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Geologic Wonders of Texas
Franklin Mountain
Dinosaur footprints
Central Texas
Galveston Island
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Dinosaur Footprints

Rock Sediment and Soil Facts

If you want to see rock formations, don't go to Galveston Island. However, if you would like to see the first stages of how some sedimentary rocks are formed, then Galveston Island is the place to be. Galveston Island is known as a sandy barrier island because it is made up mostly of sand-sized particles, with lesser amounts of finer mud- and larger gravel-sized sediments. Three main sources of sediment compose the island (1) inorganic mud and sand eroded from the continent and transported by rivers to the coast, (2) organic mud resulting from decaying vegetation and animals, and (3) sand and gravel from shells.

The ongoing processes of wind, waves, and currents sort the sediment making up the island. Little mud is deposited on the high-energy Gulf of Mexico side because it is washed away by waves. To find deposits of mud you need to go to the bay side of the island, especially in the marshy areas. This is where mud is produced by decaying vegetation and where some of the mud moving down the rivers and creeks of the bay is trapped. The marsh vegetation and relatively sheltered locations allow the deposition of mud. In many places on the bay side, however, the marsh may have a shelly beach on the fringe, where bay waves are eroding the marsh and concentrating oyster shells. Also, near San Luis Pass, strong tidal currents transport sand along the bay-side shoreline.

Sand and shelly gravel are found on the Gulf beaches and the island's interior, which is mostly sand because of the way the island advanced seaward after it formed. So as you walk landward from the Gulf beach, you are walking across earlier beach deposits. Most of the sediment on the beach and in the dunes is quartz sand that was eroded from the continent. Whole shells and shell fragments made of calcium carbonate add to the variable texture of the sediment, and small amounts of black, heavy minerals and organic matter add color. Wind and waves sort the sediment according to how easily it is transported. Wind preferentially transports small, light grains, so the sand in the dunes is relatively fine-grained and well-sorted quartz. Compared with dune sand, sand and shelly gravel moved by waves on the beach is coarser grained and more poorly sorted because waves can move all grain sizes.

Fine-grained and well-sorted sand from the dunes.

Fine-grained and well-sorted sand from the dunes.

Shelly and relatively coarse grained and poorly sorted sediment from the beach.

Shelly and relatively coarse grained and poorly sorted sediment from the beach.

Coarse and poorly sorted shelly sediment that was deposited by a storm landward of the beach.

Coarse and poorly sorted shelly sediment that was deposited by a storm landward of the beach.

Waves deposited these shells, along with sand, on the beach, then wind eroded the sand leaving a “lag” deposit of shelly gravel.

Waves deposited these shells, along with sand, on the beach, then wind eroded the sand leaving a “lag” deposit of shelly gravel.

Trench in marsh showing mud and sand sediment

Trench in marsh showing mud and sand sediment.

Sand dunes with fine sand

In these dunes wind deposits sand that is finer and better sorted than what is on the beach.

Digging a pit in the beach reveals fine laminations created by waves sorting the sediment on a small scale.
Digging a pit in the beach reveals fine laminations created by waves sorting the sediment on a small scale.

Digging a pit in the beach reveals fine laminations created by waves sorting the sediment on a small scale. In some pits, you may see coarse, shelly or heavy, dark mineral layers deposited by storms. See the two examples above.