GEOMORPHIC EVOLUTION OF GRAND MESA, WESTERN COLORADO
The late Cenozoic erosional history of Grand Mesa is documented by an impressive array of discontinuous gravel-capped surfaces (terraces) that occur on its flanks and grade towards the Colorado and Gunnison Rivers. These surfaces, which are carved primarily into the soft Mancos Shale (Cretaceous), were generated by a variety of colluvial, alluvial, and pedimentation processes. The gravel deposits (mainly basalt clasts) range in thickness from 0.2 to 45 m, and individual surfaces range in length from 0.9 to 26.9 km. At least six major terrace levels, ranging in elevation from 1,387 and 2,812 m have been mapped to date. The highest terraces project about 400 m above present-day base level. The Lava Creek B (LCB) ash, which erupted from the Yellowstone area approximately 640,000 has been tentatively identified at two locations on the southwest flank of Grand Mesa: Petrie Mesa and Paradox Mesa. The ash layer is usually 0.5 to 3 meters thick and is interstratified with the terrace gravels. It is well cemented with calcite and appears to have accumulated in low-energy depositional settings.