What is the famous example of one of these boundaries in the US?

One famous example of a boundary in the United States is the Mason-Dixon Line, which historically demarcated the northern free states from the southern slave states prior to the Civil War. Originally surveyed in the 1760s to settle a dispute between the colonies of Maryland and Pennsylvania, it has since become a cultural symbol representing the divide between the North and South. The line is often referenced in discussions about regional differences in attitudes, culture, and history.