10th Grade Young Geoscientists
John A. and Katherine G. Jackson
School of Geosciences
The University of Texas at Austin
Southwest Texas Junior College
10th Grade Young Geoscientists
Guidebook for Geoscience Field Trip
to Mustang Island, Texas
June 2007
Tiffany Hepner*
*Tiffany Hepner is a coastal scientist at the
Bureau of Economic Geology, Scott W. Tinker, Director.
The Bureau of Economic Geology is a research unit
of the Jackson School of Geosciences.
John A. and Katherine G. Jackson
School of Geosciences
The University of Texas at Austin
Southwest Texas Junior College
10th Grade Young Geoscientists
INTRODUCTION
of the delicate balance of environments on this
very narrow stretch of land that meets the sea.
What we can learn at Mustang Island applies
to much of the Texas coast and to other similar
settings throughout the world.
This field trip is not just about the
individual environments. It is also about how
these environments interact with each other
and about the processes involved in creating
and modifying each environment. We will
look at Mustang Island through the eyes of a
geoscientist.
At each stop we will make observations and
learn how the environment formed. We will
compare one place to another and discuss how
they are similar, how they differ, and how they
are interrelated. Understanding what we see
adds to our appreciation and our response to
the beauty of the coast. This understanding is
also important in protecting our environment,
responsibly using natural resources, and limiting the impact of natural hazards. The geosciences provide important information about
how people live on the surface of the Earth.
At first it may seem as if there are a lot of
new terms, but because these are related to
features we’ll actually see, they will be pretty
easy for you to remember. The technical words
are either explained in the text or in the glossary,
or both. We hope that this guidebook will help
you expand your non-technical vocabulary.
When the text uses a non-technical word that
you may not know, we’ve defined these in a
“word alert.”
This field trip is not just about the geology.
Remember that you are at the BEACH!!!
How often do you get to go on field trips to the
beach? Have a great time while you are visiting,
but remember these few key points. There is
Did you know that Texas has 600 kilometers (that’s almost 375 miles) of shoreline
along the Gulf of Mexico? Ever-increasing
numbers of visitors travel to the Texas Gulf
Coast for recreation; they also travel on business
and in search of employment opportunities in
our coastal cities and towns. The coastal zone
is also highly productive—in terms of critical
natural resources, especially crude oil and
natural gas. The rapidly expanding population
is straining the capabilities of land and natural
resources, and development is threatening
critical habitats and species. The Gulf Coast
is subject to the episodic occurrence of violent
tropical storms, as well as the less dramatic, but
more constant, impacts of wind and waves. For
the coastal zone to be managed and developed
wisely, it is vitally important that we understand
the geoscience processes that affect the coastal
zone. In other words, we need to understand
and document natural processes at work, such as
the impacts of waves, winds, rivers, storms, and
changes in sea level on the coastal environment.
We also need to monitor the impacts of our
choices in how we develop the coastal zone and
how these choices interact with and affect the
natural processes.
On this GeoFORCE Texas Young Geoscientists field trip you will learn about a very
special place in Texas. We will be visiting
Mustang Island, a barrier island along the Gulf
of Mexico coast of Texas. Many people enjoy
visiting the beach. Some enjoy baking in the Sun
and playing in the surf; others enjoy strolling
along the shore looking for shells and listening
to the soothing sounds of the waves. Still others
visit the coast to look at migrating birds during
the spring and fall. Most people are not aware
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ORIENTATION
Field Day #1
not a lot of shelter from hot Texas sunshine.
Make sure you apply plenty of sunscreen and
wear a hat. Drink plenty of water. It’s going
to be VERY HOT out here. Because we are
going to the beach, most people will think
footwear is optional. NOT TRUE! We are
going to be exploring all the environments on
a barrier island. Please wear old tennis shoes
or something very similar. Be on the lookout
for wildlife, particularly snakes. Finally, the
environments we are visiting can be very fragile.
Please watch your step and try to keep damage
to the vegetation to a minimum. THANKS!!!
Factoid: History of People on Mustang
Island People have lived in the Texas coastal
zone for at least 8,000 to 10,000 years, even
before there was a Mustang Island and at
times when sea level was much lower than
it is today. The early inhabitants of Mustang
Island, those encountered by the Spanish
explorers and the early European settlers,
were the Karankawa Indians. The Karankawa
were a fierce people. They ate mostly shellfish
and mussels, and there is some evidence
that on occasion they were cannibals.
The island is first mentioned in the records of a Spaniard, Alonso Alvarez de
Pineda, in 1519. The bay behind the island was named Corpus Christi by another
Spaniard, Diego Ortiz Parrilla, in 1756.
The name “Mustang Island” is derived from
the wild horses, the “Mestenos,” that were
the descendants of horses left here in the
1800’s. In the 1850’s settlers used the island for grazing cattle, and there were small
settlements on the island soon after the Civil
War. The town of Port Aransas was established by about 1910. (Adapted from Texas
Parks and Wildlife, Mustang Island State
Park website http://www.tpwd.state.tx.us/
spdest/findadest/parks/mustang_island/)
Where are we, what will we see? Mustang
Island is a 40-km- (~25-mile-) long barrier island
located between Corpus Christi and the Gulf
of Mexico, south of St. Joseph’s Island and
north of Padre Island (fig. 1 and 2). A barrier
island is a narrow coastal island between the
ocean and a lagoon or estuary that borders the
mainland. Mustang Island is a barrier island
with a unique and complicated ecosystem.
This morning we will visit both ends
of Mustang Island and traverse the island
from the Gulf of Mexico to the edge of
Corpus Christi Bay. As a group we will visit
Packery Channel, Corpus Christi Pass, and
the Leona Belle Turnbull Birding Center,
accompanied by a coastal geologist from the
UT Bureau of Economic Geology.
What will we learn about today? By
the end of today’s field trip, you will have seen
the dynamic environments of a typical Texas
barrier island. You will examine the island
from the waters of the Gulf of Mexico, up
the beach to the top of the foredunes, across
the back-barrier environment, to the edge of
Corpus Christi Bay.
