, watershed development, such as 

construction of surface-water reservoirs, flood-control 

structures, and freshwater diversions, have resulted in 

additional reductions in sand supply (Morton 1994).

Like height, width of the fore-island dune ridge varies 

along the island. Generally, the ridge is about 60 m to

90 m (200 ft to 300 ft) wide; however, in areas where the 

foredunes became blowouts and later stabilized 

landward, the dune ridge is wider, as much as 370 m 

(1,200 ft) across (Weise and White 1980).

Active Dunes

Active dunes exemplify barrier-island migration. Active 

dunes occur as fore-island blowout dunes or in backisland dune fields. Where vegetation is destroyed, the 

fore-island dune ridge is vulnerable to erosion by the 

strong southeasterly winds (fig. 10). Sand is blown from 

the dune ridge across the barrier flat in a northwesterly 

direction. The result is a blowout dune complex, 

composed of rapidly changing dune forms. The sediment 

in these blowout dunes is mostly fine, well-sorted sand 

derived from fore-island dunes. Stratification resulting 

from dune migration consists principally of horizontalto-inclined plane-parallel laminae, climbing-ripple 

laminae, and ripple laminae (Hunter 1977). Predominant 

wind directions cause cross-strata to dip northwest 

(Morton and McGowen 1980). As blowout dunes 

migrate, they may either become stabilized by grasses or 

merge with a back-island dune field (Weise and White 

1980). 

In the northern part of the national seashore, fine sand is 

supplied to the back-island dune fields by the migrating 

blowout dunes. In the southern part, sand reworked 

from sediments that washed into the back-island area 

during storms largely supplies the back-island dunes. 

Presently, neither washover deposits nor large blowout 

dunes occur in the central part of the national seashore; 

as a result, the back-island area is starved of the sand 

necessary to build large dune fields there. In this part of 

the national seashore, vegetated barrier flats have 

replaced what were once dune fields (Weise and White 

1980).

Stabilized Dunes

In many parts of Padre Island National Seashore, 

elongated and parabolic-shaped dunes covered in 

vegetation extend lagoonward from the foredune ridge. 

These dunes, as much as 6 m (20 ft) high, were once 

active blowout dunes that originated at the dune ridge. 

After migrating some distance towards Laguna Madre, 
22 NPS Geologic Resources Division 

they became stabilized by vegetation (fig. 11) (Weise and 

White 1980). If the grass cover is destroyed, the dunes 

may be reactivated and begin migrating again. 

Because the sediments of the stabilized blowout dunes 

were derived from the fore-island dune ridge, they 

resemble foredune sediments. The stabilized deposits 

include fine, well-sorted sand. Nearest the fore-island 

area, they contain small shells and shell fragments that 

were blown or washed from the beach (Weise and White 

1980).

Barrier Flats 

Vegetated (and sparsely vegetated) barrier flats are the 

most common terrestrial environment at Padre Island 

National Seashore (Gibeaut and Tremblay 2005). They 

spread behind the fore-island dune ridge, gently sloping 

towards Laguna Madre. They have little topographic 

relief and commonly lie less than 1.5 m (5 ft) above mean 

water level (Leatherman 1988). According to Weise and 

White (1980), the more sparsely vegetated barrier flats 

are areas where grasses have been established recently 

and have not had enough time or moisture to develop a 

dense cover. These areas were recently eroded by wind 

and have supplied sand to the active dunes. If rainfall is 

sufficient, most of the sparsely vegetated flats will evolve 

into more heavily vegetated flats (Weise and White 

1980). The majority of sparsely vegetated barrier flats 

occur on the windward (southeast) edge of migrating 

dune fields.

Most barrier flats on the Gulf of Mexico coast of South 

Texas have some percentage of barren or minimally 

vegetated deflation flats. Wind-deflation flats are the low 

areas left behind migrating dune fields. Eolian removal of 

sand and finer sediments deflates these surfaces, often 

down to the water table.

Barrier flats can host ponds and marshes, which occur in 

troughs associated with deflation flats. Troughs are 

generally linear and are located in areas through which 

dunes have migrated. Although these troughs were 

originally floored in sandy sediments, ponding and 

associated growth of marsh vegetation promoted 

deposition of mud and plant debris. Organic debris is 

mixed or interlayered with episodic additions of eolian 

sands (Brezina 2004). On Padre Island, accumulations of 

organic matter and fine sediments are limited to ponds 

and marshes (Weise and White 1980), and pond deposits 

are the only barrier-island sediments that have low 

permeability and high water-holding capacity 

(McGowen et al. 1976). 

Back-Island Sand Flats 

A sand flat is one possible last stage in the life of a dune 

field (Weise and White 1980). Non-vegetated sand flats 

fringe barrier flats and coppice-dune fields in the backisland area. Sand within the flats commonly forms small, 

rapidly migrating dunes. As the sand continues to spread 

and dunes become smaller, the elevation of the sand flat 

is reduced sufficiently so that the area is flooded by 

normal wind tides, and the environment evolves into a 

wind-tidal flat (Weise and White 1980). 

Storm Washover Channels and Fans

Created and reopened during hurricanes, washover 

channels provide a conduit for Gulf of Mexico waters to 

freshen the hypersaline waters of Laguna Madre 

(National Park Service 2000). Natural, storm washover 

channels as much as 213 m (700 ft) wide cut through the 

dune ridge (Weise and White 1980). Sediments eroded 

from the beach and adjacent dunes are spread 

lagoonward over the wind-tidal flats, forming fan-shaped 

deposits called washover fans. The fans are lobes of sand 

and shell having very low relief that are deposited where 

the washover channels widen and the storm waters lose 

velocity. Storm waters also carry some sediment parallel 

to the shoreline along the wind-deflation flats (Hayes 

1965). Where artificial channels, such as Mansfield 

Channel, connect the Gulf of Mexico and Laguna Madre 

(and in other locations with natural inlets), hurricane 

surge may produce strong currents that dramatically 

raise lagoon water levels. As the hurricane passes, 

elevated lagoon waters, which are produced by surge and 

winds, create currents that flow toward the Gulf of 

Mexico and transport sediments back through washover 

channels, artificial passes, and natural inlets (Weise and 

White 1980).

All washover channels eroded during recent hurricanes 

occur in the southern part of the national seashore. 

These are likely to be reactivated during the next large 

storm. Washover channels are much more prominent in 

the southern part of the national seashore because the 

dune ridge is low and segmented as a result of (1) higher 

shoreline erosion rates, which reduce the amount of 

sediment available for dune construction, and (2) a drier 

climate and consequently much less vegetation to bind 

the sand (Weise and White 1980). 

The first washover channel is about 27 km (17 mi) south 

of Yarborough Pass (see fig. 1). Older, healed washovers 

are located approximately 6.4 km (4 mi) south of 

Yarborough Pass and 14 km (9 mi) south of Malaquite 

Beach. These relict channels are not mapped as 

hurricane washovers on the digital geologic map of Padre 

Island National Seashore, but rather as the various 

environments that have developed in them. 

Nevertheless, the stratigraphy of Padre Island National 

Seashore, in this case relict washover channels and fans, 

documents past hurricanes as part of the geologic record. 

According to Suter and Maynard (1983), relatively thin 

deposits consisting of numerous coalescing washovers 

represent low-profile barriers in the stratigraphic record. 

Thicker sand bodies corresponding to high-profile 

barriers occur along strike. 

Major washover channels eroded below mean sea level 

may continue to pond water after normal nearshore 

processes have reconstructed the beach across the 

channel mouths, severing their connection with Gulf of 

Mexico waters. Most ponds that form in the deepest 

parts of the washover channels exist only briefly. Where 

channels are scoured below the water table, however, the 

ponds retain brackish water. Thin algal mats develop 

around the edges of the ponds and in other moist parts of 

the channels between storms (Weise and White 1980).
PAIS Geologic Resources Inventory Report 23

Washover fans that are deposited over the wind-tidal 

flats provide a source of sand for the formation of backisland dune fields (Boker 1953). In some areas, for 

example along the southern end of the national seashore, 

“pavements” composed of large shells and shell 

fragments form lagoonward of the backbeach and foreisland dunes. These deposits are evidence of storm surge 

and the effect of subsequent eolian processes, which 

deflate the surface and leave the shell concentrated there 

(Weise and White 1980). Hayes (1967) discovered shells 

4.0 km (2.5 mi) landward of the beach as a result of 

Hurricane Carla’s surge.

Lagoon System

The lagoon system includes wind-tidal flats (units L1, L2, 

and L3), lagoon-margin sand (unit L5), and grassflats 

(unit L6). Sand and shell berms (natural islands; unit L4) 

and spoil mounds (unit M1, M2, and M3) occur in 

Laguna Madre itself.

Laguna Madre is the largest hypersaline estuary in the 

world, extending 445 km (277 mi) from the mouth of 

Corpus Christi Bay, Texas, to the Rio Soto La Marina, 

Tamaulipas, Mexico. The maximum width of the lagoon 

is approximately 16 km (10 mi); however, its width in 

many places fluctuates considerably with the height of 

wind-generated tides. The lagoon is widest during 

highest wind tides, which produce maximum flooding of 

the vast adjacent tidal flats (Weise and White 1980). 

The depth of Laguna Madre ranges from a subaerial land 

bridge to a submerged bottom, where the muddy sand 

lies at depths of 2.4 m (8 ft) (Weise and White 1980). 

According to Morton (1994), lagoon sediments are less 

than 3 m (10 ft) thick and overlie relatively stiff mud of 

the Rio Grande delta (see “Geologic History”). The 

shallowest parts of the lagoon are in the central part of 

the national seashore, in the Land Cut, which forms a 

land bridge between Padre Island and mainland Texas. 

Also known as the “Salt Flats,” “Saltillo Flats,” “Kenedy 

Flats,” and “Laguna Madre Flats,” “Land Cut” was 

probably coined when the Gulf Intracoastal Waterway 

was excavated through this area (Withers et al. 2004). 

The Land Cut was formed over several thousand years as 

a result of sediment transport from Padre Island to the 

mainland (Morton and McGowen 1980). Hurricanes and 

tropical storms, in conjunction with eolian processes, 

moved sediment from Padre Island landward into 

Laguna Madre (Morton and McGowen 1980). 

Padre Island all but isolates Laguna Madre, separating it 

from the Gulf of Mexico and restricting seawater flow 

into and out of the lagoon. In addition, no rivers flow 

into the lagoon. Most fresh water enters the system 

during infrequent, but often massive, storms, when 

several inches to several feet of rain can fall within a few 

days. Although these storms are often associated with 

summer tropical weather systems, they can occur at any 

time of the year (Withers et al. 2004). During such events, 

sheet flow in the area of the Land Cut also provides a 

source of freshwater (Brown et al. 1977).

Storm surge and eolian processes contribute sand and 

shell fragments to wind-tidal flats; mud is derived from 

adjacent lagoons. Sediment transport created extensive 

wind-tidal flats on the periphery of Laguna Madre, and 

the flats are subject to flooding by wind tides (see “Wind 

Activity” and “Tidal Activity”). Wind-tidal flats occupy 

the entire lagoon in the vicinity of Land Cut (Weise and 

White 1980). 

Frequently inundated by Laguna-Madre waters driven 

by the wind, some parts of the flats support extensive 

mats of blue-green algae; the algae thrive when wet but 

die, crack, and peel when dry. Desiccation coupled with 

heaving from biogenic gases and air trapped beneath the 

algal mats produces chaotic and contorted bedding. In 

addition, biochemical precipitates of calcium carbonate 

are interlaminated with algal mats near the surface 

(Morton and McGowen 1980).

