A surprisingly large region of the western United States, far from the edges of the continent, has been uplifted by 2 km or more sometime since the Cenomanian stage of the Cretaceous period, nearly 100 million years ago, according to a paper presented at the 1985 AGU Fall Meeting by Dork Sahagian of the University of Chicago. In fact, most of this epeirogenic uplift may well have occurred in the last 5–15 million years.
Epeirogenic motions are predominantly vertical movements that produce broad features of continents and oceans (for example, plateaus and basins). Epeirogeny is usually contrasted with orogeny, the more localized and generally horizontal processes involved in the formation of mountain chains near plate margins. Since the advent of plate tectonic theory, horizontal motions have received more attention than have vertical motions like those studied by Sahagian, according to Bradford Hager of the California Institute of Technology in Pasadena, who served as tectonophysics program cochairman for the Fall Meeting.