Skeletal grainstone deposits of the Middle Ordovician in East Tennessee are of two distinct carbonate buildup complexes. Rockdell Formation comprises pelmatozoan-bryozoan grainstones and packstones and lime wackestone. These represent carbonate sand shoals, which surround relatively small lime-mud mounds. The Rockdell developed in shallow-water, high-energy conditions on a stable platform. Holston Formation is bryozoan-pelmatozoan packstones that surround bryozoan wackestone/boundstone. The Holston is a series of lime-mud bryozoan patch-reefs surrounded by skeletal sand. It developed in deeper water on the outer platform or downslope. Recognition and distinction of these is based on: color, composition and distribution of flora and fauna, bryozoan fauna, types and distribution of component lithofacies, and character of lime-mud deposits. Distinction of these buildups has major importance in East Tennessee paleogeography during the Middle Ordovician and possible hydrocarbon exploration in these deposits. Criteria applied here may be useful in making distinctions among other carbonate buildups.--Modified journal abstract.