In 1811 Thomas Hilleary purchased 355 acres of land on Great Seneca Creek, and four years later he opened Black Rock Mill, where an inscription on the east side reads "Black Rock Mill T.H. 1815." Before the Civil War, central Maryland was part of the nation's wheat basket, and Maryland grain was ground into flour at mills like Black Rock. For farmers, the opening of a nearby mill was a major event because it reduced the time and effort needed to get grain to market. Some of the flour ground at Black Rock was kept locally for bread, but most was packed into barrels and sent to seaports such as Georgetown for export around the world. Mills also served as social centers, and because of his extensive business in the region, the miller was usually among the best-informed residents in the community, making the mill a place to catch up on the latest gossip. After the opening of the Ohio Valley to agriculture, grain production began shifting westward out of Maryland. Although the grain trade declined, Black Rock Mill continued operating as a saw mill and received a large contract to saw lumber for construction of the steam-powered Bowman Brothers Mill in Germantown. Ironically, that efficient steam mill put Black Rock out of business by the middle 1920s.