The Atlantic Fall Line

The United Nations estimate that approximately 40% of the world’s population lives within 100 kilometers of an ocean. A Reddit user named gnfnrf performed some back-of-the-napkin calculations and determined this area accounts for somewhere between 1.5% and 12.15% of Earth’s total landmass (measurements of coastline are notoriously difficult to determine), meaning people pack into these regions. Historically, the reasons for this concentration are rather straightforward: the ocean provided food and a means for trading. In many portions of the world, the ocean also moderates temperatures. In warmer areas, the conditions tend to be milder; in colder climes, the seas keep things a little less frosty.

Taking a look at the world’s biggest cities, the ocean looms near most of them. Of the world’s 20 largest cities, just three sit a significant distance from the sea: Delhi, Mexico City, and Chongqing.

On smaller scales, this trend continues.

Let’s plot the largest cities in the United States. The two largest – New York City and Los Angeles – are directly on a coast. Chicago, though not on an ocean, sits on what could be considered an inland sea. Houston appears a distance from the ocean on the map below but actually resides on Galveston Bay. Phoenix, Dallas, and San Antonio are the only ones to sit significantly inland and their growth is a recent phenomenon.

If we added other significant cities to the map, the coasts would fill out nicely with dots. Boston, Baltimore, Washington, Miami, New Orleans, San Francisco, and Seattle. Basically, the coastal regions of the United States have loaded up with major metropolises.

As a general rule, it seems, the coast equates to the preponderance of big cities.

Look at the map above, however. Do you notice anything odd? A few places have large stretches without major coastal cities. For example, between Washington/Baltimore and Jacksonville, the markers are strangely absent. In a straight line, that distance is over 700 miles.

That’s a lot of ocean without a giant city. Why don’t they exist? Let’s zoom in a bit more at this region.

The first thing to note is there are cities on this coast. Traditionally, however, the likes of Virginia Beach, Wilmington, and Charleston have not been the largest centers in their respective states. These places have grown in the recent past, as people move away from the personal necessities of fishing and trading and move toward the ability to live in places they deem preferable.

Some of the cities marked above have declined in prominence in the past century, but most of them were rather significant in the 1800s. Listed from north to south are Philadelphia, Baltimore, Washington, Richmond, Raleigh, Fayetteville, Columbia, Augusta, Macon, Columbus, and Tuscaloosa. How did a string of regionally important towns emerge along what appears to be a line? And why didn’t anything like New York or Boston emerge along this portion of the seashore?

The answer lies in geology!

If your first thought went to the Appalachian Mountains, you’re sort of on the right track. The mountains are usually a barrier to building large cities. For example, the rocky shores of Maine, where crags rise directly from sea level, precluded any type of big settlement on the coasts.

However, our line of cities actually falls (future pun alert) along the boundary between a geologic region called Piedmont and another called the Atlantic Coastal Plain. The name Piedmont derives from the Italian word Piemonte, which means “foothills.” This region is a remnant of ancient, eroded mountains, characterized by rolling hills. The Coastal Plain, by contrast, is newer land, formed by sediment from the most recent and second-to-last geologic eras. Stretching roughly from Long Island (a portion of Cape Cod also technically belongs to the plain) to Florida and then west toward Texas, this region is typified by marshes, swamps, and wetlands.

When thinking about an ideal place to begin a settlement, what might you tend to avoid? Marshes, swamps, and wetlands. They carry the risk of flooding from both river and ocean. They tend to be filled with mosquitos. Their foundations are the definition of shaky and temporal. It makes a lot of sense to avoid creating large cities on this stretch of coast.

But why did so many end up on the boundary of the plain and the foothills?

The towns we highlighted above all reside on a line geologists call the Atlantic Fall Line.

The name stems from a phenomenon that tends to occur on this line: waterfalls. Many of these cities feature rapids or waterfalls in their downtown areas, a nice surprise one might not associate with municipalities on America’s east coast. These falls occur because of the rapid change between physiographic regions. Where Piedmont hits Coastal Plain, the elevation sharply drops. When rivers flow from one to the next, a waterfall often transpires.

Why did towns crop up at these locations? Several factors from the early period of the United States contributed. The dominant mode of transportation at the time was watercraft. From the ocean, boats could usually travel to the fall line and no farther, as the rapids or waterfall were impassable. Piedmont settlements could take advantage of more fertile soil, less flooding, fewer mosquito-borne illnesses, and some physical protection from higher elevations. The rivers still offered easy access to the ocean at large, which could connect them with other cities; simultaneously, they were closer to inland settlements. And, to top it all off, the falls provided them with easy hydropower for mills and factories. In addition to these positive attributes, the rapids posed a natural stopping point, which stymied the creation of other towns upriver.

The result of this combination of factors was a series of towns one might have expected to develop along the ocean.

Earlier, we considered Philadelphia, Baltimore, and Washington to be coastal cities, yet they are included among those on the Fall Line. As you can see by the graphic above, the merging of the regions happens quite close to sea level as one nears New York. In fact, that trio of cities does not lie directly on an ocean. Things get murky in Baltimore, where the Patapsco River melds into the Chesapeake Bay, which could be considered part of the ocean, but technically it does not border the Atlantic. One place where the fall line and the Potomac coincide sits Washington. Likewise, the Schuylkill River and the fall line combine in Philadelphia.

From Edison, Princeton, and Trenton in New Jersey to Wetumpka and Tuscaloosa in Alabama, the Atlantic Fall Line went a long way into shaping the early history of the United States. One can tour a slew of cities and see firsthand the confluence of geology and human history.

Bonus Fact: The largest city in Georgia – Atlanta – is even farther inland than the fall line. The city sits near the Chattahoochee River, which reaches the fall line at Columbus. So how did it end up becoming so large, if upriver navigation would have been rather difficult? Atlanta wasn’t founded until 1837 and wasn’t the largest city in the state until approximately 1880. In fact, it wasn’t even originally named Atlanta. Terminus, Georgia, was founded in a somewhat random spot away from the river at the end of the Western & Atlantic railroad. As the railroad’s prominence grew, so did Terminus, which then became Marthasville before finally morphing into Atlanta.