Evaporite, Fulgurite, and Oolite

Amidst so much sand, seeing a rock at Padre Island 

National Seashore is a memorable experience. The 

national seashore hosts three distinctive types of rocks: 

evaporite, fulgurite, and oolite. Rock specimens, 

especially those as unique as the following, might be 

targets for unauthorized collecting. However, no studies 

of human behavior regarding collection of rocks and 

minerals have been conducted at Padre Island National 

Seashore, and it is not known whether theft of these 

limited resources is a problem at the national seashore.

Evaporite

Bladelike crystals of gypsum (calcium sulfate) form in the 

wind-tidal flats of Laguna Madre. Gypsum is an 

evaporite rock, which forms as a result of extensive or 

total evaporation of a saline solution. At Padre Island 

National Seashore, gypsum crystals form when highsalinity waters periodically flood the wind-tidal flats and 

seep into the underlying sediments. Gypsum is 

precipitated in the pore spaces of the sandy substrate, 

cementing the sand grains and forming gypsum-sand 

crystals (Fisk 1959). These crystals grow larger with each 

successive flooding, infiltration, and precipitation 

(Masson 1955). Crystals increase in size with depth; they 

are less than 2.5 cm (1 in) in length near the top of the 

water table and more than 0.3 m (1 ft) at a depth of 3.7 m 

(12 ft) below sea level (Fisk 1959). 

Gypsum occurs in three forms at Padre Island: (1) clear, 

amber-colored blades or crystals (selenite); (2) gray, 

sand-textured blades or crystals; and (3) small, lightcolored, sand-textured rosettes (fig. 12) (Tunnell 2002b). 

Clusters of rosettes—also called “sand roses” or “gypsum 

roses”—can reach 50 cm (20 in) in length and weigh as 

much as 11 kg (24 lb) (McBride et al. 1991).

The Laguna Madre area is the only place in Texas where 

gypsum rosettes occur (fig. 7), namely on both sides of 

the Gulf Intracoastal Waterway in the vicinity of the 

Land Cut, and along Mansfield Channel (Tunnell 

2002b). When channels were dredged across the flats, 

the crystals were deposited along with other dredge 
24 NPS Geologic Resources Division 

material in piles. Over time, wind and rain erode the 

sandy sediment, leaving the crystals and rosettes exposed 

on the surface (Tunnell 2002b). 

Fulgurite

When lightning hits sand, it can fuse the particles 

together, forming fulgurite (fig. 12). Likely places for 

lightning strike are points of highest elevation, which in 

the case of Padre Island National Seashore are dune 

ridges. Lightning penetrates the ground and turns the 

sand into glass, which forms in the path of the strike, 

creating branches or rods below ground and crusts on 

the surface. Fusion also may vaporize the center of 

fulgurite, resulting in a tube.

Usually fulgurite is between 2 cm (1 in) and 5 cm (2 in) in 

diameter, but the national seashore’s museum collection 

contains a few exceptionally rare specimens that are 10 

cm (4 in) across. Workers discovered these pieces during 

the construction of the Malaquite Visitor Center. 

Specimens are now displayed at the visitor center, where 

they can serve as a reminder to visitors about the 

importance of staying off the dunes, particularly as 

storms approach.

Oolite

Resembling fish roe, oolite is a sedimentary rock, usually 

limestone, made of ooliths—round or oval grains formed 

by accretion around a nucleus of shell fragment, algal 

pellet, or sand grain. These laminated grains may reach 

diameters of 2 mm (0.08 in.), but 0.5 mm to 1 mm (0.02 in 

to 0.04 in) is common. Oolite is usually formed of 

calcium carbonate but may consist of dolomite, silica, or 

other minerals. 

The classic interpretation of oolite formation suggests 

that wave-agitated water is necessary for layers to 

accrete. However, the oolites in Laguna Madre defy this 

interpretation. Formation in the quiet waters of Laguna 

Madre proves that high energy is not essential, at least for 

irregular forms (Tunnell 2002b).

Like gypsum crystals, Laguna Madre is the only place 

where oolite is found in Texas. It is forming on the 

Laguna Madre shoreline just north of the Point of Rocks 

at the mouth of Baffin Bay and near two submerged sand 

bars between Baffin Bay and Padre Island (Tunnell 

2002b).

Shells 

Approximately 350 species of shell-bearing organisms 

occur along the Texas coast (Andrews 1972). About half 

of these are very minute and easily overlooked by the 

casual beachcomber, but collectors have found 37 

species of bivalves (e.g., clams, oysters, scallops, and 

mussels) and gastropods (e.g., snails or limpets) at Padre 

Island National Seashore (National Park Service 2006c). 

Specimens include bivalves such as spiny jewel box 

(Arcinella arcinella), bay scallop (Argopecten irradians), 

and incongruous ark (Anadara brasiliana Lamarck) (fig. 

8) and gastropods such as rock shell (Thais haemastoma 

floridana Conrad), sundial (Architectonica noblis

Roding), and lightning whelk (Busycon contrarium

Conrad) (fig. 13). The most sought after shell on the 

Texas coast is the beautifully spiraled Mitchell’s

wentletrap (Amaea mitchelli Dall), which washes ashore 

only on the Texas and Mexico shorelines of the Gulf of 

Mexico (Andrews 1972).

Two distinct shell assemblages occur on Padre Island 

beaches. Northern beaches in the national seashore are 

almost entirely composed of small surf clams, primarily 

Donax variabilis Say but also Donax tumida Say. 

Southern beaches are characterized by the bivalve 

mollusks southern quahog (Mercenaria campechiensis 

Gmelin) and the large clams Eontia ponderosa Say and 

Echinochama arcinella Linne. Either dead or living small 

surf clams (Donax sp.) are generally absent except for a 

small colony about 32 km (20 mi) north of Mansfield 

Channel (Watson 1968). Ark shells (e.g., Andara 

braziliana Lamarck, Andara ovalis Bruguiere, and 

Andara baughmani Hertlein) are common throughout 

the extent of Padre Island and do not exhibit any 

particular pattern of distribution. The transition zone 

between north and south has a lower total percentage of 

shells than do flanking areas, which have slightly 

increased numbers of the ark (Andara) species. 

Percentage of shell species and degree of abrasion 

suggest that the large-clam (Mercenaria-EontiaEchinochama) assemblage has a southern source and is 

transported to the north and that the small surf-clam 

(Donax) assemblage has a northern source and is 

transported to the south (Watson 1968).

Following a life span within the surf zone, Donax is 

transported southward by longshore drift to accumulate 

on Little Shell Beach. Large-clam (Mercenaria-EontiaEchinochama) shells are abraded and not present as fresh 

specimens on Big Shell Beach (Watson 1968). The shells 

on Little Shell and Big Shell beaches are much abraded as 

a result of continuous reworking by wave action 

(Morton 1977). In addition, the large-clam (Mercenaria) 

shells are commonly highly discolored. These 

observations suggest accumulation of shell fragments 

due to differential sorting by wave energy and not a 

locally rich population. The discoloration might also 

suggest an older, reworked shell assemblage, possibly 

subject to burial prior to accumulation in a beach 

environment (Brezina 2004).

Unoccupied sea shells may be collected in limited 

quantities within the national seashore for 

noncommercial use. No other natural or cultural 

resources may be collected without a research permit.
PAIS Geologic Resources Inventory Report 25

Figure 7. Geomorphic provinces at Padre Island National Seashore. Graphic redrafted from Weise and White (1980) by Trista ThornberryEhrlich (Colorado State University). Used with permission of the author and the University of Texas at Austin, Bureau of Economic Geology.

Figure 8. Beach and fore-island dune ridge. National Park Service photograph by Rebecca Beavers (NPS Geologic Resources Division).
26 NPS Geologic Resources Division 

Figure 9. Fore-island dune ridge. The fore-island dune ridge protects the barrier flats from storm impacts. National Park Service photograph 

available online 'http://www.nps.gov/pais/naturescience/Geology-Gallery.htm

Figure 10. Blowout. Blowouts occur where stabilizing vegetation has been lost or disturbed, resulting in the movement of sand. National Park 

Service photograph by Rebecca Beavers (NPS Geologic Resources Division).
PAIS Geologic Resources Inventory Report 27

Figure 11. Vegetated blow out dune. Vegetation stabilizes blow out dunes. National Park Service photograph available online 

http://www.nps.gov/pais/naturescience/Geology-Gallery.htm, accessed September 23, 2010.

Figure 12. Sand rose and fulgurite. Rare gypsum rosettes (left) form in the Laguna Madre area, the only place in Texas where they occur. 

Ruler is in inches. Fulgurites (right) form when lightning strikes sand and fuses the grains together. Sometimes the center is completely 

“vaporized” leaving a hollow tube. National Park Service photographs available online: http://www.nps.gov/pais/naturescience/Geology.htm

(accessed September 22, 2010).
28 NPS Geologic Resources Division 

Figure 13. Seashells of Padre Island. The wentletrap is one of the 

most sought-after shell along the Texas coast. Paper clip for scale. 

National Park Service graphic, available online: 

http://www.nps.gov/pais/photosmultimedia/Seashells.htm

(accessed September 22, 2010).
PAIS Geologic Resources Inventory Report 29

Map Unit Properties

This section identifies characteristics of map units that appear on the Geologic Resources 

Inventory digital geologic map of Padre Island National Seashore. The accompanying

table is highly generalized and for background purposes only. Ground-disturbing 

activities should not be permitted or denied on the basis of information in this table. 

Geologic maps facilitate an understanding of Earth, its 

processes, and the geologic history responsible for its 

formation. Hence, the geologic map for Padre Island 

National Seashore provided information for the 

“Geologic Issues,” “Geologic Features and Processes,” 

and “Geologic History” sections of this report. Geologic 

maps are two-dimensional representations of complex 

three-dimensional relationships; their color coding 

illustrates the distribution of rocks and unconsolidated 

deposits. Bold lines that cross or separate the color 

patterns mark structures such as faults and folds. Point 

symbols indicate features such as dipping strata, sample 

localities, mines, wells, and cave openings. 

Incorporation of geologic data into a Geographic 

Information System (GIS) increases the usefulness of 

geologic maps by revealing the spatial relationships 

among geologic features, other natural resources, and 

anthropogenic features. Geologic maps are indicators of 

water resources because they show which rock units are 

potential aquifers and are useful for finding seeps and 

springs. For example, on the digital geologic map for 

Padre Island National Seashore, unit B5 (sparsely 

vegetated barrier flat) has the greatest volume of 

freshwater of any map unit at Padre Island National 

Seashore. However, Boylan (1986) investigated the 

geohydrologic resources of the northern part of Padre 

Island and concluded that extensive withdrawal of 

groundwater from North Padre Island is not 

recommended because of the fragility of the ecosystems 

and dune forms.

Geologic maps are not soil maps, and do not show soil 

types, but they do show parent material—a key factor in 

soil formation. The following table incorporates

information from Soil Survey of Padre Island National 

Seashore, Texas (Brezina 2004). Furthermore, resource 

managers have used geologic maps to make connections 

between geology and biology; for instance, geologic 

maps have served as tools for locating sensitive, 

threatened, and endangered plant species, which may 

prefer a particular map unit. 

Although geologic maps do not show where earthquakes 

will occur, the presence of a fault indicates past 

movement and possible future seismic activity. Similarly, 

map units show areas that have been susceptible to 

hazards such as landslides, rockfalls, and volcanic 

eruptions. Geologic maps do not show archaeological or 

cultural resources, but past peoples may have inhabited 

or been influenced by depicted geomorphic features. For 

example, alluvial terraces may have been preferred use 

areas and formerly inhabited alcoves may occur at the 

contact between two rock units.

The geologic units listed in the following table 

correspond to the accompanying digital geologic data. 

