Figure 4. Map of the southern Uinkaret volcanic field showing the location of all reliable dates between 800 and 475 ka. Lava flows of this age are interpreted to have entered the canyon at around river mile 179, near the Toroweap fault. Numerous dates on Black Ledge remnants at Granite Park and a single date from a remnant (Spencer Canyon) at river mile 246 indicate that this oldest stage of volcanism was the most voluminous and far-traveled. A 540 ± 30 ka ⁴⁰Ar/³⁹Ar date in Whitmore Canyon indicates that volcanism likely occurred along the Hurricane fault at this time as well but may not have resulted in intracanyon flows.