Today’s geo-words: At the end of the day,
you should be able to explain
• Erosion and accretion
• Tidal inlet
• Washover channels, washover fans
• Dunes, foredunes, coppice dunes
• Beach, forebeach, and backbeach
• Marsh
• Barrier island
• Jetty
• Tide
• Vegetated barrier flat
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Figure 1. Aerial
photographs showing
Mustang Island and
surrounding water
bodies.
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Figure 2. Map showing Texas Coastal Bend barrier islands. Modified from Brown et al. (1976).
STOP #1:
The island emerges from the Gulf of Mexico
at the beach. Behind the beach are a series of
dunes. The dunes, which are mostly stabilized
by vegetation, give way to a vegetated flat that
extends back toward Corpus Christi Bay.
Highway 361, the main road down Mustang
Island, generally runs along the vegetated flat
just behind the dunes. Away from the dunes,
across the vegetated flats, is the bay margin at
the edge of Corpus Christi Bay. In the shallow
waters of the bay itself are the marine grassflats.
MUSTANG ISLAND TRAVERSE —
FROM OCEAN BEACH TO BAY
We will begin our field day at the Gulf of
Mexico shore by Packery Channel. Our walking
traverse across Mustang Island will take the
majority of our time. We will start on the beach
near the north jetty of Packery Channel and
hike across the island to Corpus Christi Bay
(fig. 3).
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Figure 3. Schematic illustration of
the major environments on
Mustang Island.
The major environments are easy to recognize.
Each has its own characteristics, each has its
own flora and fauna, and they occur in a
predictable sequence because each is linked to
the others by interdependent processes.
PACKERY CHANNEL: Packery Channel
has an interesting history and is a very hot topic
in the Port Aransas and Corpus Christi region.
In 2003 Packery Channel was a closed natural
tidal inlet located in the southeast corner of
Corpus Christi Bay. A tidal inlet is a channel
through the barrier island that allows water
driven by the tides to enter the bay at high tide
and leave the bay at low tide. The exchange of
waters between the Gulf of Mexico and Corpus
Christi Bay is very important to maintaining a
healthy ecosystem in the bay and maintaining
the fisheries of the Gulf of Mexico.
Factoid: Tides, the daily rise and fall
of the water in the ocean, are caused by the
Moon and Sun. As the Earth rotates, the
gravitational attraction of the Moon and
Sun “pull” up a bulge of ocean waters when
the ocean is facing the Moon to create a high
tide. There is a paired, high-tide bulge on
the “backside” of the Earth. Pretty simple.
In detail, tides are very complex and the tide
height depends on many factors (such as
winds, barometric pressure, rotation of the
Earth, and the shape of the ocean basin).
Texas’ tides are usually a few feet or less, but
some places outside of Texas have enormous
tides. The Bay of Fundy in eastern Canada,
for example, has a tidal range of up to 16 m;
that’s almost 53 feet!
Packery Channel, along with Newport Pass,
1852 Pass, and Corpus Christi Pass (which
we will visit at STOP #2), form a complex of
washover channels along northern Padre Island
and southern Mustang Island. A washover
channel is like a temporary tidal inlet that occurs
where Gulf of Mexico waters driven by storms
have washed over the island, eroding a channel
and depositing sand in the bay. Since currents
filled Packery Channel with sand in 1923 the
Figure 4. Construction at Packery Channel,
November 8, 2004. Photo from the City of
Corpus Christi (http://www.cctexas.com/
engineering/packery).
storm-driven waves of hurricanes have reopened
the channel in 1933, 1945, 1961, 1964, 1967,
and 1970. Each time, the natural tidal currents
were too weak to keep the channel open, and
it was blocked with sand within a few months.
For many years residents in Corpus Christi
debated the pros and cons of reopening Packery
Channel. Reopening Packery Channel would
require dredging the channel to a depth that
would allow boats to pass between the Gulf
of Mexico and Corpus Christi Bay. The pass
would also need to be stabilized and be redredged from time to time in order to keep the
pass open. As you can imagine, this project is
extremely expensive.
In 2003 the United States Army Corps of
Engineers began a project to reopen Packery
Channel (fig. 4). Most of the channel was
dredged, except for a plug of sand left to keep
out the Gulf of Mexico, and construction of two
jetties was begun. A jetty is a wall built into the
ocean, along a navigational channel, to stabilize
the channel and provide protection from waves
or currents. The south jetty was complete, but
the north jetty was under construction in 2005
when Hurricane Emily struck Mexico about
85 miles south of Brownsville, Texas. A surprising amount of damage occurred at Packery
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Channel, but before it could be repaired,
Hurricane Rita struck the upper Texas coast
causing more damage. Plans were made to
extend both jetties (fig. 5). On our field trip we’ll
check on the progress of this project, which was
scheduled for completion in late spring 2006.
Word Alert: Dredge means to dig
something up from underwater or to find
something that was lost. A construction
company may dredge a channel on the
coast, or you may dredge up old memories,
or dredge up an old pair of shoes from the
back of your closet.
Figure 5. Extension of south jetty after 2005
hurricane season. Photo taken January 29, 2006.
Photo from the City of Corpus Christi
(http://www.cctexas.com/engineering/packery).
Let’s go for a walk to
Corpus Christi Bay!
At first glance, a beach seems like a simple
environment; however, if you look closely
this narrow interface between land and sea
has many different parts. Even the sandy part
of the beach has different parts, the forebeach
and the backbeach! The forebeach is the sloping
part of the beach lying between the high-tide
mark and the low-water mark of the run-down
of the waves at low tide. This area is constantly
under the influence of waves and is exposed,
then submerged, by the rise and fall of tides.
BEACH: The beach, the gently sloping
shore of a body of water that is washed by
waves or tides, is a dynamic environment. This
narrow strip of real estate is constantly being
rearranged by the daily forces of waves, tides,
and weather. Figure 6 is a profile, or cross
section, from the water’s edge to the dunes.
You are going to label the features on this
figure as we talk about them.
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Figure 6. Schematic cross section of where the Gulf of Mexico meets Mustang Island. Dune heights may exceed
25 feet.
