In the 1700's, Chester Creek drove Thornbury Township's economy as water-powered mills ground flour, cut lumber, and made other goods for local farmers. Only remnants of those mills still exist. At John Taylor's ironworks, Sarum Forge, power from a watermill helped produce wrought iron and later iron bars that could be made into useful products like nails. Ironworks like Sarum Forge did such brisk business that they competed with iron imports from Britain, leading Britain to forbid the building of any new ironworks. In 1789, the state government divided Chester County in two, creating Delaware County, and the line separating them ran right through Thornbury. Landowners along the border could choose the county to which they would belong, and their individual choices created the jagged township boundary that still exists today.