The Hackberry wedge of the Oligocene(?) Frio Formation can be divided into two parts. The upper part ranges in thickness from zero to more than 3,000 feet, consists mainly of shale, and contains an outer-neritic microfaunal assemblage. The lower part ranges in thickness from zero to 700 feet and consists essentially of sandstone. The sequence of events in the deposition of the Frio Formation of northern Jefferson Davis and Calcasieu Parishes is summarized as follows: 1) deposition of lower Frio Textularia seligi zone; 2) development of lower unconformity; 3) deposition of lower Frio and Hartburg sediments; 4) uplift, folding, erosion, and development of 'pre-Hackberry unconformity;' 5) tilting of the unconformity surface and channel forming erosion; 6) filling of the channels with basal Hackberry sandstones; 7) deposition of Hackberry shale; and 8) deposition of the remainder of the Frio.