Highland Rim

The Highland Rim is a geographic term for the area in Tennessee surrounding the Central Basin. Nashville is largely surrounded by higher terrain in all directions.

Geologicially, the Central Basin is actually opposite, being a dome. The Highland Rim is a cuesta surrounding the basin, and the border where the difference in elevation is sharply pronounced is an escarpment, located somewhat in the northwestern corner of the Basin.

Geology and physiography
The Highland Rim is a physiographic section of the larger Interior Low Plateaus province, which in turn is part of the larger Interior Plains physiographic division.

There are sections of the Rim referred to by all four of the cardinal directions, i.e., "Northern Highland Rim", etc. It should be recognized that the Rim is rather continuous and any division of it, including the ones made below, are somewhat arbitrary. The term "Highland" here is relative: it is certainly higher than the Basin it surrounds, but it nonetheless is seldom at an elevation above 1100 ft above sea level, and never more than about 1400 ft above sea level except where interrupted, primarily to the southeast, by outliers of the Cumberland Plateau. With the exception of a few broad stream bottoms, the land is characterized by ridges and valleys with a few fairly low hills. The entire region is well watered with many perennial streams, and occasional waterfalls which sometimes delineate the Rim from the Basin which it surrounds.

Western Highland Rim
The Western Rim is similarly encountered only a few miles west of Nashville, and extends to the western valley of the Tennessee River. Underlying bedrock of the region is chiefly Mississippian limestone, chert, shale, and sandstone with exposures of Devonian, Silurian, Ordovician, and Cambrian limestone, chert, and shale. In the northern part of the Western Highland Rim, karstification or sinkholes, readily occur in an area with a southern extension of the Pennyroyal plateau of Kentucky, where the karst is best developed on the Mississippian St. Louis Limestone and the Ste. Genevieve Limestone. The area is a hilly area that is bisected by the Tennessee River and the Cumberland River valleys.

Eastern Highland Rim
The Eastern Rim rises approximately fifty miles east of Nashville, and is bordered to its east by even higher terrain, the Cumberland Plateau. Erosion has exposed carbonate bedrock of Late Paleozoic age. These carbonate rocks contain variable amounts of chert, and are often interbedded with fine grained clastic rocks. As a result, these rocks are more resistant to erosion than the underlying, purer limestones of the Lower (Early) Paleozoic. The geology is diverse, and is typically limestone at valley floors (around 500 ft elevation) and sandstone on ridges (to around 1000 feet). The constituent bedrock is composed primarily of Mississippian, St. Louis, and Warsaw limestone with Fort Payne chert underlain by Chattanooga Shale that forms a large part of the escarpment. This area is mostly undulating plains, hills, and karst.

Northern Highland Rim
The Northern Highland Rim is encountered just a few miles north of Nashville proper and extends to the Kentucky border, and the region of Kentucky adjacent to it called the Pennyroyal is in fact largely a continuation of it under another name.

Southern Highland Rim
For the most part the Southern Rim is the farthest from Nashville, rising at some points just a few miles north of the border with Alabama. Again, the landforms here are continuous with those in adjacent portions of Alabama, although perhaps the most spectacular landforms of any portion of the Rim are to be found here.

The stratigraphy of the Southern Highland Rim is primarily composed of flat-lying limestones, dolomites, and shales, and to a much lesser extent, of cherts, siltstones, mudstones, and very fine grained to conglomeratic sandstones.