 Starting 20 million years ago ( in the Cenozoic ) and lasting until 5 million years ago , a now @-@ extinct extension of Cascade Range volcanoes erupted , bringing large amounts of igneous material in the area . These igneous deposits blanketed the region north of the Yosemite region . Volcanic activity persisted past 5 million years BP east of the current park borders in the Mono Lake and Long Valley areas . 