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The backbeach is the upper zone of the beach
lying between the high-tide mark and the point
where vegetation begins or there is a change in
slope (such as a sea cliff, dunes, or a seawall). The
backbeach is nearly horizontal or slopes down
slightly landward. The backbeach is commonly
under the influence of the wind. Only during
high-water events are waves a factor in this zone.
The backbeach is where most beach goers like
to leave their blankets, umbrellas, and coolers.
We will use a few other simple terms; they
will be defined on the field trip (take notes) and
are in the glossary at the back of the guidebook.
We have included a barrier-island profile and a
beach profile for you (fig 6). You will want to
fill this in with the names of different environments and features as we mention each one.
Study Hint: You may see these profiles
again!
Beach Sand
Figure 7. Vegetated dunes at the landward edge of the
backbeach, Mustang Island State Park.
(16 to 20 feet) is a more common height. You
will also notice that the dunes are more or less
covered by vegetation. The island’s existence
depends upon sand dunes, especially those
with vegetation.
Sand dunes are formed by the wind.
The wind picks up loose, dry sand from the
beach and drops it in the dunes (fig. 8). Let’s first
look at the dunes at the edge of the backbeach.
Look at the size of the sand grains, the “texture”
of the sand. The dune sand is generally finer
than the beach sand because winds typical
of this area can’t move coarse sand like the
waves can. The dune sand is also more evenly
textured; the sand grains have a more uniform
size. Finer grained silts are carried far away
by the winds, and coarser sand and shells are
left behind on the beach.
The transition area between the open beach
and the dunes is mostly windblown sand.
There are small windshadow dunes behind, or
downwind, from almost any stationary object.
Dune Sand
DUNES: At the back of the beach are
sand dunes (fig. 7). The dunes are an almost
continuous chain of sand hills that form the
spine of Mustang Island. The first observation
you will make about the dunes is that the
topography, the shape of the land, has suddenly
changed. We are no longer on the gently upward
sloping backbeach. We’re now in the “hills”!
The dune crests are the highest elevation on
Mustang Island. The height of these dunes may
reach 11 meters (36 feet), though 5 to 6 meters
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Figure 8. Schematic figure of dunes.
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Figure 9. Coppice
dunes formed at the
back of the beach.
Why do windshadow dunes form? The offshore
winds carry the fine sand from the dry backbeach until the wind slows and the sand grains
fall to Earth. Anything that interrupts the
flow of the wind can cause the speed of the
wind to decrease and result in deposition of
sand and the creation of a windshadow dune.
There is a class of dunes that exists because
of the interplay between plants and the winds.
These coppice dunes (fig. 9) are irregular cones
of sand crowned by plants. Coppice dunes form
where a shrub, bush, or other plant interrupts
the course of the wind. The turbulence caused
by the plant slows the wind, and sand is
deposited at the base of the plant. Over time,
the sand pile grows larger, and the plant grows
taller to stay above the rising sand. Eventually
the coppice dune will merge with other coppice
dunes forming a foredune. The foredune is the
first high ridge at the landward margin of the
beach. The top of the foredune is an excellent
vantage point for a good look at the barrier
island. Try not to step on the plants that help
stabilize the dune.
Factoid: Dunes are essential to the
survival of barrier islands. Other than
people, the main threats to barrier islands are
tropical storms and hurricanes. The dunes
are the main defense against erosion by the
powerful waves of a large storm. A large
dune covered by dense vegetation is more
able to resist the power of a large storm than
a smaller dune or a dune with less vegetation.
VEGETATED BARRIER FLAT: The
largest environment on Mustang Island is
the vegetated barrier flat, a flat, low-relief but
bumpy plain covered by plants, that extends
from the back of the dunes almost to Corpus
Christi Bay (fig. 10). There are some patches
of back-island sand dunes that are covered by
grass and other low vegetation. There are also
depressions that host small freshwater ponds
and marshes. The vegetated barrier flat makes
up a significant part of the barrier island. As the
barrier island formed and grew by accretion
of sediments on the Gulf shore, older beach
and dune deposits were reworked by winds and
storms to become the extensive barrier flat we
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Figure 10. Schematic cross section of Mustang Island about 2 miles across, illustrating the major environments.
shellfish. The bays are also the wintering ground
for many species of waterfowl. We will examine
salt marshes and tidal flats during this portion
of the field trip. You will learn more about
marine grassflats on the boat tour.
Salt marshes (fig. 11) occur at the transition
between areas of generally dry land and the
areas that are commonly covered by the waters
of the bay. The bay side of Mustang Island is a
good place to see salt marsh. Low marsh soils
are covered by water for some significant part
of most days, and the high marsh generally is
only slightly above high tide. Because the soils
are water saturated, they have very little oxygen.
It takes special plants (Table 1) to survive in an
environment that has this combination of salt
water and very little oxygen in the soils.
The dominant grass of the salt marsh, both
here and throughout most of the east coast of
the United States and down into the coastal
see today. Compared with the beach, dunes,
and bay margins, the vegetated barrier flat is
a less active and more stable environment.
BAY MARGIN: Most of the bay margin
has very low relief as the land gently merges with
the shallow waters of Corpus Christi Bay. The
environments of the bay margin are controlled
by subtle changes in elevation, depth of water,
and salinity. Because water depths are so shallow
and relief is so low, a small change in water level
may cause a large change in the amount of land
that is either covered by water or exposed to the
air. Winds, tides, and storms can cause water
levels to rise or fall by a few feet.
The main environments are salt marshes,
marine grassflats, and tidal flats. These environments are of critical importance to the ecosystems of the bays and the Gulf of Mexico.
These areas are the nurseries for all of the
commercially important species of fish and
Figure 11. Salt marsh at
the edge of Fish Pass on
Mustang Island. Salt-marsh
cordgrass is the vegetation
at the waters edge. Photo
by Tom Tremblay.
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Figure 12.
Example of
salt-meadow
cordgrass
stand (left) and
characteristic
seedpods (right).
salt marshes of Latin America, is cordgrass,
or Spartina. There are three species of cordgrass
that you can easily see on Mustang Island
(figs. 11, 12, and 13). Very subtle changes
in salinity and frequency and duration of
flooding control the distribution of these important grasses and other plants common in
the salt marsh.
