The Marathon Uplift is a broad domal structure in southwest Texas spanning about 78 miles, or 126 kilometers. Formed in the Paleogene, it was later eroded at its crest, removing Cretaceous and younger rocks and creating the Marathon Basin. Inside that basin, northeast-trending ridges of much older, pre-Permian Paleozoic strata preserve evidence of a far deeper geologic history. Those ridges were shaped by the Ouachita orogeny, the same mountain-building event that formed the Ouachita Mountains in Arkansas and Oklahoma. Faulting associated with the Marathon Uplift may also be connected to the processes that produced the Llano Uplift elsewhere in Texas.