During recent geologic field mapping in the eastern Ouachitas of Arkansas, several new lithologic marker horizons have been recognized in the Ordovician rocks. The outcropping formations of Ordovician age include the Blakely, Womble, Bigfork, and Polk Creek. Meta-arkose boulders and gray-black chert layers are characteristic of the upper part of the Blakely Sandstone. The Womble Shale has been divided into 3 members. The lower member consists primarily of interbedded shale and calcareous siltstone. Individual layers often exhibit a very delicate banding. The middle member of the Womble is essentially black shale. The upper member is primarily shale but typically has beds of phosphatic conglomerate in its northern outcrops which are correlative with poorly sorted sandstones in the S. The Bigfork Chert is composed of chert and siliceous shale; commonly a black, sooty, punky, carbonaceous clay is found at the base. The Polk Creek Shale is chiefly a shale with some thin chert beds. No now lithologic markers were found in this unit.