The Trtiary lava succession of eastern Iceland dips at shallow angles towards and beneath the axial Quarternary volcanic zone. A 3-km vertical section through part of this Tertiary pile, studied during the Iceland Research Drilling Project, contains flows ranging in composition from basalt to icelandite. This succession can be divided into three distinct stratigraphic groups; upper and lower groups, 1.7 and 0.6 km thick with Zr/Y ratios of about 4.5, and a 0.7-km-thick middle group with a Zr/Y ratio of about 6.5. The middle group also has a significantly higher Ce/Yb ratio and higher elemental Sr abundances. The data on within-group variation are equally compatible with high-level fractionization of a plagioclase-olivine-clinopyroxene assemblage with accessory apatite and opaque minerals or with open system fractionization models. Dikes occur throughout the 3-km section and are comparable in trace element composition to the lavas of the upper group. The dikes are part of a N-S trending swarm originating from the Breiddalur volcanic center some 15 km to the south. These dikes were injected laterally, northward into the Reydarfjordur lava pile at the time of the eruption of the upper group lavas. The lower two major lava groups may be related to similar, but unidentified, discrete volcanic centers perhaps buried downdip. The new data on this deeper section corroborate earlier studies which indicated that on a gross scale, the eastern Iceland lava pile becomes less mafic with depth. This vertical change occurs because each major volcanic center erupted more silicic lavas close to the center and more magnesium-rich compositions toward the periphery. The formation of the lava pile occurred by tilting toward the spreading center and overlapping repetition of such major units which resulted in the more mafic edges of the lava groups forming the topographically higher parts of the crustal section.