One of Maryland's Lost Towns, an abandoned county seat for Baltimore County that isn't even in that county any more, a town which has been replaced with suburban sprawl.
1691 saw Baltimore County's seat move from the dying Baltimore Town on the Bush River to the "Fork of the Gunpowder River", which may be Joppa and may now be buried under the county landfill, no one knows. At any rate, settlement on the northwest shore of the Chesapeake Bay expanded, and in 1709 the Maryland General Assembly authorized the appropriation of 4500 pounds of tobacco for the construction of a courthouse and jail built in "Gunpowder Town" on the eastern shore of the Gunpowder near the outfall of Gunpowder Falls. The General Assembly officially moved the county seat to Gunpowder Town, now called "Joppa", in 1712 (the Act that moves the county seat is the earliest mention we have of the name).
But there were problems: The land purchased for the town was owned by a minor who had no right of conveyance. So while the courthouse and jail operated, there weren't any actual houses. In 1724, the General Assembly finally got around to passing a special act allowing the conveyance. Finally, by 1726, houses were built, on conditions that probably count as Maryland's first zoning regulations (each house had to be 400 square feet or less, and have a brick chimney).
Joppa soon turned into a thriving little international port. Rolling roads carried agricultural produce of all sorts (but especially tobacco) down to the port at Joppa, for transshipment to England.
But from the southern reaches of the County, it was a long roll. A few years later, some people down on the Patapsco River decided they wanted a tobacco port of their own. This new "Baltimore Town" started out very slowly, but eventually boomed, as it had a better harbor, closer to the Ellicott Brothers' wheat mill.
The Gunpowder Valley was one of the main thrusts of settlement in colonial Baltimore County. As more and more settlers cut trees, more and more sediment began to wash down Gunpowder Falls and deposit itself in Joppa's harbor. This made commerce difficult, and a 1766 smallpox epidemic probably spelled the end for the town. In 1768, Benjamin Rumsey completed the only large house to stand in Joppa. But a death blow came the same year: Baltimore merchants successfully petitioned the General Assembly to move the county seat to their town.
In 1773, the area east of the Gunpowder (including Joppa) was carved off to form Harford County. But the dying Joppa was not to be made the seat of the new county. Instead, it was moved to the settlement of "Harford Town" or Bush, on the Bush River, where Philadelphia Road intersects Maryland 136 today. St. John's Church moved out of town, to Kingsville.
By 1815, the only thing left standing was the shell of the Rumsey mansion. Some of the old rolling roads bore the town's name, but today, they are cut off miles from the site.
Fast forward to 1962. The postwar economic boom created a market for Baltimore residents seeking a quiet life in the country. The Panitz Company developed the site of this lost town, building one of the first planned unit developments (PUDs), Joppatowne. But that's a story for another node.
Some of the old roads to Joppa still survive, and their names bear witness to the former seat. The Benjamin Rumsey Mansion (standing in the middle of Joppatowne) has been restored, and is now on the National Register of Historic Places.
Harford County, Maryland Planning Department - Joppa/Joppatowne Community Plan
Virtual Harford - The Town of Joppatowne
Baltimore County Historical Society - Chronology