There are other plants at the bay margin.
The salt-marsh cordgrass is usually the first
emergent plant at the water’s edge (fig. 11).
Next is the low-growing, brilliant yellow-green
saltwort (fig. 14). Wherever you find saltwort,
look for marble-sized balls of sandy mud piled
up near the hole of a fiddler crab. Glasswort
(fig. 15a), which looks like long, skinny,
segmented fingers, and sea purslane (fig. 15b)
which has fleshy red stems, may also be present
near the bay margin. We are going to look for
all of these plants. Use the photos in this text
to help you identify the marsh plants you see.
The plants at the bay shore are an important
part of the ecosystem of the bay, and important
Figure 13. Gulf cordgrass.
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Figure 14.
Example of
saltwort.
(a)
(b)
Figure 15.
Example of
glasswort
(a) and sea
purslane (b).
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Figure 16. Example of
black mangroves on
Mustang Island. Notice
the pneumatophores
around the mangrove.
Table 1. Salt Marsh Vegetation
Did you see it?
Salt-marsh cordgrass (Spartina alterniflora)
Salt-meadow cordgrass (Spartina patens)
Gulf cordgrass (Spartina spartinae)
Saltwort (Batis maritima)
Glasswort (Saliconia spp.)
Sea purslane (Sesuvium portulacastrum)
Black mangroves (Avicennia germinans)
figure 11
figure 12
figure 13
figure 14
figure 15a
figure 15b
figure 16
to the geology of the bay. The plants help
stabilize the shore and also trap sediments.
Over time, the sediment trapped by the plants
may cause the shore to build out into the bay.
Along the shore of Corpus Christi Bay
you may see a treelike shrub having glossy
green leaves, which grows as high as about
6 feet (in this area they may be under 3 feet
tall). This is the black mangrove (fig. 16). The
black mangrove and other species of mangrove
are tropical trees. If there’s an especially hard
freeze, the mangroves may die, but new trees
usually develop during the next growing
season. Look on the ground near the mangrove
shrubs, and you’ll find a number of spikelike,
woody stems. These are pneumatophores that
are sent up by the roots of the mangrove. The
roots are in water-saturated mud and sand that
have no oxygen; the roots of the mangrove
breathe through the pneumatophores.
STOP #2 :
CORPUS CHRISTI PASS
Corpus Christi Pass is where the geographers
say Mustang Island begins. Corpus Christi
Pass was a natural tidal pass between Corpus
Christi Bay and the Gulf of Mexico prior to
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Figure 17. View west of Corpus Christi Pass a few days after Hurricane Allen in August 1980. Note the
washover fan in the distance and the spit that is building northward and will soon seal the Gulf entrance
to the pass. Photo by William White.
1929. Corpus Christi Pass was reopened as a
washover channel by Hurricane Carla in 1961
and by Hurricane Allen in 1980 (fig. 17).
Corpus Christi Pass, 1852 Pass, and
Newport Pass are washovers. Washovers
are just what the name implies. When a big
tropical storm or hurricane slams into the shore
from the Gulf of Mexico, the winds, waves,
and elevated tides can force waters from the
Gulf to “wash over” the island. These high
waters are called storm surge. Washover
channels are eroded as waters rush into the
bay and a large fan-shaped layer of sediment,
a washover fan, is deposited in the bay.
Washover fans are significant features in
the expansion of barrier islands. A washover
channel may transport many thousand cubic
yards of sediment in a single event and build
a large washover fan, a square mile or more
in area. The washover fans may be modified
by wind and rain, stabilized by back-island
vegetation, and merged imperceptibly with the
older parts of the barrier flat.
As the storm abates, the channels are
partially refilled with sand and other sediments
and are soon sealed from the Gulf. Low areas
of the partially filled channels and the absence
of sand dunes make these places vulnerable to
future washovers in future storms. Washover
channels obscure the beginning and end of
Padre and Mustang Islands (even some local
residents are unsure of the location of the official
boundary) and indicate the dynamic nature
of this environment.
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those of the foredune ridge behind the beach.
Most of the back-island dunes resulted from
the coincidence of two processes, one natural
and the other due to people. In the late 1800’s
a drought caused a partial dieback in the
vegetation that stabilized the fore-island dunes.
The problem was exacerbated by overgrazing by
cattle and sheep. Sand from the poorly vegetated
fore-island dunes was carried by the onshore
winds to form the back-island dunes. Until
they became stabilized by vegetation, these
dunes were “active dunes” that migrated across
the barrier flat toward the bay. Even in times
of normal rainfall, not all the windblown sand
is captured in the fore-island dunes behind the
beach. Some sand is blown back to the barrier
flat, where it is trapped by the vegetation.
Word Alert: Exacerbate means to
make worse. “I had to spend the entire
weekend working on my homework. The
situation was exacerbated because I didn’t
even have time to go to the football game or
the party afterwards at my friend’s house.”
Word Alert: Abate means to decrease
or lessen. “My dad was really angry when we
came home 20 minutes late, but his anger
abated after we told him about stopping to
help the old woman change a flat tire.”
A tidal pass and a washover channel are
similar in that both are channels from the Gulf
to the bay, although on South Padre Island the
washovers often do not reach the lagoon or bay.
The washover channels tend to be ephemeral and
are usually quickly filled with sediment after the
storm has passed. Because the Gulf processes—
winds, tides, waves, and longshore currents—are
generally more dynamic than those of the bay,
the mouth of the channel on the Gulf generally
fills quickly with sediment. Look gulfward
from where Highway 361 crosses any of the
washover channels, and you’ll see that the old
passes are now choked with sediment and a
narrow ridge of fore-island sand dunes has
developed. Some of the recently abandoned
washover channels have small freshwater
marshes with cattails and other plants.
Word Alert: Ephemeral means
temporary, brief, or not long lasting.
“Her fancy hair style looked great at the
prom, but it was ephemeral. The next day
she looked the same as usual.”
