Great Appalachian Valley

It is a gigantic trough, including a chain of valley lowlands, and the central feature of the Appalachian Mountains system.

The trough stretches about 1,200 miles (1,900 km) from Quebec in the north to Alabama in the south and has been an important north–south route of travel since prehistoric times.

There is a wide gap between the Reading Prong and South Mountain at Harrisburg, through which the Susquehanna River passes, connecting the Great Valley with the Piedmont region of southeast Pennsylvania.

To the west or continental side, a series of more impenetrable mountain regions border the northern Great Valley.

The Cumberland Gap connects the Great Valley region with Kentucky and Tennessee lands to the west.

The various gaps connecting the Great Valley to lands to the east and west have played important roles in American history.

On the east side, the wide gap in southeast Pennsylvania became the main route for colonization of the Great Valley.

By the 1730s, the Pennsylvanian Great Valley west of South Mountain was open to settlement after treaty cessions and purchases from the Indians.

Early settlers from Pennsylvania used the water drop from Culvers Lake to Branchville for a wide assortment of mills.

Immigrants continued to travel from the Philadelphia area south through the Great Valley beyond Shenandoah, to the vicinity of present-day Roanoke, Virginia.

In the decades before the American Revolution, the Piedmont "upcountry" of the Carolinas was quickly settled, mostly by recent immigrants who migrated from the north to the south via the Great Valley.

The Great Valley, especially Shenandoah Valley, played an important role during the American Civil War, including its Blue Ridge gaps and nearby Piedmont area and its northward extension to Gettysburg, Pennsylvania, where the bloodiest and most influential Civil War battle was fought in the Battle of Gettysburg from July 1 to July 3, 1863.

I-78 then continues the route through the valley parallel to the southern slopes of Blue Mountain, connecting Harrisburg with Lebanon, Kutztown, and Allentown.

At Allentown, I-78 then swings away south into the hills of the Reading Prong en route to New York City.

Heading north from Burlington, however, along the east side of Lake Champlain, I-89 runs through the valley's northernmost stretches to the Canada–US border, where it becomes Quebec Route 133 and Autoroute 35, which trace the route of the Richelieu River in its southern section, where the Great Valley finally dissipates into the plain of the Saint Lawrence River to the east of Montreal.