San Marcos br/Hays County, Texas br/29 52.174' N 97 55.711' W br/br/Text: 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.