an area of about 200,000 square miles, in places reaching 10,000 feet thick.
Periodically, volcanic eruptions spewed out batches of basalt as large as 1,200
cubic miles, forming lava lakes up to 450 miles wide in a matter of days.
The volcanic episodes might be related to the Yellowstone hot spot,
which was then positioned beneath the Columbia River Plateau region. The
hot spot moved eastward relative to the North American plate. It can be traced
Figure 180Palouse Falls
in Columbia River basalt,
Franklin-Whitman
Counties,Washington.
(Photo by F. O. Jones,
courtesy USGS)TERTIARY MAMMALS