Around thirty million years ago (millions of years after the Rocky Mountains formed), tectonic plates shifted and portions of the crust of southwestern North America began to split apart.
(tectonic plates; shifted; ) Context(tectonic plates shifted,List([86, 109))):(the Rocky Mountains; formed; T:Around thirty million years ago)
tectonic plates shifted,List([86, 109))
tectonic plates the Rocky Mountains
shifted formed
T:Around thirty million years ago