Similar lavas also form the pedestal beneath Bald Crater and the spur running northwest therefrom, and presumably Bald Crater, like Red and Desert cones, marks the site of a vent which burst into renewed activity about the close of the Pleistocene.
Finally, reference should be made to the olivine-bearing lavas forming the mesas on either side of National Creek. On Crescent Ridge these lavas are exposed to a thickness of more than 1,500 feet, and throughout this great pile there are no interbeds of fragmental debris. The source of the flows must lie to the east, probably in the vicinity of some of the ruined volcanoes already mentioned.
Hence, during the Pliocene and perhaps also during the early Pleistocene, before the cone of Mount Mazama began to form, the northwest corner of the park was occupied by a group of volcanoes the activity of which was marked by quiet effusion of liquid, wide-spreading, basic lavas, and an almost complete absence of pyroclastic explosions. Mount Mazama rose in the depression between this group of volcanoes on the north and the Union Peak volcano on the south.
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