Little is known in detail of the geologic events in Yellowstone during
Oligocene and Miocene times. Rocks of these ages have not been
recognized within the Park; if ever deposited there, they have since
been removed by erosion or buried by younger volcanic rocks. Thus, we
can only speculate as to what events took place during this
25-million-year period. No doubt the broad Absaroka volcanic plateau was
eroded, but not deeply, because the topographic relief and stream
gradients of the region remained low. There are also hints that some
volcanic activity took place, for volcanic rocks representing parts of
this time interval occur south of the Park, and some of these rocks may
have originated within the Park area. Little transpired, however, to
significantly alter the existing geological makeup of the Park; it was
indeed a quiet time, particularly when compared with the extremely
dynamic periods which immediately preceded and followed it.