Caption below states:
~ Mile 178.5: Looking downstream at remnants of massive volcanic flows that poured into the Grand Canyon in Quaternary time. The volcanic flows and intrusions are part of the Uinkaret volcanic field located on the north rim of the Grand Canyon in the Grand Canyon National Park. The oldest dated volcanic flows in the canyon are about 1.2 million years ago, whereas the youngest rocks in the volcanic field are dated about 12,500 years ago (Nash, 1990; Hamblin, 1990).
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