Final answer:
The Mississippi River is the landmark indicating one has left East Tennessee, as it forms the western boundary well beyond the state's limits.
Step-by-step explanation:
The landmark that would reveal you have left East Tennessee is the Mississippi River. The Great Smoky Mountains and the Appalachian Mountains are within East Tennessee and its bordering regions, while the Cumberland Gap is a pass through the Appalachian Mountains near where Tennessee, Kentucky, and Virginia meet. However, the Mississippi River denotes a significant geographical boundary to the west of Tennessee; crossing this river typically indicates that you have exited Tennessee.