CENOZOIC TOPOGRAPHY OF THE WESTERN UNITED STATES
Our isotopic study shows that topography of the much of this region was established in the early Tertiary. The central and northern portion of the Sierra Nevada Mountains attained its topography prior to 16 Ma. In contrast, the southern Sierra may have undergone a reduction of topography between 14 and 7 Ma. Our study of the eastern Rocky Mountains shows that the present topographic gradient has existed in this region since Eocene times. The unusual climatic regime caused by interaction of three air masses and the steep topography of the eastern Rocky Mountains has been in place since the Eocene.