Additionally, each map unit is part of a particular 

geomorphic environment (see “Geologic Features and 

Processes”). Map units cover barrier-island and lagoon 

environments, not marine environments, because 

mapping of offshore geology has not been completed 

(see “Benthic Habitat Mapping”). Map units are listed in 

the table from youngest to oldest. Geologically speaking, 

all the map units for Padre Island National Seashore are 

very young, hailing from the most recent geologic 

epoch—the Holocene (fig. 14). The table highlights 

characteristics of map units such as: susceptibility to 

erosion and hazards; the occurrence of paleontological 

resources (fossils), cultural resources, mineral resources, 

and caves or karst; and suitability as habitat or for 

recreational use. Some information on the table is

conjectural and meant to serve as suggestions for further 

investigation.

The GRI digital geologic maps reproduce essential 

elements of the source maps including the unit 

descriptions, legend, map notes, graphics, and report. 

The following reference is the source for the GRI digital 

geologic data for Padre Island National Seashore: 

Gibeaut, J., and T. Tremblay. 2005. Padre Island natural 

environments map (scale 1:5,000). Unpublished data. 

University of Texas, Bureau of Economic Geology, 

Austin, Texas, USA.

The GRI team implements a geology-GIS data model 

that standardizes map deliverables. This data model 

dictates GIS data structure including data layer 

architecture, feature attribution, and data relationships 

within ESRI ArcGIS software, and increases the overall 

utility of the data. GRI digital geologic map products 

include data in ESRI personal geodatabase and shapefile 

GIS formats, layer files with feature symbology, Federal 

Geographic Data Committee (FGDC)-compliant 

metadata, a Windows help file that contains all of the 

ancillary map information and graphics, and an ESRI 

ArcMap map document file that easily displays the map 

and connects the help file directly to the map document.

GRI digital geologic data are included on the attached 

CD and are available through the NPS Data Store at 

http://science.nature.nps.gov/nrdata/. Data will be 

available on the Natural Resource Information Portal 

when the portal goes online. As of September 2010, 

access is limited to NPS computers at 

http://nrinfo/Home.mvc.
PAIS Geologic Resources inventory Report 30

Map Unit Properties Table: Padre Island National Seashore

Age Unit Name

(Symbol) Features and Description Suitability for Infrastructure and Recreation Hazards Habitat

QUATERNARY

(Holocene Epoch)

Man-made Units

Vegetated spoil mound

(M1) Partially vegetated, subaerial deposits of dredge material from the Gulf Intracoastal Waterway

Suitable for recreation but somewhat limited suitability 

for paths, trails, and 4WD trails. Very limited suitability 

for camp and picnic areas.

Flooding Saline-water wetland plants and riparian shrubs, vines, and trees. Popular nesting 

grounds for a variety of bird species.

Barren spoil mound

(M2)

Barren, subaerial deposits of dredge material from the Gulf Intracoastal Waterway. Contains 

gypsum crystals.

Suitable for recreation but somewhat limited suitability 

for paths, trails, and 4WD trails. Very limited suitability 

for camp and picnic areas.

Flooding Algal mats

Subaqueous spoil

(M3) Submerged dredge material from the Gulf Intracoastal Waterway Unsuitable, submerged None Marine grass

Modified land

(M4) Land modified by human activity Disturbed by oil and gas exploration. Existing development.

Potential contamination 

from oil and gas spills or 

leaks

Alteration of natural environment evident on aerial photographs. Devegetation 

scars.

Barrier System Units

Beach

(B1)

Sand and shell of varying compositions from north to south. Varying profile shapes and widths.

Predominant internal structure (gently seaward-dipping planar laminations) produced by wave 

swash.

Mapping notes: Bounded to the east by marine open waters of the Gulf of Mexico and landward 

by a prominent fore-island dune ridge. Gulfward shoreline captured from 2000 LIDAR data.

Tolerant of recreation but limited suitability for paths, 

trails, and 4WD trails. Very limited suitability for camp 

and picnic areas. Frequent wave surges and subsequent 

saltwater contamination.

Flooding

Foreshore is ecotone. Kemp’s ridley turtle (Lepidochelys kempii) nesting site. Habitat for 

ghost crab (Ocypode quadrata), mole crab (Emerita), snails (Olivella and Terebra),

staphylinid beetles, polychaete worm (Lepidopa), coquina clam (Donax variabilis).

Texas pocket gopher (Geomys personatus) in backshore. Grasses. Freshwater and 

saline-water wetland plants in backbeach.

Coppice dunes

(B2)

Composed of fine beach sand. Characteristic mound appearance. Commonly found in 

backbeach, landward of fore-island dune ridge, and within sand flats. Forms broad fields where 

fore-island dune ridge is absent.

Unsuitable for development and recreation. Sensitive to 

vehicular and pedestrian traffic.

Blowing sand and migrating 

dunes Animal species similar to beach habitat

Fore-island dune ridge

(B3)

Grass-covered or sparsely vegetated. Fine, well-sorted sand containing rare shells. Parallels the 

Gulf of Mexico shoreline, becoming less prominent farther south on the island.

Mapping notes: Vegetated surfaces interpreted primarily from 2003 photographs. Boundary 

frequently interpreted through comparison with 2000 LIDAR DEM. Aerial photography 

signature alone did not provide sufficient information to delineate dune extent; however, the high 

vertical resolution of the LIDAR allowed delineation of the dune ridge throughout the entire 

length of the island.

Unsuitable for development and recreation, should not 

be breached. Lens of fresh groundwater underneath, 

thinning towards back-island dune system.

Inundation and erosion 

during hurricanes. First 

landward defense against 

floods and storms.

Plants zoned by elevation on the dunes. Lower: marshhay cordgrass (Spartina 

patens), morning glory (Ipomoea spp.), and sea purslane (Sesuvium portulacastrum).

Middle and upper: sea oats (Uniola paniculata), bitter panicum (Panicum amarum), 

and gulf croton (Croton punctatus). Backside of foredunes: seacoast bluestem 

(Andropogon scoparius littoralis).

Ghost crab, burrowing mammals, and reptiles 

Vegetated barrier flat

(B4)

Most common terrestrial environment. Fine and coarse sand and shells. Includes heavily 

vegetated stabilized dunes. Located behind fore-island dune ridge. 

Remains of three (ranching) line camps. Suitable for 

recreation but very limited suitability for paths, trails, 

4WD trails, and camp and picnic areas. Groundwater 

withdrawal and water diversion could easily kill 

vegetation, resulting in dune migration.

Flooding and ponding

Grasses. Riparian shrubs, vines, and trees. Freshwater wetland plants. Pocket 

gophers, moles, weasels, ground squirrels, mice, and snakes; occasionally coyotes, 

shrews, bats, raccoons, skunks, rats, jackrabbits, and armadillos. Insects abound.

Temporarily flooded brackish 

to fresh marsh

(B4a)

Forms in deeper, troughlike wind-deflation flats. Floored by mostly sand, also mud and plant 

debris.

Mapping notes: Lightest tone of marsh signatures. Visible on 2003 photographs but absent on 2002 

and 1995 photographs.

Sensitive to recreation Flooding and ponding

Various forms of algae. Plants such as common cattail (Typha domingensis), 

American bulrush (Scirpus americanus), and spikerushes (Eleocharis sp.) in marshy 

areas.

Salt marsh

(B4b)

Forms in deeper, troughlike wind-deflation flats. Floored by mostly sand, also mud and plant 

debris.

Mapping notes: Characteristic dark signature adjacent to saltwater bodies or areas frequently 

flooded with saltwater. Undivided.

Sensitive to recreation Flooding and ponding Shoregrass (Monanthochloe littoralis), inland saltgrass (Distichlis spicata), bushy seaoxeye (Borrichia frutescens), and perennial forbs

Seasonally flooded brackish to 

fresh marsh

(B4c)

Forms in deeper, troughlike wind-deflation flats. Floored by mostly sand, also mud and plant 

debris.

Mapping notes: Visible on 2003 and 2002 photographs. Mostly absent from 1995 photographs.

Sensitive to recreation Flooding and ponding

Various forms of algae. Plants such as common cattail (Typha domingensis),

American bulrush (Scirpus americanus), and spikerushes (Eleocharis sp.) in marshy 

areas.

Semipermanently flooded 

brackish to fresh marsh

(B4d)

Forms in deeper, troughlike wind-deflation flats. Floored by mostly sand, also mud and plant 

debris.

Mapping notes: Darkest marsh signature. Visible on all vintages of photographs.

Sensitive to recreation Flooding and ponding

Various forms of algae. Plants such as common cattail (Typha domingensis),

American bulrush (Scirpus americanus), and spikerushes (Eleocharis sp.) in marshy 

areas.

Sparsely vegetated barrier flat

(B5) Highly fragmented vegetation. Barren areas expose oldest sand in the national seashore. Suitable water source for groundwater withdrawal. Greatest volume of freshwater of any map unit.

Flooding and ponding. 

Active blowout dunes.

Potential habitat for grasses. Riparian shrubs, vines, and trees. Freshwater wetland 

plants. Pocket gophers, moles, weasels, ground squirrels, mice, and snakes; 

occasionally coyotes, shrews, bats, raccoons, skunks, rats, jackrabbits, and 

armadillos.
PAIS Geologic Resources inventory Report 31

Age Unit Name

(Symbol) Features and Description Suitability for Infrastructure and Recreation Hazards Habitat

QUATERNARY

(Holocene Epoch)

Barrier Island Units

Stabilized dune

(B5a)

Moderately vegetated blowout dunes in elongate or parabolic forms. Sediment derived from foreisland dune ridge.

Mapping notes: Distinct from vegetated barrier flat through shape and bright signature of 

underlying dune sand.

Unsuitable for development and recreation. Should not 

be disturbed.

Second defense against 

floods and storms

Grasses such as bitter panicum (Panicum amarum), bitter panicum (Panicum 

amarum), gulf dune paspalum (Paspalum vaginatum), and dropseed (Sporobolus 

silveanus)

Storm washover channel

(B7)

Non-vegetated conduit as much as 213 m (700 ft) wide for sand, shells, and water during storms.

Mud in ponded areas.

Mapping notes: Found exclusively in the southern third of the map area (South Padre Island).

Unsuitable for developments. Hurricane-formed feature. 

Cuts through fore-island dune ridges.

Inundation during 

hurricanes. Ponding. Algal mats around edges of ponds

Storm washover fan

(B7a)

Non-vegetated sand deposit from washover channel. Slight topographic relief on gulfward end 

but grades imperceptibly into back-island environments. Unsuitable for developments. Hurricane deposit. Inundation during hurricanes Non-vegetated

Wind-deflation flat

(B8)

Sand and finer sediments with shell and cobble lag deposit. Mud drapes. Transitional between 

barrier and lagoon environments.

Mapping notes: Found in southern third of the map area (South Padre Island).

Unsuitable for development. Serves as storm-water 

runway.

Storm waters deposit 

sediment on unit. Flooding 

and ponding in troughs.

Algal mats

Sand flat

(B8a)

Non-vegetated, high sand flat. Commonly fringing back-island vegetated barrier flats and coppice 

dunes. Unsuitable for development Blowing sand and rapidly migrating dunes. Flooding. Non-vegetated

Water

(B9) Larger ponds and marshes. Ephemeral. Forms in deeper troughs of wind-deflation flats.

Ponds serve as discharge areas during fall and spring 

(wet seasons) and recharge areas during summer and 

winter (dry seasons) of groundwater system.

Ponds contain pathogenic 

organisms. Should not be 

used for potable water.

Various forms of algae. Plants such as common cattail (Typha domingensis),

American bulrush (Scirpus americanus), and spikerushes (Eleocharis sp.) in marshy 

areas.

Dredged channel

(B9a) Gulf Intracoastal Waterway and Mansfield Channel. Contains gypsum crystals.

Suitable for boating and fishing. Maintenance dredging 

required to keep channels open. Laguna Madre sensitive 

to changing salinity and hydrological conditions.

Sedimentation, turbidity, 

and resuspension of 

contaminants

Negative impacts to grassflats, vertebrate and invertebrate marine species, and 

nesting-bird habitat.

