Where does the mason dixon line start on the east coast?

Answer 1

There's the actual, historical boundary line surveyed by Mason and Dixon, and then there's what people think of when they use the term "Mason-Dixon Line", as a shorthand for the boundary between slave states and free states. This is not entirely accurate though, because the line has Delaware and New Jersey on the "free" side, whereas Delaware was a slave state and there was still very small-scale slavery in New Jersey until after the Civil War, when slavery was ended everywhere by the 13th Amendment. Additionally, when the line was surveyed Pennsylvania still had slavery, though Pennsylvania is on the "free" side too.

People THINK of the Mason-Dixon line as being between Maryland and Delaware, so the eastern end of it is thought to be at the Atlantic Ocean, where the common border of those states meet the sea.

But Mason and Dixon were hired to do the surveying to settle boundary disputes between Maryland, Pennsylvania, and the three counties that became Delaware which at that time were a part of Pennsylvania, though they had previously been part of the original grant of Maryland. In essence the three counties that are Delaware were granted twice - first to Maryland, then later again to Pennsylvania. This naturally made Maryland unhappy. So part of the job for which Mason and Dixon were hired was to locate the boundary between Maryland and what was to become Delaware. But they did not start at the ocean. If you look at a map, the southern boundary of Delaware is a line running inland from the sea, straight west. Then, the boundary makes a ninety degree turn and runs straight north. It was this north-south running line that was the easternmost portion of the surverying work done by Mason and Dixon. So in actual fact the end of the line surveyed by Mason and Dixon is at that ninety degree corner at the southwestern corner of Delaware, not at the sea.