Arlington and Gillespie shield volcanoes erupted near the Gila River in the late Pliocene to Early Pleistocene and are located 60 and 70 km west of Phoenix, AZ, respectively. Each shield consists of 3–4 alkali olivine basalt units emplaced in three eruptive phases of decreasing volume. The lava flows of the initial phase of each shield flowed into the river channel. At Arlington the channel was shifted ∼0.5–1 km south. At Gillespie the channel was dammed by lavas and possibly formed a lake. The river cut a new channel located ∼1.5 km to the east. The northern margin of Arlington is mantled by alluvium, possibly due to a change in the Hassayampa River base level related to the formation of the paleolake. Although alluvial sediments deposited on Arlington probably indicate that it is older than Gillespie, both shields exhibit comparable weathering, incision by ephemeral washes, and pedogenic carbonate accumulation. There is no evidence of weathering between basalt units. This suggests that the eruptions were essentially monogenetic and that some of the radiogenic dates that span from 3.28–1.28 Ma for Arlington and 4.3–1.35 Ma for Gillespie are inaccurate.