Active dunes

(B10)

Devegetated. Fine sands. Back-island dune field and fore-island blowout dunes. Exemplifies 

barrier-island migration toward the Texas mainland.

Mapping notes: Bright, nearly white signature on photographs.

Unsuitable for development and recreation. Should not 

be breached.

Blowing sand and migrating 

dunes Burrowing mammals and reptiles

Back-island sand flat

(B11) Non-vegetated, small migrating dunes. Transitional from back-island dune field to wind-tidal flat. Unsuitable for development and recreation. Sensitive to vehicular and pedestrian traffic.

Rapid changes in dune 

forms. Flooding. Algal mats. Texas pocket gopher.

Lagoon System Units

Wind-tidal flat with small 

dunes

(L1)

Loose wind-blown sand. Contains gypsum crystals. Forms in Land-Cut area.

Unsuitable for development and recreation. Highly 

susceptible to vehicular and pedestrian traffic. Cut by 

Gulf Intracoastal Waterway. Brine (hypersaline) 

groundwater.

Inundation during 

hurricanes. Migrating dunes. 

Rarely flooded by wind 

tides.

Saline-water wetland plants. Macroinvertebrates. Piping plover. Texas pocket 

gopher.

Wind-tidal flat with firm sand 

and mud

(L2)

Higher areas rarely flooded. Lagoonward fringes of back-island area.

Unsuitable for development and recreation. Highly 

susceptible to vehicular and pedestrian traffic. Cut by 

Gulf Intracoastal Waterway. Brine (hypersaline) 

groundwater.

Inundation during 

hurricanes. Rarely flooded 

by wind tides.

Saline-water wetland plants. Macroinvertebrates. Piping plover.

Wind-tidal flat with algal mats

(L3) Sand and mud. Alternately emergent/submergent.

Unsuitable. Highly susceptible to vehicular and 

pedestrian traffic. Cut by Gulf Intracoastal Waterway. 

Brine (hypersaline) groundwater.

Inundation during 

hurricanes. Frequent 

flooding. Ponding.

Algal mats. Saline-water wetland plants. macroinvertebrates. Piping plover.

Vegetated sand and shell 

berms

(L4)

Vegetated, accretionary, subaerial, natural islands (i.e., North Bird and South Bird islands) Development and recreation prohibited. Wildlife 

sanctuary. Storm waves

Bitter panicgrass (Panicum amarum), perennial forbs, sea oats (Uniola paniculata), 

camphorweed (Pluchea Cass.), and seacoast bluestem (Schizachyrium scoparium). 

Popular nesting grounds for a variety of bird species.

Lagoon-margin sand

(L5)

Subaqueous to emergent, usually submerged under as much as 0.9 m (3 ft) of water. Sand waves 

and ripples common. Unsuitable. Generally submerged. Flooding and ponding High-energy lagoon margin. Typically unvegetated.

Grassflat

(L6)

Mud including shell and grass. Generally covered by <1.2 m (4 ft) of water. Shallowest parts 

exposed at low tide.

Unsuitable. Submerged. High biologic productivity easily 

disturbed by changes in turbidity, salinity, and water 

depth.

None

Algae. Marine grasses, primarily shoalgrass (Halodule wrightii), also widgeon-grass 

(Ruppia maritima), turtlegrass (Thalassia testudinum), clovergrass (Halophila 

engelmannii), and manateegrass (Cymodocea manatorium). Saline-water wetland 

plants. Extensive invertebrate population. Breeding grounds for fish and waterbird 

species.

PAIS Geologic Resources Inventory Report 33

Geologic History

This section describes the rocks and unconsolidated deposits that appear on the digital 

geologic map of Padre Island National Seashore the environment in which those units were 

deposited, and the timing of geologic events that created the present landscape.

Along the gently sloping continental shelf of eastern 

Texas, near the Rio Grande, Colorado River, and Brazos 

River deltas, many factors converged to favor barrierisland development, namely an ample sediment supply 

and waves that break far from the shoreline. Add to this a 

change between regressive and transgressive conditions 

(table 4), and Padre Island literally took shape.

Between 125,000 and 110,000 years ago, the continental 

shelf built up as rivers deposited deltaic mud and fluvial 

sand (Beaumont Formation) during the Sangamon 

interglacial stage (Morton and Price 1987). The 

Sangamon was the “warm period” between the Illinoian 

and Wisconsinan glaciations. The Wisconsinan was the 

last major advance of continental-scale glaciers on North 

America. Strand-plain deposits (sand reworked by waves 

and currents at the prograded shoreline) marked the 

seaward extent of delta advance (Morton and McGowen 

1980). About 50,000 years ago, sea level dropped during a 

regression (table 4). Rivers eroded the fluvial-deltaic 

deposits on the exposed continental shelf, creating a 

network of deep valleys. Depths of these valleys range 

from 15 m to 38 m (50 ft to 125 ft) below present-day sea 

level (Fisk 1959). 

Approximately 18,000 years ago, sea level began to rise 

rapidly, flooding the entrenched river valleys. Estuaries 

formed in the lower valleys, and the upper valleys filled 

with fluvial-deltaic deposits. Large rivers with large 

watersheds (e.g., the Rio Grande and Brazos River) 

carried enough sediment to fill their valleys and build 

deltas into the Gulf of Mexico. The unfilled, former river 

valleys became today’s major bays (e.g., Galveston, 

Mobile, and Pensacola) (Morton 1994). 

Rapid sea level rise (transgression) persisted until about 

4,500 years ago when the rate of sea-level rise slowed. 

Barrier islands began forming, initially as submerged 

shoals on drainage divides (topographic highs) between 

the river valleys that cut into the continental shelf. 

Erosion of Pleistocene (2.6 million to 11,700 years old) 

strand-plain sand supplied sediment for barrier-island 

development (Morton and McGowen 1980). The shoals 

became a series of discontinuous, sandy islands (Morton 

and McGowen 1980). About 3,700 years ago, these small 

islands coalesced into a continuous barrier that ran 

parallel to the mainland. This barrier enclosed Laguna 

Madre. 

As hurricanes and prevailing southeasterly winds 

transported sand across the island, a deeper, open 

lagoonal system evolved into a shallow, closed 

hypersaline system. The width of Laguna Madre 

decreased, and finer sediments accumulated in the center 

(Morton and McGowen 1980). Fisk (1959) estimated 

that about 170 years ago central Laguna Madre became 

filled with sand derived from Padre Island. The fill, 

which is characteristic of the Land Cut, divided Laguna 

Madre into a northern and southern section. Laguna 

Madre will eventually become filled from the Land Cut 

northward to Baffin Bay (Morton and McGowen 1980).

Since their inception, various parts of the barrier chain 

have (1) prograded seaward by the deposition of sand on 

their shorefaces, (2) remained stationary (aggraded), or 

(3) moved landward by shoreface erosion (retrograded). 

Aggradational barriers occupy the same general 

geographic position and grow vertically in response to 

abundant sediment supply and a relative rise in sea level, 

rather than growing seaward as do progradational 

barriers (Morton 1994). Progradational and 

aggradational barriers are wide, thick, and have high 

topographic profiles that effectively prevent storm surges 

from washing over the islands (Morton 1994). 

Retrogradational barriers are narrow, thin, and have low 

topographic profiles that allow repeated overwash by 

even minor storm surges (Morton 1994).

Two of these geomorphic modes are apparent at Padre 

Island National Seashore. The northern and central 

segments of the national seashore have remained 

stationary and grown upward as an aggradational barrier 

(fig. 15). The southern (retrogradational) part has 

migrated lagoonward. 

Both progradational and aggradational barrier types 

began as retrogradational, but the availability of sand 

from the converging Brazos-Colorado and Rio Grande 

deltas allowed the central and northern parts of Padre 

Island National Seashore to become aggradational 

(Morton 1994). The shoreline curvature (concave 

seaward), predominant wind directions, and deficit in 

annual rainfall also contributed to development of an 

aggradational barrier. Eolian processes prevent 

progradation, that is, significant seaward migration and 

loss of sand from the beach. Under relatively stable sealevel conditions, this central segment of the barrier island 

continues to aggrade as windblown sand is transported 

onto the barrier and stored on it (Morton 1994).

The southern part of Padre Island National Seashore is a 

retrogradational barrier (fig. 15). Such barriers typically 

form along shoreline segments that are either straight or 

convex seaward. This configuration focuses wave energy 

and promotes sediment transport away from the barrier, 

leaving it starved for sand (Morton 1994). 

Retrogradational barriers are also known as 

“transgressive” or “low-profile” barriers. The southern 

retrogradational barrier of the national seashore 
34 NPS Geologic Resources Division 

originated from, and is located at, the retreating flank of 

the Rio Grande delta. The diagnostic feature of this type 

of island is the sharp contact of the sand with the 

underlying lagoonal sediments (Dickinson et al. 1972). At 

Padre Island National Seashore, the modern barrier 

island is a thin wedge of sand situated over Holocene and 

Pleistocene mud of the founding Rio Grande delta 

(Morton 1994). Presently, little sand is available for 

maintenance of South Padre Island because of dams on 

the Rio Grande and jetties at Brazos Santiago Pass 

(Morton 1994).

Texas barriers are entering a new phase of their 

evolution as a result of recent decreases in sediment 

supply and increases in the rate of relative sea-level rise 

(Morton 1994). Erosion of both the Gulf of Mexico and 

Laguna Madre shorelines is causing most barriers to 

narrow. Moreover, retrogradational barriers, including 

the southern part of Padre Island National Seashore, are 

continuing to migrate landward. Central Padre Island, 

including the central and northern parts of Padre Island 

National Seashore, is an exception among the Texas 

barriers; it remains an actively aggrading island as a result 

of converging littoral currents. The sediment supplied to 

the beaches, dunes, and wind-tidal flats of this part of 

Padre Island is sufficient to maintain a stable landform 

even as relative sea level is rising (Morton 1994).

Table 4. Transgressive and regressive characteristics of shorelines

Transgression Regression

Interglacial stage Glacial stage

High (or rising) sea level Low (or decreasing) sea level

Warmer climatic conditions Colder climatic conditions

Low (or decreasing) precipitation Abundant (or increasing) precipitation

Low (or decreasing) sediment supply Abundant (or increasing) sediment supply

Decreasing streamflow Increasing streamflow

Fluvial valley aggradation or non-erosion Fluvial valley entrenchment or erosion

Shoreface retreat (landward) Shoreface progradation and advance (seaward)

Delta retreat (landward) Delta progradation and advance (seaward)

Sediments thickening landward (updip) Sediments thickening seaward (downdip)

Barrier-lagoon formation Strand-plain formation
PAIS Geologic Resources Inventory Report 35

Figure 14. Geologic timescale. Included are major life history and tectonic events occurring on the North American continent. Red lines 

indicate major unconformities between eras. Radiometric ages shown are in millions of years (Ma). Compass directions in parentheses 

indicate the regional location of individual geologic events. Adapted from the U.S. Geological Survey, http://pubs.usgs.gov/fs/2007/3015/

with additional information from the International Commission on Stratigraphy. http://www.stratigraphy.org/view.php?id=25.
36 NPS Geologic Resources Division 

Figure 15. Barrier island morphology. Barriers vary as a result of climate, shoreline geometry, patterns of longshore drift, past changes in sealevel rise, and sediment supply. Padre Island National Seashore is primarily an aggradational barrier, but the southern part is a 

retrogradational barrier. Graphic redrafted after Morton (1994) by Trista Thornberry-Ehrlich (Colorado State University).
PAIS Geologic Resources Inventory Report 37

Glossary

This glossary contains brief definitions of technical geologic terms used in this report. Not all 

geologic terms used are referenced. For more detailed definitions or to find terms not listed

here please visit: http://geomaps.wr.usgs.gov/parks/misc/glossarya.html. Definitions are 

based on those in the American Geological Institute Glossary of Geology (fifth edition; 2005).

amphibole. A common group of rock-forming silicate 

minerals. Hornblende is the most abundant type.

aquifer. A rock or sedimentary unit that is sufficiently 

porous that it has a capacity to hold water, sufficiently 

permeable to allow water to move through it, and 

currently saturated to some level.

astronomical tide. Also called “equilibrium tide.” A 

hypothetical global form of the sea-surface elevation 

which would be in equilibrium with the lunar and solar 

tide-generating forces in the absence of inertia and 

currents, and no continents.

barchan dune. A crescent-shaped dune with arms or 

horns of the crescent pointing downwind. The 

crescent or barchan type is most characteristic of 

inland desert regions.

barrier island. A long, low, narrow island formed by a 

ridge of sand that parallels the coast.

beach. A gently sloping shoreline covered with sediment, 

commonly formed by the action of waves and tides.

beach face. The section of the beach exposed to direct 

wave and/or tidal action.

bed. The smallest sedimentary strata unit, commonly 

ranging in thickness from one centimeter to a meter or 

two and distinguishable from beds above and below.

bedding. Depositional layering or stratification of 

sediments.

blowout. A geomorphology term for a small saucer-,

cup-, or trough-shaped hollow or depression formed 

by wind erosion on a preexisting dune or other sand 

deposit. Commonly applies to areas of shifting sand or 

loose soil, where vegetation is disturbed or destroyed. 

