The Trinity time-stratigraphic unit is defined in this paper as that interval above the top of the Sligo formation or its time-stratigraphic equivalent and below the top of the Glen Rose (as recognized in the Austin, Texas, area) or its time-stratigraphic equivalent. The Trinity unit (Trinity group throughout much of the area) can be subdivided in ascending order as follows.

Pearsall formation--divisible into three members which are in ascending order: Pine Island shale member, Cow Creek or James limestone member, Bexar (proposed) shale member

Rodessa formation

Ferry Lake formation

Rusk formation (proposed)

Beyond the limits of the Ferry Lake anhydrite, the Trinity group above the Pearsall formation or its equivalent is referred to as the Glen Rose subgroup or the Glen Rose undifferentiated.

Time-stratigraphic units which coincide with the type sections of these formations and members are extended areally by electrical-log correlations. Cross sections from the Rio Grande to the Florida Keys illustrate the stratigraphic relationships of these units.