During Eocene time a volcanic field was built up on a north-south axis that had persisted since mid-Cretaceous in the region from central Idaho into Nevada. Eocene subalpine conifer forest recovered from the higher parts of the axis implies a minimum altitude near 4000 ft, and structural evidence suggests that locally some peaks may have been 2000 ft higher. The successive appearance of conifer-deciduous hardwood forest, mixed deciduous hardwood forest, and evergreen broadleaved forest to the east and west of the central axis indicates lower altitudes and warmer climates there.

Cross-collapse of the axis at the site of the Snake River Plain probably commenced during the Oligocene because the basin was present by Miocene time. This is shown by Miocene floras from the trough that represent deciduous hardwood forest, whereas those on its flanks comprise conifer-hardwood forest, indicating higher altitudes and cooler climates there.

The Snake River basin appears to be a tensional rift that widened and deepened as the Idaho batholith drifted northward (Hamilton and Myers, 1966). This movement, which presumably initiated collapse of the Cretaceous-Eocene divide during Oligocene time and has continued down to the present, was accompanied by uplift of the bordering highlands of approximately one mile.

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