Generally includes the adjoining accumulation of sand 

derived from the depression. Some blowouts maybe 

many kilometers in diameter. 

biotite. A widely distributed and important rock-forming 

mineral of the mica group. Forms thin, flat sheets.

carbonate. A mineral that has CO3

-2 as its essential 

component (e.g., calcite and aragonite).

carbonate rock. A rock consisting chiefly of carbonate 

minerals (e.g., limestone, dolomite, or carbonatite).

clastic. Describes rock or sediment made of fragments of 

pre-existing rocks (clasts).

clay. Can be used to refer to clay minerals or as a 

sedimentary fragment size classification (less than 

1/256 mm [0.00015 in]).

claystone. Lithified clay having the texture and 

composition of shale but lacking shale’s fine layering 

and fissility (characteristic splitting into thin layers).

continental shelf. The shallowly submerged part of a 

continental margin extending from the shoreline to the 

continental slope with water depths less than 200 m 

(660 ft).

coppice dune. Also called “shrub-coppice dune,” 

“coppice mound,” or “nebkha.” A small, streamlined 

dune that forms to the lee of bush-and-clump 

vegetation on a smooth surface of very shallow sand.

coquina. Limestone composed of cemented shell 

fragments.

cross bedding. Uniform to highly varied sets of inclined 

sedimentary beds (“cross beds”) deposited by wind or 

water that indicate flow conditions such as water flow 

direction and depth.

cross section. A graphical interpretation of geology, 

structure, and/or stratigraphy in the third (vertical) 

dimension based on mapped and measured geological 

extents and attitudes depicted in a vertically oriented 

plane.

cross stratum. A cross bed.

delta. A sediment wedge deposited where a stream flows 

into a lake or sea.

dip. The angle between a bed or other geologic surface 

and horizontal.

dune. A low mound or ridge of sediment, usually sand, 

deposited by wind. Common dune types include 

“barchan,” “longitudinal,” “parabolic,” and 

“transverse” (see respective listings).

eolian. Describes materials formed, eroded, or deposited 

by or related to the action of the wind. Also spelled 

“Aeolian.”

estuary. The seaward end or tidal mouth of a river where 

freshwater and seawater mix; many estuaries are 

drowned river valleys caused by sea-level rise 

(transgression) or coastal subsidence.

eustatic. Relates to simultaneous worldwide rise or fall of 

sea level.

evaporite. A sedimentary rock composed primarily of 

minerals produced from a saline solution as a result of 

extensive or total evaporation of the solvent (usually 

water).

feldspar. A group of abundant (more than 60% of Earth’s 

crust), light-colored to translucent silicate minerals 

found in all types of rocks. Usually white and gray to 

pink. May contain potassium, sodium, calcium, 

barium, rubidium, and strontium along with 

aluminum, silica, and oxygen.

formation. Fundamental rock-stratigraphic unit that is 

mappable, lithologically distinct from adjoining strata, 

and has definable upper and lower contacts.

garnet. A hard mineral that has a glassy luster, often with 

well defined crystal faces, and a variety of colors, dark 

red being characteristic. Commonly found in 

metamorphic rocks.

glauconite. A green mineral, closely related to the micas. 

It is an indicator of very slow sedimentation.
38 NPS Geologic Resources Division 

hornblende. The most common mineral of the 

amphibole group. Hornblende is commonly black and 

occurs in distinct crystals or in columnar, fibrous, or 

granular forms.

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes of 

rocks—igneous, metamorphic, and sedimentary.

Interglacial stage. A subdivision of a glacial epoch 

separating two glaciations. Characterized by a 

relatively long period of warm or mild climate during 

which temperature rose to at least that of the present 

day; especially an interval of the Pleistocene Epoch, as 

the “Sangamon Interglacial Stage.”

lamination. Very thin, parallel layers.

longitudinal dune. Dune elongated parallel to the 

direction of wind flow.

longshore current. A current parallel to a coastline 

caused by waves approaching the shore at an oblique 

angle.

marl. An unconsolidated deposit commonly with shell 

fragments and sometimes glauconite consisting chiefly 

of clay and calcium carbonate that formed under 

marine or freshwater conditions.

metamorphic. Describes the process of metamorphism 

or its results. One of the three main classes of rocks—

igneous, metamorphic, and sedimentary.

metamorphism. Literally, a change in form. 

Metamorphism occurs in rocks through mineral 

alteration, formation, and/or recrystallization from 

increased heat and/or pressure.

mineral. A naturally occurring, inorganic crystalline solid 

with a definite chemical composition or compositional 

range.

mud cracks. Cracks formed in clay, silt, or mud by 

shrinkage during dehydration at Earth’s surface.

parabolic dune. Crescent-shaped dune with horns or 

arms that point upwind.

progradation. The seaward building of land area due to 

sedimentary deposition.

pyroxene. A common rock-forming mineral. It is 

characterized by short, stout crystals. 

regression. A long-term seaward retreat of the shoreline 

or relative fall of sea level.

ripple. The undulating, approximately parallel and 

usually small-scale ridge pattern formed on sediment 

by the flow of wind or water.

rock. A solid, cohesive aggregate of one or more minerals.

rutile. A reddish brown titanium oxide mineral. It occurs 

as a primary mineral in some igneous rocks, in 

metamorphic rocks, and as residual grains in beach 

sands.

Saffir-Simpson hurricane intensity scale. A five-category 

hurricane intensity scale developed by H. S. Saffir and 

R. H. Simpson that rates hurricanes from 1 to 5 on the 

basis of maximum sustained wind speed. Categories 

are 1 (weak), 2 (moderate), 3 (strong), 4 (very strong), 

and 5 (devastating).

sand. A clastic particle smaller than a granule and larger 

than a silt grain, having a diameter in the range of 1/16 

mm (0.0025 in) to 2 mm (0.08 in).

sandstone. Clastic sedimentary rock of predominantly 

sand-sized grains.

scour and fill. A sedimentary structure consisting of a 

small erosional channel, generally ellipsoidal, that is 

subsequently filled; a small-scale washout.

sediment. An eroded and deposited, unconsolidated 

accumulation of rock and mineral fragments.

sedimentary rock. A consolidated and lithified rock 

consisting of clastic and/or chemical sediment(s). One 

of the three main classes of rocks—igneous, 

metamorphic, and sedimentary.

serpulid. A round, segmented ocean worm of the Family 

Serpulidae with a crown of tentacles and a flat coiled 

shell.

silt. Clastic sedimentary material intermediate in size 

between fine-grained sand and coarse clay (1/256 to 

1/16 mm [0.00015 to 0.002 in]).

slope. The inclined surface of any geomorphic feature or 

measurement thereof. Synonymous with “gradient.”

soil. Surface accumulation of weathered rock and 

organic matter capable of supporting plant growth and 

often overlying the parent material from which it 

formed.

staurolite. A dark reddish-brown, blackish-brown, 

yellowish-brown, or blue mineral. Twinned crystals 

often resemble a cross. It is a common constituent of 

rocks such as mica schist and gneiss that have 

undergone metamorphism. Also called: staurotide, 

cross-stone, grenatite, and fairy stone.

strand plain. A shore built seaward by waves and 

currents and continuous for some distance along the 

coast.

strata. Tabular or sheet-like masses or distinct layers of 

rock.

stratigraphy. The geologic study of the origin, 

occurrence, distribution, classification, correlation, 

and age of rock layers, especially sedimentary rocks.

swash. The rush of water up the beach face after a wave 

has broken.

tourmaline. A crystal silicate mineral occurring in three-, 

six-, or nine-sided prisms composed of elements such 

as sodium, calcium, magnesium, iron, aluminum, 

lithium, and sometime fluorine in small amounts. 

Tourmaline is classed as a semi-precious stone and the 

gem comes in a wide variety of colors.

transgression. Landward migration of the sea as a result 

of a relative rise in sea level.

transverse dune. Dune elongated perpendicular to the 

prevailing wind direction. The leeward slope stands at 

or near the angle of repose of sand whereas the 

windward slope is comparatively gentle.

water table. The upper surface of the saturated zone; the 

zone of rock in an aquifer saturated with water.

zircon. A common accessory mineral in siliceous igneous 

rocks, crystalline limestone, schist, and gneiss, also in 

sedimentary rocks derived from and in beach and river 

placer deposits. When cut and polished, the colorless 

varieties provide exceptionally brilliant gemstones. 

Very durable mineral, often used for age-dating.
PAIS Geologic Resources Inventory Report 39

Literature Cited

This section lists references cited in this report. A more complete geologic bibliography is 

available from the National Park Service Geologic Resources Division.

Andrews, J. 1972. Shell collecting in the Padre Island 

National Seashore. Pages 31–35 in R. F. Travis, R. N. 

Tench, W. D. Hodgson, D. C. Callaway, and H. 

Wyneken, editors. Padre Island National Seashore 

Field Guide. GCAGS Convention Field Trip, October 

14, 1972. Gulf Coast Association of Geological 

Societies, Corpus Christi, Texas, USA.

Babcock & Brown. 2008. Harvesting the wind of the 

South Texas gulf coast. Online presentation. Babcock 

& Brown, Sydney, Australia. http://www.kenedy.org/

KenedyRanch/tabid/1093/Default.aspx (accessed 

September 1, 2010).

Baccus, J. T., and J. K. Horton. 1979. An ecological and 

sedimentary study of Padre Island National Seashore. 

Southwest Texas State University, San Marco, Texas, 

USA.

Baccus, J. T., and J. K. Horton. 1982. Pedestrian impacts: 

Padre Island. Pages 89–102 in B. Mayo and L. Smith, 

editors. Proceedings of Barrier Island Forum and 

Workshop, Provincetown, Massachusetts, May 28–30, 

1980. National Park Service, North Atlantic Region, 

Boston, Massachusetts, USA.

Baccus, J. T., J. K. Horton, and P. D. Carangelo. 1977. A 

study of beach and dunes floral and faunal 

interrelations as influenced by recreational and user 

impact on Padre Island National Seashore. Southwest 

Texas State University, San Marcos, Texas, USA.

Barajas, M. 2010. South Padre Island largely unscathed 

by Hurricane Alex. Online news article. The Monitor 

(2010-07-01 20:06:09). Freedom Communications, 

Inc., Irvine, California, USA. 

http://www.themonitor.com/common/printer/view.p

hp?db=monitortx&id=40402 (accessed August 31, 

2010).

Beavers, R. 2005. Sea-level impacts to coastal parks. Page 

31 in J. Selleck, editor. Natural Resource Year in 

Review—2004. Technical Information Center (TIC) 

publication number D-1609. U.S. Department of the 

Interior, National Park Service, Natural Resource 

Information Division, Denver, Colorado, USA.

