...starting 20 million years after the death of the dinosaurs.
When researching the history of the Yellowstone volcano for "Ashfall" I was floored to learn that Nevada had been a much skinnier region before the volcano's birth.
Around seventeen million years ago something massive struck the continental crust from below and created not only the Yellowstone and Newberry hot spots, and buried much of Oregon and Washington miles deep under basalt flows, but also lifted what is now the Great Basin and Range area to nearly a mile above sea level and stretched out Nevada from east to west. So much so that it nearly tore Nevada in half. The basaltic remains of the resulting rift still bisect the state, running from near the original Yellowstone caldera at McDermitt on the Nevada - Oregon border south to Nevada's midpoint.
This video shows the overall development of the Earth's continents over the past half billion years, and presents an accurate depiction of the western United States essentially being a cluster of micro continents, uplifted ocean floor segments and volcanic arc island chains that gradually fused with the granitic core that underlies the central US and Canada.
A much faster version that extends 250 million years into the future