A buried ridge or series of buried hills, known as the Red River arch, in northern Texas and southern Oklahoma, extends from northern Denton County, Texas, west to central Foard County, Texas. A number of productive geological structures are located on this arch. The Petrolia dome is the oldest of these fields and one of the most pronounced structures. Drilling has revealed that beneath the producing horizons in this field lies a series of thick limestones of Ordovician age, supported by a core of igneous rock; also, that beds of only two formations, Permian and Cisco, extend completely over the crest of the dome, but that those of Canyon, Strawn, and possibly Bend are deposited on the flanks. The chief source of production is near the base of the Cisco. Thick bodies of w ter sands are found in the lower formations around the edge of the field. One distinct unconformity is known to exist. This is between the Ordovician and beds of overlying formations. The Petrolia field is an excellent example of the relation of petroleum accumulation to anticlinal structure.