The boundary between the Piedmont and the Coastal Plain is known as the Fall Line. This is not a sharp, clearly defined line, but rather a transition zone that stretches from New Jersey to Alabama.

Here's a breakdown of how the Fall Line is characterized:

* Geologically: The Fall Line marks the boundary between the harder, crystalline rocks of the Piedmont and the softer, sedimentary rocks of the Coastal Plain. This difference in rock types leads to a distinctive change in topography.

* Topographically: The Piedmont is generally higher and hillier, while the Coastal Plain is flatter and lower. The Fall Line is characterized by a series of waterfalls and rapids, as rivers flow from the Piedmont to the Coastal Plain.

* Historically: The Fall Line played a significant role in the development of the American Southeast. Due to the rapids and waterfalls, early settlements tended to be located along the Fall Line, as these locations provided ideal sites for mills and other industries that relied on water power.

Major cities located on the Fall Line:

* Philadelphia, Pennsylvania

* Baltimore, Maryland

* Washington, D.C.

* Richmond, Virginia

* Raleigh, North Carolina

* Columbia, South Carolina

* Augusta, Georgia

* Montgomery, Alabama

Note: While the Fall Line marks a significant transition, it's not a rigid line. There are areas where the Piedmont extends further east, and the Coastal Plain stretches further west. However, the Fall Line serves as a general marker for the boundary between these two physiographic regions.