The boundary between the piedmont and coastal plain is known as the

Fall zone

Explanation:

The fall zone is also referred to as a fall line, is an area where a plateau region meets a coastal plain. It is shown more where a river runs along it. The plateau region or upland is usually hard rock while the coastal plain is usually made of softer sediments.

The Atlantic seaboard fall zone of 1400 km is a boundary between the Piedmont and the Atlantic coastal plain which meets in eastern parts of the United states. Another example of a fall zone is the Little Falls of Potomac Rivers.

The boundary between the Piedmont and the coastal plain is called the fall line.