STOP #3:
LEONA BELLE TURNBULL BIRDING CENTER
(FIG. 18)
The fresh-water to brackish-water marsh
is created by the waste water treatment plant.
This is a popular site for birding enthusiasts.
As we make our way out to the observation
platform, please stay very quiet. We don’t
want to disturb the birds or alligators. That’s
right, be on the lookout for ‘gators. Once on
the boardwalk you will see a cattail marsh
surrounding a large body of water. Look closely
in the areas where the cattails look like they have
been mowed down. This has been done by the
alligators. They like to hide among the cattails
and are very good at camouflaging themselves.
Word Alert: Brackish means slightly
salty. The salinity of brackish water is higher
than that of fresh water but less than that
of seawater.
ALONG THE WAY:
MUSTANG ISLAND VEGETATED
BARRIER FLAT AND BACK- ISLAND DUNES
As we drive along the island on Highway
361, we are driving along the southeastern edge
of the vegetated barrier flat (fig. 10). On the
Gulf side, you can see the hilly shapes of the
dune ridge. Toward the back side of Mustang
Island, to the northwest, is the hummocky
surface of the barrier flat. In the distance, the
barrier flat approaches Corpus Christi Bay.
Back-island sand dunes, which you can
see north of Corpus Christi Pass and near the
bayshore at Fish Pass, are much smaller than
14
Figure 18. Boardwalk at the Leona Belle Turnbull Birding Center on Mustang Island.
STOP # 5:
S OUTH
AFTERNOON
We will split into two groups for the
afternoon to explore the waters surrounding
Mustang Island. The first group will go on a
boat to explore part of Corpus Christi Bay with
a scientist from the UT Marine Science Institute.
The second group will be visiting the Texas
State Aquarium in Corpus Christi. Tomorrow
afternoon these groups will switch places.
JETT Y AND THE BEACH
If we have time today we will visit the
South Jetty (fig. 19) at the north end of
Mustang Island. This is a very popular place
among fisherman. It’s also a great place to
watch the sunrise over the Gulf of Mexico
and to watch boats and tankers enter and
exit Aransas Pass. The jetty, constructed
in the early 1900’s of large granite blocks
quarried from Marble Falls, Texas, extends
approximately ½ mile from the northern tip
of Mustang Island into the Gulf of Mexico,
providing stabilization to the entrance of
the Corpus Christi Ship Channel.
DAILY REVIEW
What did we learn today? Today we
looked at Mustang Island with the eyes of
a geoscientist. As geoscientists, we made
observations and described the features we
15
Figure 19. The South Jetty on Mustang Island is a popular destination for visitors to the island.
and the environment is the most
stable of any on the island.
• Washovers from large storms
temporarily destroy the beach
and dunes, but the washover fans
build the back side of the island
at the bay margin.
saw, but we also tried to understand how
these features developed and the relationships
among them. We viewed the barrier island as
an integrated system of related features and
natural Earth processes.
• We started at the beach. This area is
dominated by the waves. The waves
and tides of the ocean are the main
forces causing erosion or accretion
on the beach.
• Landward of the beach, above the reach
of the waves, the dominant process is
the wind. The wind erodes sand from
the beach and deposits dunes. This
is also where we see the first plants,
and plants are a major control on the
development and stability of the dunes.
Plants anchor the sand and disrupt
the smooth flow of the winds.
• Most of the wind-carried sand is
deposited in the main dunes, but
some sand is blown from the dunes
or beach to the vegetated barrier flat.
The plants here are more numerous
Today’s geo-words:
• Erosion and accretion
• Tidal inlet
• Washover channels, washover fans
• Dunes, foredunes, coppice mounds
• Beach, forebeach, and backbeach
• Marsh
• Barrier island
• Jetty
• Tide
• Vegetated barrier flat
Why is what we saw today important?
The Texas coast is a very dynamic and produc16
LECTURE #1:
tive environment that is of enormous importance to us as Texans, but also to our nation.
This marvelous part of Texas hosts vast resources of oil and gas, is home to many thousands
of people, is the nursery for many of the shellfish and fin fish in the Gulf of Mexico, and annually sees the migration of millions of birds.
This area is also the part of Texas that is most
vulnerable to the devastating effects of tropical
storms and hurricanes.
Geoscientists play an important role in protecting these environments. By understanding the physical processes of the coast, we can
better manage development and protect critical
wetlands. Coastal geologists can anticipate the
effects of storms and save lives and property;
they can also provide guidance on clean-up
activities related to oil spills or releases of
other chemicals from ships or pipelines.
Two natural resources that are essential to
our society, crude oil and natural gas, can best
be produced from porous and permeable rocks.
Sandstones, such as those deposited in coastal
settings, can be very porous and permeable and
great reservoirs for these fluids. There is a pile
of sandstone and other types of sedimentary
rock that is over 3 miles (!) thick directly
below us at Mustang Island. The Texas coastal zone has numerous wells producing oil
and gas from these coastal zone sedimentary
rocks. Geoscientists that explore for oil and
gas must understand modern depositional environments to find ancient ones effectively.
WAVES , LONGSHORE CURRENTS , AND TIDES
Tomorrow we’ll spend part of the morning
looking at processes that deposit and erode
sediment on the Gulf shoreline. The shoreline
advances or retreats depending on the actions of
the waves, currents, and tides and the availability
of sediments. The changes in the location of the
shoreline are of enormous importance to Texas.
Shoreline erosion and retreat can cause millions of
dollars in damages to homes, businesses, roads,
pipelines, and any other development that is
lost or damaged by shoreline change.
Factoid: Texas is the only state in the
nation that has an Open Beaches Act. But
what exactly does this mean? It means that
Texas Gulf Coast beaches can be used by
the public. The beach is defined as the area
between the mean low-water line to the
vegetation line. This law allows the public
free and unrestricted access to, and use of, the
beach. Unfortunately, Texas also has one
of the highest rates of coastal erosion in the
country. Today, sections of the Texas coast
experience significant erosion of its shorelines because there is not enough sediment
in the depositional system to balance the
effects of rising relative sea level and the
impacts from coastal storms. Understanding how coastal processes (waves, currents,
and tides) affect Texas beaches helps coastal
planners in developing management plans
that will allow future generations of beachgoers to enjoy our coast.
