The Great Eastern Wall

The process of European colonization of North America largely took place (with the exception of Spanish/Mexican and Russian colonization) on an east to west axis.  North America, meanwhile, was and is rather notable for having a distinct lack of that particular direction in mountain ranges.  The thirteen colonies were prevented from spread west of the Appalachian wall by the British, but for the most part found that particular terrain to be an obstacle at best, at least early on.  French colonization, based largely on trade rather than independent settlement, developed alongside rivers that pushed deep into the heart of North America, and even then only founded permanent settlements on navigable stretches of those rivers.  Eventually, their expansion too found the wall of the Rockies, and did not penetrate far into the mountains.  

Now, the Appalachians are a different affair from many of the other civilization containment ranges of the world.  They are not extremely tall (Mt. Mitchell in North Carolina is the tallest point on the range, at 6,684 feet, which is considered to be a "foothill" out west), they are not that expansive on the east to west axis, and they have numerous low passes and gaps that facilitate travel to the lands beyond.  

The long yellow area of the Appalachians stretches from Atlanta and Birmingham in the south all the way to the Gaspe Peninsula in Quebec.  The orange range is the different body of the Adirondacks, Laurentian, and Algonquin mountains, which are far older (in fact some of the oldest rock anywhere on Earth).  As noted, the French largely circumvented the wall by staying on the St. Lawrence valley which ran north of it and through the other, older and more eroded, mountains.  The thirteen colonies and a young United States, however, did not have that option.  Most of their "passes" ran north into more mountains, or when they did have reasonable connections, as in the level route of New York, the Potomac passage to the Ohio river, or the foot path in the Cumberland Gap, well, they found French territory instead of more room.  

Even if they wanted to build in the mountains while they were not able to expand into enemy territory, the colonists found the landscape rugged and rocky.  While not nearly to the scale of the Rockies, the Appalachians were far less suitable for digging into the land and farming than their coastal plains and accessible port cities had thus far been.  Most of the founding fathers' surviving homes stand in testament to this pattern of settlement, with only a more rugged, Rousseau-influenced Thomas Jefferson choosing to maintain his estate in "frontier" lands.  Take a look at the view from his porch:

The allure of these more unspoiled forests and ridges would come to symbolize the pioneer spirit well into more recent times.  In all his travels, for instance, Theodore Roosevelt loved no place more than the Adirondacks and the ranges of Maine.  Perhaps he, and many outdoor enthusiasts like him, could not help but enjoy the fact that some primitive part of America was still so rugged and wooded, despite being so close at hand to the hectic pace and development of the eastern cities.  Certainly, when it came time to preserve the lands east of the Mississippi as national parks, these mountains came to mind.  Shenandoah, the Great Smoky Mountains, and Acadia became the first such parks to be created in the east, all in the Appalachians.  Such parks also became early battlegrounds in the war between recreation and wilderness preservation that continues to be waged in parks throughout the world today.  The relative sharp relief of the terrain, in contrast to the surrounding lowlands, managed to largely win the battle for the preservationist camp.  And what a wall does even the first range encountered make.

As you can see, the eastern front of the mountains are often a very long ridge.  Even modern highways do not thrust directly into the range, but mainly follow the existing river gaps and various established passes that had been traveled for centuries before they were sealed in cement.  As seen, the wall escaped the plow, and served instead to form a regional boundary between "east coast" and the mis-labeled "midwest".  Even in Canada, while French Canadians dwell on both sides of the Gaspe, Quebec and New Brunswick can often seem like worlds apart.  Buffalo, Erie, and Pittsburgh are certainly different from Albany, Philadelphia, and Harrisburg.  Ultimately, the barrier has been overcome, but the wall remains, towering above the gates and serving as a memorial to times of past exploration and frontier.