Behrens, E. W., P. D. Carangelo, and H. S. Finkelstein. 

1976. Effect of vehicular and pedestrian traffic on 

backshore vegetation and beach development, beach 

impact study, Padre Island National Seashore. Final 

report for contract CX700050448. National Park 

Service, Southwest Regional Office, Santa Fe, New 

Mexico, USA.

Behrens, E. W., R. L. Watson, P. D. Carangelo, W. H. 

Sohl, and H. S. Finkelstein. 1975. Effect of vehicular 

and pedestrian traffic on backshore vegetation and 

dune development, beach impact study, Padre Island 

National Seashore. Final report for contract 

CX700040146. Technical Information Center (TIC) 

publication number D-88. National Park Service, 

Southwest Regional Office, Santa Fe, New Mexico, 

USA.

Blum, M., and J. R. Jones. 1985. Variation in vegetation 

density and foredune complexity at North Padre 

Island, Texas. Texas Journal of Science 37:63–73.

Boker, T. A. 1953. Sand dunes on northern Padre Island, 

Texas. Thesis. University of Kansas, Lawrence, 

Kansas, USA.

Boylan, D. M. 1986. The hydrologic resources of North 

Padre Island, coastal South Texas. Thesis. Baylor 

University, Waco, Texas, USA.

Breuer, J. P. 1957. An ecological survey of Baffin and 

Alazan bays, Texas. Publications of the Institute of 

Marine Science 4(2):134–155.

Brezina, D. N. 2004. Soil survey of Padre Island National 

Seashore, Texas, special report. Natural Resources 

Conservation Service, National Park Service, and 

Texas Agricultural Experiment Station, Washington, 

D.C., USA.

Brown, L. F., J. L. Brewton, T. J. Evans, J. H. McGowen, 

W. A. White, W. L. Fisher, and C. G. Groat. 1976. 

Environmental geologic atlas of the Texas coastal 

zone: Corpus Christi area. University of Texas, Bureau 

of Economic Geology, Austin, Texas, USA.

Brown, L. F., J. L. Brewton, T. J. Evans, J. H. McGowen, 

W. A. White, C. G. Groat, and W. L. Fisher. 1977. 

Environmental geologic atlas of the Texas coastal 

zone: Kingsville area. University of Texas, Bureau of 

Economic Geology, Austin, Texas, USA.

Brown, L. F., J. L. Brewton, T. J. Evans, J. H. McGowen, 

W. A. White, C. G. Groat, and W. L. Fisher. 1980. 

Environmental geologic atlas of the Texas coastal 

zone: Brownsville-Harlingen area. University of Texas, 

Bureau of Economic Geology, Austin, Texas, USA.

Bullard, F. M. 1942. Source of beach and river sands on 

gulf coast of Texas. Geological Society of America 

Bulletin 53(7):1021–1043.

Callaway, B. 1972. History of Padre Island. Pages 28–30 

in R. F. Travis, R. N. Tench, W. D. Hodgson, D. C. 
40 NPS Geologic Resources Division 

Callaway, and H. Wyneken, editors. Padre Island 

National Seashore Field Guide, GCAGS Convention 

Field Trip, October 14, 1972. Gulf Coast Association of 

Geological Societies, Corpus Christi, Texas, USA.

Chaney, A. H., B. R. Chapman, J. P. Karges, D. A. Nelson, 

and R. R. Schmit. 1978. Use of dredged material islands 

by colonial seabirds and wading birds in Texas. Texas 

A&I University, Kingsville, Texas, USA. 

Christensen, B. 2009. Port Mansfield Channel navigable 

once again. Online news article. The Monitor (April 

11, 2009). http://www.themonitor.com/articles/

channel-25295-mansfield-wilson.html (accessed 

September 1, 2010).

Cochran, S. A., A. E. Gibbs, and J. B. Logan. 2007. 

Geologic resource evaluation of Pu‘uhonua O 

Hōnaunau National Historical Park, Hawai‘i: Part II—

Benthic habitat mapping. Scientific Investigations 

Report 2006-5258. U.S. Geological Survey, Reston, 

Virginia, USA.

Conti, R. 2007. When dunes walk over dune walkovers. 

Gulf Coast Association of Geological Societies 

Transactions 57:151–163.

Dansby, L., D. Echols, and E. Martinez. 2008. Benthic 

invertebrate diversity and density on the Gulf of 

Mexico beach, Padre Island National Seashore, 

October 30, 2008. Draft funding proposal for Youth 

Partnership Program (YPP). National Park Service, 

Intermountain Regional Office; Padre Island National 

Seashore; and New Mexico Highlands University; 

Santa Fe, New Mexico; Corpus Christi, Texas; and Las 

Vegas, New Mexico, USA.

Dickinson, K. A., H. L. Berryhill, Jr., and C. W. Holmes. 

1972. Criteria for recognizing ancient barrier 

coastlines. Pages 192–214 in J. K. Rigby and W. K. 

Hamblin, editors. Recognition of ancient barrier 

environments. Special Publication 16. Society of 

Economic Paleontologists and Mineralogists (SEPM), 

Tulsa, Oklahoma, USA.

Ditton, R. B., and J. H. Gramann. 1987. A survey of 

down-island visitors and their use patterns at Padre 

Island National Seashore. Cooperative Park Study 

Unit Technical Report 6. Texas A&M University, 

Department of Recreation and Parks, Corpus Christi, 

Texas, USA.

Drawe, D. L. 1990. Succession of vegetation on Padre 

Island National Seashore. Progress report. Welder 

Wildlife Foundation, Sinton, Texas, USA.

Drawe, D. L., and I. M. Ortega. 1996. Impacts of 

geophysical seismic survey vehicles on Padre Island 

national seashore vegetation. Texas Journal of Science 

48(2):107–118.

Echols, D., and E. Kassman. 2004. Regulations help 

endangered sea turtles make a comeback. Pages 92–93 

in J. Selleck, editor. Natural Resource Year in 

Review—2003. Technical Information Center (TIC) 

publication number D-1533. National Park Service, 

Denver, Colorado, and Washington, D.C., USA.

Fisk, H. N. 1959. Padre Island and the Laguna Madre 

flats, coastal South Texas. Pages 103–151 in R. J. 

Russell, editor. Proceedings of the 2nd Coastal 

Geography Conference, Louisiana State University, 

Baton Rouge, Louisiana, April 6–9, 1959. National 

Academy of Sciences, Washington, D.C., USA. 

Gibbs, A. E., S. A. Cochran, J. B. Logan, and E. E. 

Grossman. 2007. Benthic habitats and offshore 

geological resources of Kaloko-Honnokōhau National 

Historical Park, Hawai`i. Scientific Investigations 

Report 2006-5256. U.S. Geological Survey, Reston, 

Virginia, USA.

Gibeaut, J., and T. Tremblay. 2005. Padre Island natural 

environments map (scale 1:5,000). Unpublished data. 

University of Texas, Bureau of Economic Geology, 

Austin, Texas, USA.

Godfrey, P. J. 1976. Management guidelines for parks 

and barrier beaches. U.S. Department of the Interior, 

National Park Service, Washington, D.C., USA.

Hart, L. 2003. Park Service names names in trashing 

Texas seashore. Online news article. Los Angeles 

Times (October 5, 2003). 

http://articles.latimes.com/2003/oct/05/nation/napadre5 (accessed September 1, 2010).

Hayes, M. O. 1963. Grain size characteristics of Padre 

Island (Texas) sediments: A study of effects of 

bimodality on grain size parameters. The Texas 

Journal of Science 15:407–409.

Hayes, M. O. 1965. Sedimentation on a semiarid, wavedominated coast (South Texas) with emphasis on 

hurricane effects. Dissertation. University of Texas, 

Austin, Texas, USA.

Hayes, M. O. 1967. Hurricanes as geologic agents: Case 

studies of Hurricane Carla, 1961, and Hurricane 

Cindy, 1963. Report of Investigations 61. University of 

Texas, Bureau of Economic Geology, Austin, Texas, 

USA.

Hill, G. W., and R. E. Hunter. 1976. Interaction of 

biological and geological processes in the beach and 

nearshore environments, northern Padre Island, 

Texas. Pages 169–187 in R. A. Davis Jr. and R. L. 

Ethington, editors. Beach and Nearshore 

Sedimentation. Special Publication 24. Society of 

Economic Paleontologists and Mineralogists (SEPM), 

Tulsa, Oklahoma, USA.

Hill, G. W., and R. E. Hunter. 1987. Interaction of 

biological and geological processes in the beach and 

nearshore environments, northern Padre Island, 

Texas. Pages 187–205 in R. A. Davis Jr., editor. Beach 

and Nearshore Sediments and Processes. Reprint 
PAIS Geologic Resources Inventory Report 41

Series 12. Society of Economic Paleontologists and 

Mineralogists (SEPM), Tulsa, Oklahoma, USA.

Hunter, R. E. 1977. Basic types of stratification in small 

eolian dunes. Sedimentology 24:361–387.

Hunter, R. E., R. L. Watson, G. W. Hill, and K. A. 

Dickinson. 1972. Modern depositional environments 

and processes, northern and central Padre Island, 

Texas. Pages 1–27 in R. F. Travis, R. N. Tench, W. D. 

Hodgson, D. C. Callaway, and H. Wyneken, editors. 

Padre Island National Seashore Field Guide, GCAGS 

Convention Field Trip, October 14, 1972. Gulf Coast 

Association of Geological Societies, Corpus Christi, 

Texas, USA.

IPCC. 2007. Climate Change 2007: Synthesis Report. 

Contribution of Working Groups I, II and III to the 

Fourth Assessment Report of the Intergovernmental 

Panel on Climate Change. Intergovernmental Panel on 

Climate Change Geneva, Switzerland. 

http://www.ipcc.ch/publications_and_data/ar4/syr/en/

contents.html. Accessed 9 September 2010.)

The John G & Marie Stella Kenedy Memorial 

Foundation. 2008. Kenedy Ranch overview. Online 

information. The John G & Marie Stella Kenedy 

Memorial Foundation, Corpus Christi, Texas, USA. 

http://www.kenedy.org/KenedyRanch/tabid/1093/Def

ault.aspx (accessed September 1, 2010).

Jones, W. D. 1999. Padre Island National Seashore: An 

administrative history. National Park Service, Austin, 

Texas, USA. http://www.nps.gov/history/history/

online_books/pais/index.htm (accessed September 1, 

2010).

Karl, T. R., Melillo, J. M., and Peterson, T. C., 2009, 

Global climate change impacts in the United States. 

Cambridge University Press, New York, New York, 

USA. http://www.globalchange.gov/publications/

reports/scientific-assessments/us-impacts. (accessed 

September 9, 2010.)

Kattner, K. R. 1973. Secondary successional vegetation 

on Padre Island National Seashore, Texas. Texas 

A&M University, Kingsville, Texas, USA.

Kenworthy, J. P., V. L. Santucci, and C. C. Visaggi. 2007. 

Paleontological resource inventory and monitoring, 

Gulf Coast Network. Technical Information Center 

(TIC) publication number D-750. National Park 

Service, Lakewood, Colorado, USA. 

Leatherman, S. P. 1988. Barrier island handbook. 3rd 

edition. Coastal Publications Series. The University of 

Maryland, Laboratory for Coastal Research, College 

Park, Maryland, USA.

Leatherwood, A. 2008. The handbook of Texas online: 

Port Mansfield Channel. The Texas State Historical 

Association, Denton, Texas, USA. 

http://www.tshaonline.org/handbook/online/articles/

PP/rup2.html (accessed September 1, 2010). 

Lonard, R. I., F. W. Judd, J. H. Everitt, D. E. Escobar, M. 

