Conversation so far:
what large plateau is in the north-eastern part of california
The Modoc Plateau lies in the northeast corner of California as well as parts of Oregon and Nevada.
how high above sea level is the modoc plateau
The Modoc Plateau's landform is volcanic table land ranging from 4,000 to 6,000 feet above sea level.

Question: how old is the plateau
Self-contained rewrite: how old is the modoc plateau