This chapter examines the Mio-Pliocene volcanic materials exposed along the western margin of the Middle Awash area, with a primary focus on the late Miocene deposits of the Adu-Asa Formation. The primary goals of this chapter are to: define the different types of volcanic material preserved in the Adu-Asa Formation; present a working composite stratigraphic view of the Adu-Asa Formation volcanic horizons; present the existing geochemical data for western margin volcanic samples, including some with uncertain chronostratigraphic context; illustrate the data and observations that provided the basis for previous correlations and naming of key volcanic marker horizons; and illustrate the data and observations that provide new regional correlations and hence the establishment of newly named volcanic horizons.