A. Alaniz, I. Cavazos, and M. R. Davis. 1999. Vegetative 

change on South Padre Island, Texas, over twenty 

years and evaluation of multispectral videography in 

determining vegetative cover and species identity. 

Southwestern Naturalist 44:261–271.

Lundelius, E. L. Jr. 1972. Fossil vertebrates from the late 

Pleistocene Ingleside fauna, San Patrico County, 

Texas. Report of Investigations 77. University of 

Texas, Bureau of Economic Geology, Austin, Texas, 

USA. 

Masson, P. H. 1955. An occurrence of gypsum in 

southwest Texas. Journal of Sedimentary Petrology 

25:72–79.

McAtee, J. W. 1975. Human impact on the vegetation 

and microclimate on the beach and foredunes of Padre 

Island National Seashore. Thesis. Texas A&I 

University, Kingsville, Texas, USA.

McAtee, J. W., and D. L. Drawe. 1980. Human impact on 

beach and foredune vegetation on North Padre Island, 

Texas. Environmental Management 4(6):527–538.

McAtee, J. W., and D. L. Drawe. 1981. Human impact on 

beach and foredune microclimate on North Padre 

Island, Texas. Environmental Management 5(2):121–

134.

McBride, E. F., and H. Honda. 1994. Carbonate 

sediments in shallowly buried Pleistocene and 

Holocene sandstone and limestone, South Texas gulf 

coast. Transactions of the Gulf Coast Association of 

Geological Societies 44:467–476.

McBride, E. F., H. Honda, and A. A. Abdel-Wahab. 1991. 

Fabric and origin of gypsum sand crystals, Laguna 

Madre, Texas. Abstract. AAPG Bulletin 75(9):1532.

McCoy, C. 2009. Protecting parks while achieving energy 

independence. Earth, Wind, & Water 2(4):1, 6–7. 

http://www1.nrintra.nps.gov/nrpc/newsletters/pdf/20

09/NRPCNewsSpring_2009.pdf (NPS intranet access 

only, August 28, 2009).

McGowen, J. H., L. F. Brown Jr., T. R. Calnan, J. L. Chin, 

J. P. Herber, and C. L. Lewis. 1977. History and 

processes involved in development of South Padre 

Island, Laguna Madre, and Los Bancos de en Medio. 

Report to Texas General Land Office. University of 

Texas, Bureau of Economic Geology, Austin, Texas, 

USA.

McGowen, J. H., C. G. Groat, L. F. Brown Jr., W. L. 

Fisher, and A. J. Scott. 1970. Effects of Hurricane Celia: 

A focus on environmental geologic problems of the 

Texas coastal zone. Circular 70-3. University of Texas, 

Bureau of Economic Geology, Austin, Texas, USA.

McGowen, J. H., C. V. Proctor, L. F. Brown Jr., T. H. 

Evans, W. L. Fisher, and C. G. Groat. 1976. 
42 NPS Geologic Resources Division 

Environmental geologic atlas of the Texas coastal 

zone: Port Lavaca area. University of Texas, Bureau of 

Economic Geology, Austin, Texas, USA.

McGowen, J. H., and A. J. Scott. 1975. Hurricanes and 

geologic agents on the Texas coast. Pages 23–43 in

Estuarine Research. Volume 2: Geology and 

Engineering. Academic Press, New York, New York, 

USA.

Meehl, G. A., Stocker, T. F., Collins, W. D., 

Friedlingstein, P., Gaye, A. T., Gregory, J. M., Kitoh, A., 

Knutti, R., Murphy, J. M., Noda, A., Raper, S. C. B., 

Watterson, I. G., J, W. A., and Zhao, Z.-C. 2007. Global 

Climate Projections. in S. Solomon, D. Qin, M. 

Manning, Z. Chen, M. Marquis, K. B. Averyt, M. 

Tignor, and H. L. Miller, editors. Climate Change 

2007: The Physical Science Basis. Contribution of 

Working Group I to the Fourth Assessment Report of 

the Intergovernmental Panel on Climate Change. 

Cambridge University Press, Cambridge, United 

Kingdom and New York, New York, USA. 

http://www.ipcc-wg1.unibe.ch/publications/wg1-

ar4/wg1-ar4.html (accessed September 9, 2010).

Miller, G. W., and R. E. Hunter. 1979. Distribution of 

macroinvertebrates from subsurface Quaternary shell 

beds, northern Padre Island, Texas. Open-File Report 

OFR 79-1324. U.S. Geological Survey, Reston, 

Virginia, USA.

Miller, J. E., and E. R. Jones. 2003. Shoreline trash: 

Studies at Padre Island National Seashore, 1989–1998. 

Padre Island National Seashore and Texas A&M 

University, Corpus Christi, Texas, USA.

Morton, R. A. 1977. Historical shoreline changes and 

their causes, Texas gulf coast. Gulf Coast Association 

of Geological Societies Transactions 27:352–364.

Morton, R. A. 1994. Texas barriers. Pages 75–114 in R. A. 

Davis, editor. Geology of Holocene Barrier Islands. 

Springer-Verlag, Berlin, Germany.

Morton, R. A. 2003. An overview of coastal land loss with 

emphasis on the southeastern United States. Open-File 

Report 2003-337. U.S. Geological Survey, Center for 

Coastal and Watershed Studies, St. Petersburg, 

Florida, USA.

Morton, R. A., and C. W. Holmes. 2009. Geological 

processes and sedimentation rates of wind-tidal flats, 

Laguna Madre, Texas. Gulf Coast Association of 

Geological Societies Transactions 59:519–538.

Morton, R. A., and J. H. McGowen. 1980. Modern 

depositional environments of the Texas coast. 

Guidebook 20. University of Texas, Bureau of 

Economic Geology, Austin, Texas, USA.

Morton, R. A., T. L. Miller, and L. J. Moore. 2004. 

National assessment of shorelines change: Part 1—

Historical shoreline changes and associated coastal 

land loss along the U.S. Gulf of Mexico. Open-File 

Report 2004-1043. U.S. Geological Survey, Center for 

Coastal and Watershed Studies, St. Petersburg, 

Florida, USA.

Morton, R. A., and J. G. Paine. 1985. Beach and 

vegetation-line changes at Galveston Island, Texas: 

Erosion, deposition, and recovery from Hurricane 

Alicia. Geological Circular 85-5. University of Texas, 

Bureau of Economic Geology, Austin, Texas, USA. 

Morton, R. A., and W. A. Price. 1987. Late Quaternary 

sea-level fluctuations and sedimentary phases of the 

Texas coastal plain and shelf. Pages 181–198 in D. 

Nummedal, O. H. Pilkey, and J. D. Howard, editors. 

Sea Level Fluctuations and Coastal Evolution. Special 

Publication 41. Society of Economic Paleontologists 

and Mineralogists (SEPM), Tulsa, Oklahoma, USA.

Morton, R. A., and F. M. Speed. 1998. Evaluation of 

shorelines and legal boundaries controlled by water 

levels on sandy beaches. Journal of Coastal Research 

14(4):1373–1384.

National Park Service. 1973. Master plan, Padre Island 

National Seashore, Texas. U.S. Department of the 

Interior, National Park Service, Denver Service Center, 

Denver, Colorado, USA.

National Park Service. 1994. Padre Island National 

Seashore statement for interpretation. Padre Island 

National Seashore, Corpus Christi, Texas, USA.

National Park Service. 1995. Padre Island National 

Seashore parkwide development concept plan and 

environmental assessment. Public draft (January 2005). 

Technical Information Center (TIC) publication 

number D-58. Padre Island National Seashore, Corpus 

Christi, Texas, USA.

National Park Service. 2000. Final oil and gas 

management plan / environmental impact statement, 

February 2000, Padre Island National Seashore, 

Klegberg, Kenedy, and Willacy counties, Texas. U.S. 

Department of the Interior, National Park Service, 

Corpus Christi, Texas, USA.

National Park Service. 2001. Management policies 2001. 

NPS D1416 (December 2000). U.S. Department of the 

Interior, National Park Service, Washington, D.C., 

USA.

National Park Service. 2006a. Management policies 2006. 

U.S. Department of the Interior, National Park Service, 

Washington, D.C., USA.

National Park Service. 2006b. Padre Island National 

Seashore: Coasts / Shorelines. Online information. 

Padre Island National Seashore, Corpus Christi, Texas, 

USA. http://www.nps.gov/pais/naturescience/

coasts.htm (accessed September 1, 2010).

National Park Service. 2006c. Padre Island National 

Seashore: Mollusks. Online information. Padre Island 

National Seashore, Corpus Christi, Texas, USA. 
PAIS Geologic Resources Inventory Report 43

http://www.nps.gov/pais/naturescience/coasts.htm

(accessed September 1, 2010).

National Park Service. 2006d. Padre Island National 

Seashore: The oil and gas program at Padre Island 

National Seashore. Online information. Padre Island 

National Seashore, Corpus Christi, Texas, USA. 

http://www.nps.gov/pais/parkmgmt/white-paper.htm

(accessed September 1, 2010). 

National Park Service. 2007. Padre Island National 

Seashore: Geologic formations. Online information. 

Padre Island National Seashore, Corpus Christi, Texas, 

USA. http://www.nps.gov/pais/naturescience/

geologicformations.htm (accessed September 2, 2010).

National Park Service. 2009. Padre Island National 

Seashore: History & culture. Online information. 

Padre Island National Seashore, Corpus Christi, Texas, 

USA. http://www.nps.gov/pais/historyculture/

index.htm (accessed August 30, 2010).

National Park Service. 2010a. Explore nature: Explore 

geology. Coastal geology in our national parks: Global 

climate change and sea-level rise. Online information. 

http://www.nature.nps.gov/geology/coastal/

gw_slr.cfm (accessed September 2, 2010).

National Park Service. 2010b. Padre Island National 

Seashore: Bird Island Basin. Online information. Padre 

Island National Seashore, Corpus Christi, Texas, USA. 

http://www.nps.gov/pais/planyourvisit/bird_island_ba

sin.htm (accessed August 31, 2010). 

Norby, L. 2008. Updated reasonably foreseeable 

development scenario for oil and gas development at 

Padre Island National Seashore, October 31, 2008. 

National Park Service, Geologic Resources Division, 

Denver, Colorado, USA.

Nummedal, D., S. Penland, R. Gerdes, W. Schram, J. 

Kahn, and H. Roberts. 1980. Geologic response of 

hurricane impact on low-profile gulf coast barriers. 

Gulf Coast Association of Geological Societies 

Transactions 30:183–195.

Parry, M. L., O. F. Canziani, J. P. Palutikof, P. J. van der 

Linden, and C. E. Hanson. 2007. Climate Change 2007: 

Impacts, adaptation and vulnerability. Contribution of 

Working Group II to the Fourth Assessment Report of 

the Intergovernmental Panel on Climate Change 

(IPCC). Cambridge University Press, Cambridge, 

United Kingdom, and New York, New Yor, USA. 

http://www.ipcc-wg2.org/ (accessed September 1, 

2010).

Pendleton, E. A., R. Thieler, S. J. Williams, and R. L. 

Beavers. 2004. Coastal vulnerability assessment of 

Padre Island National Seashore (PAIS) to sea-level 

rise. Open-File Report 2004-1090. U.S. Geological 

Survey, Reston, Virginia, USA.

Pinet, P. R. 1992. Oceanography: An introduction to the 

planet oceanus. West Publishing Company, St. Paul, 

Minnesota, USA.

Price, W. A. 1958. Sedimentology and Quaternary 

geomorphology of South Texas. Gulf Coast 

Association of Geological Societies Transactions 8:41–

75.

Price, W. A. 1987. Range cattle, critical factor in eolian 

plain stabilization. Abstract. Page 67 in SEPM Annual 

Midyear Meeting, Volume 4. Society of Economic 

Paleontologists and Mineralogists (SEPM), Tulsa, 

Oklahoma, USA.

