Philadelphia's Coastal Plain

Atop Philadelphia's City Hall looking east over the Delaware River- a broad flat coastal plain
Atop Philadelphia’s City Hall looking east over the Delaware River – a broad flat coastal plain

You are looking at the land where I live. My home, the City of Philadelphia, is located on a broad, flat sandy expanse of land known as the Atlantic Coastal Plain.

Most of the city sits on this plain except for the northwest section. The northwest section (Chestnut Hill, Mt. Airy, West Oak Lane, Roxoborough, Manayunk and Germantown) sits on the geological region known as the Piedmont Plateau.

Philadelphia looking west toward Phila Art Museum, Schuylkill River on left side of photo
Philadelphia looking west toward Phila Art Museum (‘the Rocky Steps”), Schuylkill River on left side of photo.

The Coastal Plain includes most of Philadelphia, southeastern parts of Bucks and Delaware Counties, all of southern New Jersey (from Newark to Cape May) and most of Delaware except a small area at the very top of the state.

Stretching along the Atlantic Ocean, the Coastal Plain covers the Eastern seaboard from New York City  to the Rio Grande. The plain widens as it extends southward.

In Philadelphia, the plain is just five miles wide – the widest part in Pennsylvania. In Pennsylvania the plain runs along the Delaware river, from Levittown to Chester. The Delaware River flows in to the Atlantic Ocean.

You can row a boat from the Atlantic Ocean 98 miles up the Delaware and land in Philadelphia. Philly is the only city in Pennsylvania with access to the Atlantic Ocean.

Elevation on the Pennsylvania section of the Coastal Plain ranges from sea level to just 60 above sea level.

Atlantic Ocean (at Cape May, NJ) looking over the Continental Shelf
Atlantic Ocean (at Cape May, NJ) looking over the Continental Shelf

There are two parts of the Coastal Plain, the Continental Shelf Province and the Coastal Province.

The Continental Shelf is the submerged part of the Atlantic Coastal Plain. It is a broad belt sloping away from the land’s edge to a depth of 600 feet. This is the part that we don’t see. It is covered by the Atlantic Ocean.

The Coastal Province is the part that is above sea level. It was formerly sea bottom. This is what Philadelphia, south Jersey and Delaware lie on. The Coastal Plain meets the Piedmont Plateau, a broad area of gently rolling hills. The area where the two landforms meet  is called the Fall Line.

The Fall Line is marked by waterfalls and rapids. When water flowing from the higher Piedmont Plateau drops onto the lower Coastal Plain, waterfalls are created. They can easily be seen along the Schuylkill River. Particularly, just below the Waterworks in Philadelphia.

The rolling Piedmont continues until it meets the Appalachian Mountains in Central Pennsylvania.

Hawk Mountain - overlooking the rolling mountains on the Piedmont Plateau
Hawk Mountain – overlooking the rolling mountains on the Piedmont Plateau

What did Philadelphia look like before the building of the City? Probably, much looked like the wetlands and riverine forests at the John Heinz National Wildlife Refugee in South Philly.

John Heinz NWR at Tinicum
John Heinz NWR at Tinicum – flat wetlands

The City of Philadelphia was established on a raised area of the Coastal Plain. This helped the city escape some of the flooding of the Schuylkill and Delaware rivers. But, as the city spread, homes and buildings were built on places of lower elevation and the wetland area of the Delaware River. This is why Southwest Philly floods during heavy rains.

I have stood in wetlands within the city limits that are lower than sea level.  City Hall is about thirty feet above sea level.

Since, Philadelphia and the surrounding area is on such flat land, climate change and rising seas will directly affect the City.

The common phrase said around here, is that most of Philly will become beach front property. I love the seashore. But, I think the seas rising up to meet me is a scary way to finally have the beach house I have always dreamed of.

News Story:

Stream of Trouble Runs Though Region – Philly.com, May 06 2012. Again the Darby Creek Appears on the verge of flooding homes.  “Philadelphia is the best case study in the United States for storm-water management.”

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