The inland sea receded, giving way to lush jungle. 37 to 23 million years ago (Oligocene) the climate changed to a drier, cooler environment, and jungles gave way to grasslands. To the west stood the young Rocky Mountains. Flash floods periodically swept out from the Rockies to the plains below, and the mud trapped and buried many animals. Volcanic eruptions in that mountain range spewed forth millions of tons of ash that periodically blanketed the area.