In north-central New Mexico the rugged Cimarron Range marks the eastern margin of the Southern Rocky Mountains, abruptly rising more than 5000 feet above the adjacent Great Plains. Structurally the range is a northward-plunging anticline with a core of pre-Cambrian crystalline rocks. Faulting along the eastern and western margins of the anticline separates the mountains from the Great Plains and the Moreno Valley. North of Cimarron Canyon the anticline plunges beneath the Tertiary sediments of the Raton Coal Basin. To the south the structure is obscured by the basaltic flows of the Ocaté Mesa. Upturned Paleozoic to Tertiary sedimentary rocks crop out along the eastern and northern margin of the anticlinal uplift. Regional relations are obscured by extensive Tertiary intrusions in the northern portion of the area and by the lava flows to the south. In the valley of Rayado Creek a volcanic plug which fed some of the lava flows has been exposed by erosion. In this region there is evidence for a Mid-Tertiary surface of low relief in the central portion of the mountains, a later lava-capped surface believed to be the equivalent of the Broad Valley Stage, the surface of the Park Plateau, and the Ocate Mesa. Three well-developed gravel-capped Pleistocene pediment surfaces extend steplike from the mountain front into the Great Plains. Both structurally and topographically the Cimarron Range is similar to the Colorado Front Range. An attempt is made to reconstruct the geologic history of the Cimarron Range and to fit it into the broader story of the development of the Southern Rocky Mountains in New Mexico. © 1943 Geological Society of America.