Thompson's Islands
San Marcos, Hays County, Texas
Marker Inscription
William A. Thompson brought his family and slaves to Texas from Louisiana in 1850. They established plantations in Caldwell and Hays counties. With slave labor, the family built a mill on the San Marcos River near this site. Construction and operation of the mill resulted in the creation of three land masses in the river channel which became known as Thompson's Islands. The land remained in the family for generations, and the mill, which provided power for local lumber operations and a cotton gin, became an important element in the area's economy.
Erected by Texas Historical Commission
The Story
William A. Thompson moved his family and the people he enslaved from Louisiana to Texas in 1850, establishing plantations in Caldwell and Hays counties. Enslaved laborers built a mill on the San Marcos River whose construction reshaped the channel into three small islands still called Thompson's Islands. The mill powered local lumbering and a cotton gin, anchoring the area's antebellum economy for generations.
Why it matters
The site is a candid reminder that the prosperity of antebellum Texas rested on enslaved labor, whose work literally reshaped the land it stood on.
The story behind this marker
AI contextThe era
When William A. Thompson brought his household from Louisiana in 1850, Texas had been a state for only five years. Cotton land was cheap, the frontier was moving west fast, and planters from the older Southern states were pouring in. Many of them brought enslaved people with them, and those people were forced to make the move whether they wanted to or not.
The San Marcos River was a prize in that landscape. It rises from springs at the edge of the Balcones Escarpment and runs clear, steady and cool all year. In the 1800s, before steam engines were common in rural Texas, a dependable river meant power. Anyone who could dam it and turn a wheel could saw lumber, grind grain and gin cotton for a whole district.
Across the South in these years, enslaved labor built the infrastructure as well as working the fields. Dams, mills, levees, roads and houses were often built by people who were never paid and never owned what they built. Texas was no exception, and on the eve of the Civil War slavery was growing in the state, not shrinking.
People & events
The marker names one man, William A. Thompson. The story it tells, though, belongs just as much to people whose names it does not record. Thompson's family set up plantations in both Caldwell and Hays counties. Enslaved men and women did the work: clearing land, planting crops, and building a mill on the San Marcos River.
Building a mill on a living river is heavy, dangerous work. It means moving earth and stone, setting timbers in moving water, and changing where the current runs. That work, and the mill's running afterward, reshaped the channel itself. Three pieces of land formed in the river, and people came to call them Thompson's Islands. They are a strange kind of monument. The landscape still holds the shape that enslaved hands gave it, under a name that honors the man who owned them.
The mill turned out to be a lasting success. It powered local lumber operations and a cotton gin and became an important part of the area's economy. The land stayed in the Thompson family for generations. The marker doesn't say what became of the people who were enslaved there after emancipation reached Texas in 1865. That silence is common in the records of the time, and it is worth noticing.
Its place in the American story
Thompson's Islands is a small, concrete example of a big American truth: in the slave states, wealth and infrastructure were often built by people held in bondage. Visitors sometimes picture slavery only as cotton picking. Here it shows up as engineering. Enslaved workers built a mill, changed a river, and laid the economic foundations of a town.
The site also fits the larger story of Texas's westward expansion in the 1850s, when slavery moved onto new land along with settlement. Political arguments over whether slavery would spread into the West were tearing the country apart in exactly those years. In Texas it simply spread, and places like this mill are what that looked like on the ground.
Last, the marker shows how public history has been changing. It says plainly that the mill was built with slave labor. Remembering early Texas industry in that way gives credit to the people who actually did the work, not only to those who owned the land.
If you visit
The marker stands near the San Marcos River in the town of San Marcos. Today the river is known for its clear spring-fed water and its tubers, swimmers and paddlers. Take a few minutes to look at the channel and think about how a mill dam and its works could have built up land in the middle of the stream. Then picture the labor that took, done without wages or choice.
The upper San Marcos is also an unusually delicate ecosystem. It is home to species found almost nowhere else, including Texas wild rice. If you get in the water, follow posted rules and stay off sensitive plant beds.
San Marcos is an easy stop on the Interstate 35 corridor between Austin and San Antonio. Pair this marker with the town's other historic sites and a walk along the river parks, and you get a fuller picture of how the town grew. Remember the people this marker doesn't name, too. Their work is the reason the place is here at all.
Written by AI to add context, grounded in the marker’s inscription and the historical record. The inscription above is the original, unaltered text.
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Related people
- · William A. Thompson
Themes & tags
Nearby & related markers
Thompson's Island Homeplace
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In 1850 Dr. William Alexander Thompson struck an agreement with his neighbors to harness the San Marcos River, and he and his sons used enslaved laborers to build a gristmill, sawmill, and cotton gin. The channels dug for the waterwheel created the parcels known as "Thompson Islands." The family ran the mills until 1911, and the arrival of electricity in 1942 finally ended the site's dependence on water power.
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