The Midway Group has been considered a monotonous section of marine shale which was deposited on the continental shelf during the initial Cenozoic transgression. However, recent studies show the Midway to be a complex group of sediments deposited in environments ranging from deltaic to bathyal.

Sedimentary rocks of the Midway Group crop out along the northern and western edges of the Rio Grand embayment. Siltstones deposited as a beach occur in the northwest corner of the Rio Grande embayment in Uvalde County, Texas. To the east and south, richly fossiliferous carbonate rocks indicate a nearshore shallow-marine environment. Basinward, the Midway Group grades into shale deposited on the shallow continental shelf. Sandstones, up to 10 ft (3 m) thick, occur intermittently throughout the shale. These shallow-marine sands are most likely derived by longshore currents paralleling the coast.

Sandstone in the Midway is increasingly abundant toward the shelf edge near Laredo, Texas. The sand was probably deposited as barrier islands and shallow-marine shelf sands. The source of the sand was probably to the east. Significant quantities of gas have been discovered in these sandstones near Laredo in both the United States and Mexico.

Recent gas discoveries have been made in the Midway rocks of eastern Texas which were deposited in deltaic to bathyal environments. It appears that a delta prograded over the shallow continental shelf, and sediments were deposited on the shelf edge and upper slope. Sands from the delta complex were probably carried by longshore currents to the south and deposited near the shelf edge in the vicinity of Laredo.

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