Waves: Waves may erode or deposit
sediment on the beach. A 10-foot-high wave is
not uncommon during a major tropical storm
or hurricane, and a few hours of big waves
crashing on the coast of Mustang Island can
erode a lot of the beach—back to the dunes—
or even farther. The shoreline has retreated.
If the eroded sand is carried into deep water,
the sediment is not available for the smaller
waves to carry back to reconstruct the beach
after the storm has passed. Unless additional
EVENING ACTIVITY
We’ll have a short lecture tonight that will
give you some information on waves, currents,
and tides that will help you better understand
tomorrow’s field exercise. Tomorrow we will
spend the morning back on the beach. The
beach is one of the most active environments
on a barrier island. We’ll look more closely at
how water moves sediment along the shore.
Following the lecture we will review what
you learned today.
17
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Figure 20. Ocean-wave motion.
sand is available, the post-storm shoreline will
be established many feet inland of its former
position before the storm.
column to a depth of about one-half the
wavelength (L/2).
• Breaking occurs when crest angle is less
than 120º or steepness (H/L) greater
than 1/7.
Breaker Types: The slope of the lower
beach depends on the type of waves (in beaches
of uniform composition).
• Spilling Breaker: Gentle beach slope;
waves break far from shore, and surf
gently rolls over the front of the wave.
• Plunging Breaker: Moderately steep
beach slope, less steep wave, slightly
longer period; wave curls over, forming
a tunnel until the wave breaks.
• Surging Breaker: Steep beach slope, low
wave of long period, doesn’t actually
break; wave rolls onto the beach.
Longshore Current: Sand moves to
and from the beach in an onshore-offshore
direction, but it also moves along the shoreline
(“longshore”). This longshore sand transport
is referred to as longshore drift (fig. 21), and
because of it, beaches have been called “rivers
of sand.”
The longshore current moves the sand along
the beach. At a particular time, the longshore
A SUMMARY OF WAVES
• There are three forces that form waves:
wind, earthquakes (forming tsunamis),
and gravitational attractions between
Earth, Sun, and Moon (forming tides).
• Ocean waves are formed by the wind
(friction between the wind and the
water’s surface).
• The size of a wave (fig. 20) depends
on wind speed, length of time wind
blows in the same direction, and
the distance over which the wind
blows (fetch).
• Wind waves will continue as long as
the wind is blowing. Once the wind
stops, the waves will continue until
their energy is dissipated.
• Water particles in waves move in a nearly
stationary circular motion (fig. 20).
• At the surface, the orbit of the water
particles in a wave is about equal to
the wave height. The orbits decrease
in size downward through the water
18
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Figure 21. Schematic of
longshore current.
drift of the sand by the longshore current may
be in either direction along the beach. Over
the course of a year and at most locations, the
volume of sand moved in one direction will
exceed the amount moved in the opposite
direction. The difference in the amounts of
longshore drift between the two directions
over a period of 1 year is called the annual
net longshore drift. Knowing the annual net
longshore drift is extremely important because
the major source of sand for Texas beaches is
from updrift beaches
The ultimate sources of the sand for our
beaches are either “new” sand introduced to the
coastal zone by rivers or “old” sand reworked
by the waves and currents from deposits in
shallow offshore areas. Not much sand is added
from the offshore because it is too deep, and
little sand comes from the rivers because dams
on the rivers trap the sand, and today most
Texas rivers discharge into bays where the sand
is trapped. A less than satisfactory alternative
is to dredge sand from deeper water in the
Gulf of Mexico and pump it onto our beaches;
dredging is expensive.
The longshore current, the movement of
water along the shoreline, is caused by:
• Waves approaching at an angle to
the shoreline.
• Tidal currents.
• Wind pushing water along the beach.
Tides: We discussed tides briefly in the
Factoid related to our stop at Packery Channel.
Tides are the periodic rising and falling of
ocean waters caused by the gravitational forces
of the Sun and Moon (fig. 22).
• Spring tides, the tides having the largest
tidal ranges, occur when the Earth, Sun,
and Moon line up (new and full Moon,
or twice during the lunar month).
• Neap tides, the tides having the smallest
tidal ranges, occur when the Moon is
in the first and third quarters.
As the water level rises and falls, it generates
a tidal current that flows horizontally. Tidal
currents caused by the dropping water level
(as the tide “goes out”) are called ebb currents.
19
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ORIENTATION
Field Day #2
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Where are we, what will we see?
We’ll spend the morning on the beach at
Mustang Island State Park, swim in the Gulf
of Mexico, have lunch at UT Marine Science
Institute, and then split up for a second day
of boat trip/aquarium visit. On the beach
we’ll look at coastal processes that erode and
deposit sediments, and we’ll see how coastal
geologists take measurements to monitor the
changes in the beach and the shoreline.
Factoid: Mustang Island State Park is
almost 4,000 acres in size (a little over
6 square miles), including about 5 miles of
beach on the Gulf of Mexico. It’s located
in Nueces County, near the south end of
Mustang Island, and was opened to the
public in 1979.
What will we learn about today? Today
you will see how we take measurements to
monitor the beach, shoreline, and longshore
currents and how we make observations of
active coastal processes. We also hope to take
a closer look at a jetty. A jetty is a “hard” shore
compared to the “soft” shore of the loose
sand of the beach. Different animals live on a
hard shore than a soft shore. A jetty also alters
the currents, and we’ll see what happens to
the beach near a jetty.
Today’s geo-words: By lunchtime today,
you should be able to explain
• Longshore drift
• Vegetation line
• Berm and berm crest
• Wet/dry line and scarp
• Global Positioning System
• Longshore current
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Figure 22. Earth, Moon, and Sun alignment
during spring and neap tides.
The rising tide generates flood currents. Tidal
currents affect how sediments are transported
along the Gulf shore and the nearshore
environments in the bays. This is especially
true for shores where the tidal range, the
difference in elevation between high tide and
low tide, is large. The tidal range in Texas is
just a few feet, but this is enough to affect the
shoreline environment.
