Miocene Epoch: Tectonics and Paleoclimate  The Miocene saw a
change in global circulation patterns due to slight position
changes of the continents and globally warmer climates.
Conditions on each continent changed somewhat because of these
positional changes, however it was an overall increase in
aridity through mountain building that favored the expansion of
grasslands. Because the positions of continents in the Miocene
world were similar to where they lie today, it is easiest to
describe the plate movements and resulting changes in the
paleoclimate by discussing individual continents.  In North
America, the Sierra Nevada and Cascade Mountain ranges formed,
causing a non-seasonal and drier mid-continent climate. The
increasing occurrences of drought and an overall decrease in
absolute rainfall promoted drier climates. Additionally,
grasslands began to spread, and this led to an evolutionary
radiation of open habitat herbivores and carnivores. The first
of the major periods of immigration via the Bering land