Price, W. A., and G. Gunter. 1942. Certain recent and 

geological and biological changes to South Texas, with 

consideration of probable causes. Texas Academy of 

Science Proceedings and Transactions 26:138–156.

Prouty, J. S. 1989. Historical back-barrier shoreline 

changes, Padre Island National Seashore, Texas. 

Abstract. AAPG Bulletin 73(9):1190.

Rabalais, N. N. 1977. Gulf beach national environmental 

study area. Padre Island National Seashore, Corpus 

Christi, Texas, USA.

Rahmstorf, S. 2007. A semi-empirical approach to

projecting future sea-level rise. Science 315:368–370.

Roth, D. 2004. Texas hurricane history: Featured 

storm—Bret (1999). National Weather Service, Lake 

Charles, Louisiana, USA. 

Roth, D. 2009. Texas hurricane history. Online 

document. National Weather Service, Camp Springs, 

Maryland, USA. http://www.hpc.ncep.noaa.gov/

research/txhur.pdf (accessed September 1, 2010).

Rusnak, G. A. 1960. Sediment of the Laguna Madre. 

Pages 153–196 in F. P. Shepherd, F. B. Phelger, and T. 

J. van Andel, editors. Recent Sediments, Northwestern 

Gulf of Mexico. American Association of Petroleum 

Geologists, Tulsa, Oklahoma, USA.

Scott, A. J., Hoover, R. A., and McGowen, J. H. 1969. 

Effects of Hurricane Beulah, 1967, on Texas coastal 

lagoons and barriers. Pages 221–236 in A. A. 

Castañares, F. B. Phleger, and D. F. México, editors. 

Lagunas Costeras: Un Simposio. UNAM-UNESCO, 

Memorandum Simposio Internacional Lagunas 

Costeras. Universidad Nacional Autónoma de México, 

Mexico City, Mexico.

Sheire, J. W. 1971. Padre Island National Seashore 

historic resource study. Technical Information Center 

(TIC) publication number D-1. U.S. Department of the 

Interior, National Park Service, Office of History and 

Historic Architecture, Washington, D.C., USA.

Smith-Engle, J. M., R. G. Hay, and D. D. Williams. 2006. 

Padre Island National Seashore, Bird Island Basin 

beach restoration project—Phase one. Final report. 
44 NPS Geologic Resources Division 

Texas A&M University, College of Science and 

Technology, Corpus Christi, Texas, USA.

Smith-Engle, J. M., R. Hay, and D. Williams. 2007. 

Models for beach renourishment of a back-barrier 

shoreline, Padre Island National Seashore, Texas. Gulf 

Coast Association of Geological Societies Transactions 

57:679–683.

Suter, J. R., and A. K. Maynard. 1983. Regional variability 

of washover deposits on South Texas coast. Abstract. 

AAPG Bulletin 67(9):1473.

Teerling, J. 1970. The incidence of the ghost crab 

Ocypode quadrata (Fabr) on the forebeach of Padre 

Island, and some of its responses to man. Thesis. Texas 

A&I University, Kingsville, Texas, USA.

Texas Center for Policy Studies. 2001. The Gulf 

Intracoastal Waterway from Corpus Christi to 

Brownsville: Little value, big cost. An update to the 

1994 report: “Subsidized destruction.” Online 

document. Texas Center for Policy Studies, Austin, 

Texas, USA. http://www.texascenter.org/publications/

littlevaluebigcost.pdf (accessed September 2, 2010).

Thayer, P. A., A. La Rocque, and J. W. Tunnell Jr. 1974. 

Relict lacustrine sediments on the inner continental 

shelf, southeast Texas. Gulf Coast Association of 

Geological Societies Transactions 24:337–347.

Tunnell, J. W. Jr. 2002a. Geography, climate, and 

hydrology. Pages 7–27 in J. W. Tunnell Jr. and F. W. 

Judd, editors. The Laguna Madre of Texas and 

Tamaulipas. Texas A&M University Press, College 

Station, Texas, USA.

Tunnell, J. W. Jr. 2002b. Origin, development, and 

geology. Pages 28–37 in J. W. Tunnell Jr. and F. W. 

Judd, editors. The Laguna Madre of Texas and 

Tamaulipas. Texas A&M University Press, College 

Station, Texas, USA.

Tunnell, J. W. Jr., K. Withers, and E. H. Smith. 2002. 

Conservation issues and recommendations. Pages 275–

288 in J. W. Tunnell, Jr. and F. W. Judd, editors. The 

Laguna Madre of Texas and Tamaulipas. Texas A&M 

University, College Station, Texas, USA.

U.S. Army Corps of Engineers. 2003. Final 

environmental impact statement, Volume 1: 

Maintenance dredging of the Gulf Intracoastal 

Waterway Laguna Madre, Texas, Nueces, Kleberg, 

Kenedy, Willacy, and Cameron counties, Texas 

(September 2003). U.S. Army Corps of Engineers, 

Galveston District, Southwestern Division, Galveston, 

Texas, USA. http://www.swg.usace.army.mil/items/

Laguna/eis/Laguna%20Madre%20FEIS%20-

%20Volume%20I.pdf (accessed September 2, 2010).

U.S. Army Corps of Engineers. 2005. Laguna Madre 

project. Online information. U.S. Army Corps of 

Engineers, Galveston District, Galveston, Texas, USA. 

http://www.swg.usace.army.mil/items/Laguna/special_

studies/ (accessed September 2, 2010).

U.S. Geological Survey. 2008. Coastal change hazards: 

Hurricanes and extreme storms. Hurricane Ike: Initial 

assessment of potential coastal-change impacts. Online 

information. U.S. Geological Survey, St. Petersburg, 

Florida, USA. http://coastal.er.usgs.gov/hurricanes/

ike/coastal-change/ (accessed September 2, 2010).

van Andel, T. H. 1960. Sources and dispersion of 

Holocene sediments, northern Gulf of Mexico. Pages 

34–55 in F. P. Shepard, F. B. Phleger, and T. H. van 

Andel, editors. Recent Sediments, Northwest Gulf of 

Mexico. American Association of Petroleum 

Geologists (AAPG), Tulsa, Oklahoma, USA.

Watson, R. L. 1968. Origin of shell beaches, Padre Island, 

Texas. Thesis. University of Texas, Austin, Texas, 

USA. 

Weise, B. R., and W. A. White. 1980. Padre Island 

National Seashore: A guide to the geology, natural 

environments, and history of a Texas barrier island. 

Guidebook 17. University of Texas, Bureau of 

Economic Geology, Austin, Texas, USA.

Wicksten, M. K., T. M. Green, and M. H. Sweet. 1987. A 

quantitative study of sandy beach organisms at Padre 

Island National Seashore. Texas A&M University, 

Department of Biology, College Station, Texas, USA.

Withers, K. 1996. An evaluation of recovery of benthic 

invertebrate communities in vehicle tracks and 

restored oil and gas impacted areas on wind-tidal flats 

in the upper Laguna Madre, Padre Island National 

Seashore, Texas. Contract 1443PX749050162. U.S. 

Department of the Interior, National Park Service, 

Natural Resource Program Center, Water Resources 

Division, Fort Collins, Colorado, USA..

Withers, K., E. Smith, O. Romez, and J. Wood. 2004. 

Assessment of coastal water resources and watershed 

conditions at Padre Island National Seashore, Texas. 

Technical Report NPS/NRWRD/WRTR—2004/323. 

U.S. Department of the Interior, National Park Service, 

Natural Resource Program Center, Water Resources 

Division, Fort Collins, Colorado, USA.

Zervas, C. 2001. Sea level variations of the United States 

1854–1999. Technical Report NOS CO-OPS 36. 

National Oceanographic and Atmospheric 

Administration (NOAA), Silver Spring, Maryland, 

USA. http://tidesandcurrents.noaa.gov/publications/

techrpt36doc.pdf (accessed September 2, 2010).
PAIS Geologic Resources Inventory Report 45

Additional References

This section lists additional references, resources, and web sites that may be of use to 

resource managers. Web addresses are current as of September 2010

Geology of National Park Service Areas

National Park Service Geologic Resources Division 

(Lakewood, Colorado): http://nature.nps.gov/geology/

NPS Geologic Resources Inventory:

http://www.nature.nps.gov/geology/inventory/gre_pu

blications.cfm

U.S. Geological Survey Geology of National Parks 

(includes 3D photographs):

http://3dparks.wr.usgs.gov/

Harris, A. G., E. Tuttle, and S. D. Tuttle. 2003. Geology of 

National Parks. Sixth Edition. Kendall/Hunt 

Publishing Co., Dubuque, Iowa, USA.

Kiver, E. P. and D. V. Harris. 1999. Geology of U.S. 

parklands. John Wiley and Sons, Inc., New York, New 

York, USA.

Lillie, R. J. 2005. Parks and Plates: The geology of our 

national parks, monuments, and seashores. W.W. 

Norton and Co., New York, New York, USA.

[Geared for interpreters].

NPS Geoscientist-in-the-parks (GIP) internship and 

guest scientist program: http://www.nature.nps.gov/

geology/gip/index.cfm

Resource Management/Legislation Documents

NPS 2006 Management Policies (Chapter 4; Natural 

Resource Management): http://www.nps.gov/policy/

mp/policies.html

NPS-75: Natural Resource Inventory and Monitoring 

Guideline: http://www.nature.nps.gov/

nps75/nps75.pdf.

NPS Natural Resource Management Reference Manual 

#77: http://www.nature.nps.gov/Rm77/

Geologic Monitoring Manual

R. Young and L. Norby, editors. Geological 

Monitoring. Geological Society of America, Boulder, 

Colorado.

[Website under development]. Contact the Geologic 

Resources Division to obtain a copy.

NPS Technical Information Center (Denver, repository 

for technical (TIC) documents): http://etic.nps.gov/

National Park Service Climate Change

National Park Service Climate Change Response 

Program:

http://www.nature.nps.gov/climatechange/index.cfm

Global climate change impacts in the United States 

report (Karl et al. 2009) 

http://www.globalchange.gov/publications/reports/sci

entific-assessments/us-impacts.

Geological Survey Websites

Texas Bureau of Economic Geology

http://www.beg.utexas.edu/

U.S. Geological Survey: http://www.usgs.gov/

Geological Society of America: 

http://www.geosociety.org/

American Geological Institute: http://www.agiweb.org/

Association of American State Geologists: 

http://www.stategeologists.org/

Other Geology/Resource Management Tools

Bates, R. L. and J. A. Jackson, editors. American 

Geological Institute dictionary of geological terms (3rd 

Edition). Bantam Doubleday Dell Publishing Group, 

New York.

U.S. Geological Survey National Geologic Map Database 

(NGMDB): http://ngmdb.usgs.gov/

U.S. Geological Survey Geologic Names Lexicon 

(GEOLEX; geologic unit nomenclature and summary): 

http://ngmdb.usgs.gov/Geolex/geolex_home.html

U.S. Geological Survey Geographic Names Information 

System (GNIS; search for place names and geographic 

features, and plot them on topographic maps or aerial 

photos): http://gnis.usgs.gov/

U.S. Geological Survey GeoPDFs (download searchable 

PDFs of any topographic map in the United States): 

http://store.usgs.gov (click on “Map Locator”).

U.S. Geological Survey Publications Warehouse (many 

USGS publications are available online): 

http://pubs.usgs.gov

U.S. Geological Survey, description of physiographic 

provinces: http://tapestry.usgs.gov/Default.html
46 NPS Geologic Resources Division 

Appendix A: Overview of Digital Geologic Data

The following pages are an overview of the digital geologic data for Padre Island 

National Seashore. For poster-size PDFs of this overview and complete digital data, 

please see the included CD or visit the Geologic Resources Inventory publications web

site: http://www.nature.nps.gov/geology/inventory/gre_publications.cfm.




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Communities. 

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