The everyday movement of tidal currents
is an important factor in transporting sediments along a beach or through tidal channels.
But there are also instances when the height of
the tides can enhance the impact of storms, or
the opposite can be true. During a major storm,
like a tropical storm or hurricane, waves will
be significantly bigger. There is also the threat
from the storm surge, which is simply water
that is pushed toward the shore by the force
of the winds swirling around the storm. Now
imagine the difference in impact between this
storm surge arriving during a spring high tide
or neap low tide.
20
STOP #1:
hazards, and many other coastal issues that are
important to Texas.
Students make several field trips to their
study sites during the school year. Working
in teams, they conduct topographic surveys
(beach profiles) of the foredune and beach,
map the vegetation line and shoreline, collect
sediment samples, and observe weather and
wave conditions. And you will too!
An important part of this program is
the repeated mapping of the shoreline and
measurement of beach profiles. The data are
used to determine the rate and direction of
shoreline change and are made available to
other researchers and coastal managers.
MUSTANG ISLAND STATE PARK
AND FISH PASS
Texas High School Coastal Monitoring
Program: The methods of measurement and
observation that we’ll show you this morning
are the same ones we use every year with
students from several Texas coastal middle
and high schools. This program is a long-term
research project to investigate and monitor
changes on the Texas coast. We collect valuable
data that contribute to our more than 30 years
of research on shoreline changes, effects of
storms, gains and losses of wetlands, coastal
Notes
21
The following describes
our four tasks.
TASK 2—Estimate processes acting on
the beach: wind direction and speed; wave
breaker type; surf zone width; wave direction,
height, and period; number of apparent longshore bars; longshore current; shoreline and
foredune trends (fig. 24).
Purpose:
Explore relationship between processes
and beach changes. Over time these data
may be used to “calibrate,” for a specific
beach location, the data acquired by
weather and wave stations in the area.
Method:
Make visual observations and average
results among three observers. Use a
float and a stopwatch to measure the
longshore current. Use handheld wind
gauge to measure wind speed.
Equipment:
• tape measure
• sighting compass
• data forms and clipboard
• wind gauge
• 3 floats (oranges)
TASK 1—Measure a topographic transect
oriented perpendicular to the shoreline. For
comparison through time, profiles are measured from the same starting point landward
of the beach and oriented in the same
direction (fig 23).
Purpose:
Provide quantitative data on the shape
of the beach and the location of the
shoreline and vegetation line.
Method:
Measure relative changes in
beach topography using
Emery rods and tape measure.
Equipment:
• Emery rods
• tape measure
• sighting level • sighting compass
• survey flags
• data forms and clipboard
(a)
Figure 23. Texas
High School Coastal
Monitoring Program
(THSCMP) participants
(a) using a sighting level to
determine vertical change
along a topographic
profile and (b) using a
tape measure to determine
the horizontal distance
between Emery rods.
(a)
(b)
(c)
(b)
Figure 24. Students
participating in THSCMP
determining (a) longshore
current, (b) wave
direction, and (c) dune
orientation.
22
look at natural systems in detail, as well as at
a larger scale, and we need to document what
we observe by careful measurements.
This is important because similar processes operate on all the beaches of the
world, and some of the basics of erosion and
accretion that you see at the coast can also
be applied to other systems such as rivers.
You’ve been thinking like a geoscientist.
TASK 3—Make a map, a horizontal
(no elevation) survey, of the location of the
vegetation line and shoreline.
Purpose:
Provide quantitative data on
the position and trend of the
shoreline and vegetation line.
Method:
Use a handheld Global Positioning
System to make a walking survey.
Equipment:
• Global Positioning System
(GPS) receiver
• Data forms and clipboard
FINAL REVIEW
TEST
THE END
It’s been a busy couple of days at the shore.
We hope you had fun, and we hope you enjoyed learning something about the geoscience of this part of Texas. Protecting our coast
is really important, and we need more people
who understand the geoscience of the coast.
You can do it! In fact, you have done it!!
You learned how to look at part of the coast
as a geoscientist does. There’s still a lot we
don’t know about this environment and we
need more smart young people to become
scientists. You can do it!
TASK 4—Examine the jetties at Fish Pass.
Look for barnacles, green algae, and
other critters living among the rocks
of the jetties. Make observations about
the beach adjacent to the jetties, and
make a sketch of the shoreline.
DAILY REVIEW
What did we learn today? Today
we learned several different ways to make
measurements to document coastal processes.
A FINAL THANKS
Today’s geo-words:
• Longshore drift
• Vegetation line
• Berm and berm crest
• Wet/dry line and scarp
• Global Positioning System
• Longshore current
This guidebook is the result of the efforts
of many people. The Jackson School provided
financial support, and Doug Ratcliff and Julie
Spink directed this project and managed the logistics. Dr. Julie Jackson, formerly at The University of Texas at Austin and now at Texas State
University, is our educational mentor. Fellow
scientists at the Bureau of Economic Geology—
Bill White, Sigrid Clift, and Jay Raney—reviewed
the manuscript and made helpful suggestions.
Figures and text were enhanced by the efforts of
Joel Lardon, Jamie Coggin, Lana Dieterich, and
others at the Bureau. Some of the figures used
here are from Down to Earth at Mustang Island,
a previous publication by the Bureau.
Why is what we saw today important?
Yesterday we looked at the major environments
on Mustang Island, the big picture, and today
we looked at the beach in more detail and
made measurements to document what we
saw. That’s how science operates. We need to
23
Geo-Glossary
Accretion:
The gradual addition of new land
(or beach) to old by the deposition
of sediment carried by the ocean.
Berm:
A low shelf or narrow terrace on
the backbeach formed of material
deposited by storm waves.
Algal mat:
Layer of algae typically found
on a tidal flat.
Berm crest:
The seaward limit and generally
the highest point of a berm on
a beach. The crest of the most
seaward berm separates the
foreshore from the backshore.
Breaker:
A wave that has become so steep
that the crest outpaces the body
of the wave and collapses into a
turbulent mass.
