MDMaryland Geography

At Hancock, in Washington County, you can stand on the Mason-Dixon Line and see the Potomac. The state measures about 1.83 miles from its northern border to its southern one at that point, narrower than a good many airport runways are long. Two hundred miles east, the same state is a maze of tidal creeks on the Delmarva Peninsula. Maryland ranks 42nd in area and manages to be almost unrecognisable from one end to the other.

Five physiographic provinces in one small state

Maryland crosses every physiographic province of the Atlantic seaboard, which no other state its size does. Starting from Ocean City and moving west: the Atlantic Coastal Plain of sand and marine sediment; the Piedmont of hard crystalline rock; the Blue Ridge, a narrow band through Frederick and Washington counties carrying Maryland's stretch of the Appalachian Trail; the Ridge and Valley, where folded Paleozoic rock produces the long parallel ridges that squeeze the state to under two miles at Hancock; and the Appalachian Plateau in Garrett County.

The junction between the first two is visible on any city map. The fall line, where hard Piedmont rock gives way to soft coastal sediment, is where rivers drop and boats stop, and Baltimore, Washington and the older mill towns along the Patapsco all sit on it. Just upstream, the Potomac falls 76 feet in under a mile at Great Falls. Maryland's ceiling is Hoye-Crest on Backbone Mountain, 3,360 feet, in Garrett County within a thousand feet of the West Virginia line. Its floor is the Atlantic.

The Chesapeake is a drowned river valley

The bay is not a bay in the ordinary sense. It is the valley of the Susquehanna River, cut when sea level was hundreds of feet lower during the last glaciation and then flooded as the ice melted. That is why it is 200 miles long, unusually shallow at an average of about 21 feet, and shaped like a river with tributaries rather than a rounded inlet. It is the largest estuary in the United States, and its watershed drains about 64,000 square miles across six states and the District of Columbia.

The bay is also why Maryland's shoreline statistics look wrong. The state has roughly 31 miles of general Atlantic coastline, essentially the strip from Ocean City down to the Virginia line. Its tidal shoreline, counting the bay and every river and creek feeding it, runs to about 3,190 miles. The Chesapeake and its tributaries together carry nearly 11,700 miles of shoreline. That water splits Maryland into an Eastern Shore of nine counties and a Western Shore holding almost all of the population, a division that stayed a genuine barrier until the first Bay Bridge span opened in 1952.

Land in the bay is going the other way. Smith Island is the only inhabited Maryland island in the Chesapeake with no road connection to the mainland, and it has been losing ground steadily to erosion and rising water. Holland Island, a few miles north, held a village of several hundred people around 1910; its last standing house collapsed in 2010.

Boundaries argued over for three centuries

Almost every side of Maryland is the residue of a lawsuit. The northern border exists because the Calvert and Penn families fought for eighty years over overlapping charters, and Charles Mason and Jeremiah Dixon were hired to settle it, surveying westward from 1763 to 1767 and setting the line at roughly 39 degrees 43 minutes north. The stones they planted became the reference point for the free state and slave state divide a century later.

The northeast corner is stranger. Delaware's northern boundary is an arc struck twelve miles from the courthouse cupola in New Castle, the only circular state boundary in the country, and where that arc failed to meet the Mason-Dixon line cleanly it left a sliver called the Wedge, which was not definitively assigned to Delaware until 1921.

The Potomac is stranger still. Maryland's 1632 charter granted the river up to the low-water mark on the Virginia side, so the entire river belongs to Maryland rather than being split at the channel. Virginia challenged it and lost: the Supreme Court ruled for Maryland's ownership in 2003 while confirming Virginia's right to build a water intake from its own bank. And the District of Columbia is a hole cut out of Maryland, ceded in 1790, with the Virginia portion given back in 1846, so every square foot of the capital was once Maryland.

Assateague, Deep Creek and the weather range

Maryland's ocean edge is a barrier island system that people accidentally rearranged. A hurricane in August 1933 cut a new inlet at Ocean City; jetties built to keep it open then cut off the southward drift of sand, starving Assateague Island, whose northern end has since migrated several hundred yards landward of Fenwick Island. The offset is obvious from the air.

In the far west, Deep Creek Lake was created in 1925 for hydroelectric power and remains the largest inland body of water in the state. Garrett County sits high enough on the Appalachian Plateau to average around a hundred inches of snow a year, while Ocean City on the same latitude band rarely holds snow at all. The record high is 109 degrees, set at Cumberland and Frederick on July 10, 1936; the record low is 40 below zero at Oakland on January 13, 1912, a 149-degree spread in a state you can cross in a morning. All of Maryland keeps Eastern Time.

Neighboring states and more Maryland pages