The Foster sand represents local stream channels that cut into Morrowan strata while draining southeastward in the McAlester basin, Oklahoma. Four channel systems, ranging from 1 to 6 mi wide and up to 20 mi long, have been mapped. During the hiatus these streams brought detritus to the southern part of the basin where marine sedimentation was continuous and the Spiro sand was forming in a shore zone environmental complex that probably included beaches, bars, tidal flats, tidal channels, and lagoons. During a northward transgression the Spiro sand was deposited across the basin and locally overlies the Foster sand.

The Foster sand is a fine-grained quartz sandstone with well-rounded, moderately sorted grains. The Spiro sand is most commonly supermature, but is submature or mature locally. This sandstone has subangular to well-rounded, poorly to very well-sorted (1.05 to 0.31 phi), very fine to medium grains (0.11 to 0.30 mm).

Intergranular pore space in the Foster and Spiro sands has been reduced by pressure solution and formation of quartz overgrowths, carbonate cement, and chlorite cement. Chlorite is authigenic and apparently of two different ages. The chlorite coats the surface of detrital quartz and preserves intergranular porosity by retarding pressure solution and formation of overgrowths.

The Spiro sand has been subdivided into eight facies with limestone, shale, and quartz sandstone end members. Analysis of these facies, primarily by isopach maps and grain size, indicates that dry gas production limits have been established, with the possible exception of reservoirs south of the Choctaw fault in the structurally complex frontal zone of the Ouachita Mountains. The Foster sand is a good dry gas reservoir, but has limited distribution.