It is an account of 75 million years of accumulation with intermittent periods of erosion that began when the Rocky Mountains reared up in the West and spread sediments over vast expanses of the plains.
(It; is; an account of 75 million years of accumulation with intermittent periods of erosion) (erosion; began; T:when the Rocky Mountains reared up in the West and spread sediments over vast expanses of the plains) (the Rocky Mountains; reared up; L:in the West) (the Rocky Mountains; spread; sediments; L:over vast expanses of the plains)
It erosion the Rocky Mountains the Rocky Mountains
is began reared up spread
an account of 75 million years of accumulation with intermittent periods of erosion
T:when the Rocky Mountains reared up in the West and spread sediments over vast expanses of the plains
L:in the West
sediments L:over vast expanses of the plains