ABSTRACT: The Sappington Formation is a Late Devonian to Early Mississippian age, primarily siliciclastic, formation that outcrops in southwestern Montana. The Sappington Formation has been presented in the past as broadly analogous in lithologic character and age to other formations in the region, the Bakken and Exshaw Formations. The Sappington Formation was the focus of several studies in the 1950s through 1970s that extensively analyzed the fauna and distribution of the formation and established a lithostratigraphic and biostratigraphic framework. This study provides a needed present day look at the Sappington Formation, by incorporating extensive previous work with present day field observations and modern geologic concepts. The methodology applied to this work includes literature review, outcrop observations, and interpretations on lithofacies, depositional environments, and sequence stratigraphy. Results from this work include a sequence stratigraphic model, depositional models, and a summary of the effects of allocylic processes on deposition. Three sequences were identified using previous biostratigraphy and correlated throughout the study area using lithostratigraphy. Recently, intracratonic Late Devonian and Early Mississippian age formations have become of economic importance as “unconventional” hydrocarbon plays, owing to success in the Bakken Formation.