Coppice dune:
Dunes formed by the accumulation of windblown sand on
the backshore just seaward of
the foredunes and anchored by
vegetation. With an adequate
sediment supply these mounds
could grow larger, become
continuous, and form a new
foredune system.
Annual net longshore drift:
Difference in the amount of
longshore drift between two
directions over a period of
1 year.
Back-island sand dunes:
Dunes located on the bay side
of barrier island, usually adjacent
to the bay margin but may be
located anywhere back of the
fore-island dunes.
Backshore or backbeach:
The upper zone of the shore or
beach, lying between the highwater line of mean spring tides
and the upper limit of shore-zone
processes (edge of dunes): it is
acted upon by waves or covered
by water during exceptionally
severe storms or unusually high
tides. It is separated from
foreshore by the crest of the
most seaward berm.
Barrier island:
Bay margin:
Beach:
Crest (dune or wave):
The highest point on a dune;
the highest part of a wave.
Ebb-tidal current: Water movement, generally
seaward, associated with the
decrease in the height of a tide.
Erosion:
The wearing away of sediments,
rocks, or other materials by the
action of water and wind.
Estuary:
Area adjacent to the bay or other
body of water that is influenced
by tidal waters.
A partially enclosed coastal body
of water or bay that receives fresh
water from rivers and streams,
as well as saltwater from the sea.
Fetch:
The extent of open water wind
travels across.
The gently sloping shore of a
body of water that is washed
by waves or tides.
Flood-tidal current: The movement of water toward
the shore or up an estuary with
increase in the height of a tide.
A long, narrow coastal island
separated from the mainland by
a lagoon or estuary. It commonly
has dunes, vegetated zones, and
marshy terranes extending
toward the bay.
24
Forebeach:
The zone of the shore or beach that
is regularly covered and uncovered
by the rise and fall of the tide.
Foredune:
A coastal dune at the landward
margin of a beach, more or less
stabilized by vegetation.
Foredune crest:
Highest part of foredune.
Neap tide:
Pneumatophores: Erect roots that are an upward
extension of the underground root
system. Because these roots are
exposed at least part of the day and
not submerged under water, the
root system can obtain oxygen.
Fore-island dunes: System of coastal dunes roughly
parallel to the Gulf shoreline
and immediately landward
of the beach.
Global Positioning System (GPS):
Satellite-based navigation system.
High tide:
The tide at its highest: the
maximum level reached during
a tidal cycle.
Hurricane:
A tropical cyclone in which the
maximum sustained surface wind
is 74 mph (119 km/hr) or more.
Jetty:
A wall built into the ocean, along
a navigational channel, to stabilize
the channel and provide protection
from waves or currents.
Longshore bar:
A low sand ridge built mainly by
wave action, occurring at some
distance from and parallel with
the shoreline.
Salt marsh:
A saturated, poorly drained area,
intermittently or permanently
flooded with saltwater, having
aquatic and grasslike vegetation.
Scarp:
An almost vertical slope fronting
a berm on a beach or dune caused
by wave erosion. It may range in
height from several centimeters
to a few meters.
Spring tide:
A tide occurring twice each
month, at or near the times of
new Moon and full Moon, when
the gravitational pull of the Sun
reinforces that of the Moon.
It has an unusually large or
increased tide range.
Storm surge:
An abnormal, sudden rise of sea
level along an open coast during a
storm caused primarily by onshore
winds, resulting in water piled up
against the coast. It is most severe
when accompanied by a high tide.
Surf zone:
The area bounded by the landward
limit of wave uprush and the
farthest seaward breaker.
Swash zone:
Zone over which water rushes
up onto the beach following
the breaking of a wave.
Longshore current: Ocean current caused by the
approach of waves to a coast at
an angle. It flows parallel to and
near to the shore.
Longshore drift:
The transport of material
(sand, shell fragments, etc.)
along the coast by longshore
currents.
Marine grassflat:
Area of seagrass growth in
marine waters.
A tide occurring at the first and
third quarters of the Moon, when
the gravitational pull of the Sun
opposes that of the Moon, and
having an unusually small or
reduced tide range.
25
Tidal flat:
An extensive, nearly horizontal
barren tract of land that is
alternately covered and uncovered
by the tide, consisting of
unconsolidated sediment.
Tidal inlet:
Any inlet through which water
flows alternately landward with
the rising tide and seaward with
the falling tide: specifically a
natural inlet maintained by
tidal currents.
Tidal range:
The difference between the level
of water at high and low tide.
Tide:
The rhythmic, alternate rise and
fall of the surface of the ocean
resulting from the gravitational
attraction of the Moon (and,
to a lesser degree, of the Sun)
acting unequally on different
parts of the rotating Earth.
Vegetation line:
Seaward extent of vegetation,
usually seaward of the foredunes.
Some beaches have more than
one vegetation line (such as
one line marking the extent of
continuous landward vegetation,
another marking the boundary
of a sparsely vegetated area).
Washover channel: The channel, cut by storm waves
and currents, through which
tidal waters can alternately flow
landward and seaward on a
washover.
Trough (dune or wave):
The lowest point between two
dune crests; the lowest point
between two wave crests.
Vegetated barrier flat:
A flat, low-relief but bumpy plain
that extends from landward of
the fore-island dune system to the
bay margin on a barrier island,
vegetated with grasses and other
low vegetation. May have patches
of back-island dunes, as well as
depressions that may host small
fresh-water ponds and marshes.
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Washover fan:
Deltas built on the landward
side of a barrier island produced
by storm waves and currents
breaking over low places and
depositing sediment on the back
side of the island or in the bay
behind the island.
Wave:
A repeated movement in a body
of water seen as an alternate rise
and fall of the surface.
Wave height:
The vertical distance between
a wave’s crest and trough.
Wavelength:
The distance between
successive wave crests
or other equivalent points.
Wave period:
The time it takes successive wave
crests to pass a fixed point.
Wet/dry line:
The line between wet sand
and dry sand on the forebeach.
Marks highest level of wave
swash and is typically associated
with the berm crest.