DEDelaware Geography

Delaware's northern boundary is an arc, and it is the only state line in the country deliberately drawn as one. The 1682 deed transferring New Castle and its surroundings to William Penn described the land lying within twelve miles of the town's courthouse, so surveyors eventually swung that radius on the ground. Almost everything odd about the shape of Delaware follows from a property description written before anyone imagined a state would be built on it.

The Twelve Mile Circle and the Wedge

The circle is centred on the cupola of the courthouse in New Castle. Its arc forms the whole border with Pennsylvania, curving from the Delaware River around to the west, and the rest of the state hangs below it.

The straight lines came from Charles Mason and Jeremiah Dixon, who worked here between 1763 and 1768 to settle the Penn and Calvert families' quarrel. Their Delaware assignment had three parts: the Transpeninsular Line already run across the bottom of the peninsula in 1751, the Tangent Line running north from its middle point to touch the twelve mile circle at exactly one place, and the North Line continuing on to meet the arc again.

Curves and straight lines do not close neatly. Where the North Line, the arc and the Pennsylvania boundary failed to meet, they left a triangle of about 1.068 square miles known as the Wedge. Both Pennsylvania and Delaware claimed it, neither policed it, and for decades it drew prizefights, gambling and people with reasons to be somewhere no sheriff would come. Pennsylvania relinquished its claim in 1897, Delaware formally accepted the ground in 1921, and Congress confirmed the arrangement the same year.

Owning the river all the way to the far bank

Inside the twelve mile circle, Delaware's boundary is not the middle of the Delaware River. It runs to the low water mark on the New Jersey shore, which makes it the only state line in the United States drawn along the opposite riverbank rather than the channel.

The practical effects are strange. A pier built out from New Jersey soil stands in Delaware once it clears the tide line, and needs a Delaware permit. The Supreme Court affirmed the arrangement in 1934 and tested it again in 2008, ruling six to two that Delaware could refuse permission for a liquefied natural gas terminal planned off the New Jersey bank at Logan Township.

Dredging the ship channel made it stranger still. Spoil pumped ashore built up Artificial Island near the mouth of the Salem River, and some of that new ground falls on Delaware's side of the old boundary while being physically attached to New Jersey. Those parcels can only be reached by land through another state.

Ebright Azimuth and the lowest average ground in America

The highest point in Delaware is a survey mark called Ebright Azimuth, 447.85 feet above sea level, sitting at a road junction in a residential part of northern Wilmington. Only Florida's Britton Hill is a lower state high point. Mean elevation across Delaware is around 60 feet, the lowest figure of any state.

The fall line crosses at Wilmington and does all the geological work. North of it lies the Piedmont, hard crystalline rock, rolling ground, and roughly a tenth of the state's area. South of it everything is Atlantic Coastal Plain, unconsolidated sand, gravel and clay laid down in shallow seas, and it makes up the other ninety percent. Total relief from Ebright Azimuth to tidewater is under 450 feet spread across 96 miles of length.

A peninsula sliced in two

Delaware occupies the eastern side of the Delmarva Peninsula, sharing it with the Eastern Shore counties of Maryland and Virginia. A low ridge, mostly under 80 feet, runs down the peninsula and decides where water goes: the Nanticoke and Choptank drain west into Chesapeake Bay, while the Christina, Brandywine, Murderkill, Mispillion and Broadkill run east into Delaware Bay.

The Chesapeake and Delaware Canal opened in 1829 and cut the peninsula clean across, 14 miles at sea level, now maintained at 35 feet deep and 450 feet wide. Everything south of it, including most of Delaware, is functionally an island. Every road crossing is a bridge.

Delaware Bay itself is a drowned river valley of about 782 square miles, and its beaches host the largest spawning population of horseshoe crabs on earth each May. Red knots flying from Tierra del Fuego to the Arctic stop there to eat the eggs, roughly doubling their body weight in two weeks. Inland at Trap Pond stands the northernmost natural stand of bald cypress in the country. Both of the state's temperature records come from the same town: 110 degrees Fahrenheit at Millsboro on July 21, 1930, and minus 17 there on January 17, 1893.

Delaware geography, answered

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