Physiography: The science of physical geography.
Courtesy Tennessee Division of Geology.
As you travel across Tennessee, you may notice that one part of the state looks very different than other parts of the state. That's because, there are six distinct physiographic regions within Tennessee.
Moving west to east, the regions are:
Gulf Coastal Plain. The western one-third of the state is relatively flat, and the elevation is relatively low. At the far west is the Mississippi River which forms the border of Tennessee and Missouri and Tennessee and Arkansas.
The Nashville Basin. If you are traveling along the Tennessee/Kentucky border (as you would on TN52), you may miss this region. However, it's definitely there! Nashville and the surrounding areas are actually in a low spot -- the Central Basin or Nashville Basin as it's called. If you want to see this, go south a few miles from Portland on TN109. You'll drop down a 6% grade [which the locals call "The Ridge" into Gallatin. You just went from the Highland Rim into the Nashville Basin. You may also go south on TN10 from TN52 in Lafayette. The drop "down the ridge" starts just a few hundred yards south of the intersection.
The Highland Rim. This is the area surrounding the Nashville Basin. It is several hundred feet higher than the Nashville Basin. In the summer, it is often a few degrees cooler on the Highland Rim. In the days before air conditioning, some people used to leave Nashville and venture up to the Highland Rim because it was a little cooler.
The Cumberland Plateau. If you are on I-40, you'll notice a big change in elevation just east of Cookeville. This is the Cumberland Plateau. If you are on I-24, you'll be on top of the Cumberland Plateau after you've crested the mountain at Monteagle. Monteagle Grade, by the way, is one of the notorious roads in the United States for truck drivers. It has even been featured in country music songs! The next time you're on the mountain, notice the two runaway-truck lanes for trucks that might lose their brakes heading down the grade.
The Great Valley of Tennessee. This is a series of low valleys and ridges that covers most of East Tennessee.
The Unaka Mountains. This region consists of rugged mountains on the Tennessee/North Carolina border. The Great Smoky Mountains are part of the Unaka Mountains.
These divisions are explained nicely in this article for Middle School