The Simpson group of central Oklahoma has long been recognized as including five formations: the Joins, Oil Creek, McLish, Tulip Creek, and Bromide. However, as they are traced northward into Kansas, formations become less clearly defined and their relationships more vague. The stratigraphic interval between the Viola limestone and Arbuckle group in south-central Kansas is referred to as "The Simpson" and is considered erratic and unpredictable.

Detailed study of the Simpson rocks in south-central Kansas has revealed a normal sequence of deposition, and any anomalous or erratic characteristics that may be present were acquired as the result of local tectonic movements, rather than as a result of a change in deposition. A correlation of Simpson units in south-central Kansas and central Oklahoma has been made by electric-log cross sections. It is concluded that the Bromide and McLish formations are the only representatives of the Simpson group in south-central Kansas. The thin Simpson section is due to intraformational thinning and onlap, rather than regional truncation.

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