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The Highland Rim
The Highland Rim is an undulating plateau that surrounds the Nashville Basin (fig. 5). The Rim is bounded on the east by the Cumberland Plateau, on the west by the Western Valley of the Tennessee River, and extends north and south into Kentucky and Alabama, respectively. Traditionally, the Highland Rim in Tennessee has been divided into eastern and western subdivisions (Miller, 1974). West of the Central Basin, the Highland Rim can be further subdivided into a smaller section (the Pennyroyal Plateau) north of the Cumberland River and a larger section south of the Cumberland River (Kemmerly, 1980; Smalley, 1980).

The entire Highland Rim in Tennessee is underlain by carbonate rocks of Mississippian age (Fenneman, 1938; Miller, 1974). From youngest to oldest, major rock units include the Ste. Genevieve Limestone (Pennyroyal Plateau only), the Monteagle Limestone (stratigraphic equivalent of Ste. Genevieve Limestone in the Eastern Highland Rim), the St. Louis Limestone, the Warsaw Limestone, and the Fort Payne Formation. The Ste. Genevieve, Monteagle, and St. Louis Limestones are relatively pure, mechanically strong limestones that develop extensive cave and sinkhole systems (Piper, 1932; Kemmerly, 1980; Mills and Starnes, 1983). The Warsaw Limestone is a variable unit consisting of thickly to thinly bedded limestone of variable purity with interbedded calcareous shales and sandstones (Piper, 1932). The Fort Payne Formation is even more variable, consisting of "an extremely heterogeneous and variable assemblage of siliceous and calcareous shale and sandy, cherty, and earthy limestone" (Piper, 1932). The base of the Mississippian sequence, and of the Highland Rim aquifer system, which encompasses this sequence (Brahana and Bradley, 1986), is the Upper Devonian to Mississippian Chattanooga Shale (Piper, 1932; Miller, 1974).

Three distinct topographic styles are characteristic of the Highland Rim as a whole and recur in varying proportions throughout the Rim's extent:

Sinkhole plains are areas of low to moderate relief with a high density of sinkholes and such related karst features as deep vertical shafts, blind valleys, and perennially flowing disappearing streams. Sinkhole plains are prominent in the Pennyroyal Plateau and at the base of the Cumberland Plateau escarpment. Ground-water flow systems include very responsive karst aquifers with point recharge, low storage, and efficient conduit systems as well as transitional aquifers with more diffuse flow regimes or higher storage.
Barrens are weakly dissected uplands of low relief that include broad, well-drained ridge-tops, poorly drained valley bottoms cut by weakly incised, intermittently flowing channels, perennially wet shallow pans, and short valley sideslopes. Weathering profiles are generally greater than 6 m thick, and thicknesses of greater than 30 m are common (Haugh and others, 1992). The largest area of barrens in Tennessee is The Barrens of the Eastern Highland Rim (Wolfe, 1996b). Other notable concentrations occur in southern Franklin County and the western Pennyroyal Plateau (Wolfe, 1996a). Recharge and ground-water flow are generally much more diffuse than in the sinkhole plains, though examples of concentrated recharge and conduit ground-water flow have been documented at some sites in The Barrens (Haugh and Mahoney, 1994; Wolfe 1996a, b).
Fluvially dissected escarpments and valleys are areas of high slope and relief that have been extensively cut by fluvial erosion. Fluvial dissection of the Eastern Highland Rim is strongest in the northeastern part where the Rim is cut by the Cumberland River and its tributaries (White and White, 1983) and along the escarpment that marks the Rim's transition to the Central Basin. A similar dissected escarpment leads from the Pennyroyal Plateau to the Cumberland River valley. Extensive areas of the Western Highland Rim are deeply incised by numerous large and small streams. Because fluvial dissection is an external process affecting both sinkhole plains and barrens areas, ground-water hydrology is somewhat more variable in dissected areas than in other topographic settings of the Highland Rim. Recharge and ground-water flow vary from concentrated to diffuse, depending on local lithology. Numerous springs and seeps discharge along the dissected escarpments, notably at confining unit outcrops. In general, ground-water storage in areas within 1 kilometer of the escarpments is depleted each year (Piper, 1932; Smith, 1962; Burchett, 1977; Johnson, 1995).
Topography, especially the distribution of sinkholes and barrens, is strongly correlated with bedrock geology (fig. 8). Sinkholes are most common where the greatest thicknesses of relatively pure limestones such as the Ste. Genevieve and St. Louis Limestones crop out. Barrens are concentrated in areas underlain by the impure limestones, shales, and cherts of the Warsaw Limestone or Fort Payne Formation. The most extensive barrens areas are in the Eastern Highland Rim and Pennyroyal Plateau, but they occur throughout the Highland Rim (Wolfe, 1996a). In general, the Western Highland Rim is the most dissected of the three subdivisions, the Pennyroyal Plateau is the least dissected, and the Eastern Highland Rim is the most topographically variable (slope, table 4).

In the Grand Canyon, there is a formation called the Redwall Limestone, because it forms <drum roll> red walls. However, the material is not red itself. The color comes from iron oxide leached from the overlying rocks, which are likely siliclastic.http://www.bobspixels.com/kaibab.org/geology/gc_layer.htm#rl