Northwestern New Mexico's Table Mesa is a moderately large, flat-topped hill bordered by subvertical cliffs and sloping talus piles. Mesas are flat-topped due to the presence of rocks relatively resistant to weathering and erosion. Table Mesa rocks represent a portion of the upper Mancos Shale, a moderately widespread, marine, generally fine-grained siliciclastic unit deposited during the Late Cretaceous. The Mancos Shale Formation is divided into three parts: 1) lower Mancos Shale - dominated by fine-grained siliciclastics; 2) Gallup Sandstone - a coarse-grained siliciclastic unit; 3) upper Mancos Shale - dominated by fine-grained siliciclastics.
Table Mesa, consisting of fine- to coarse-grained siliciclastics of the upper Mancos Shale (Coniacian to Santonian to Campanian Stages, middle Upper Cretaceous). Locality: western side of Rt. 491/Rt. 666, between Newcomb and Shiprock, San Juan County, northwestern New Mexico, USA. Looking ~NNW.
Table Mesa - the lower cliffs (partly obscured by talus slopes) are dominated by shales, with minor interbedded siltstones and sandstones. The upper cliffs are dominated by relatively hard sandstones, which weather slowly to form the flat-topped mesa. The coarsening-upward sequence shown here represents a minor regression during a time of global sea level highstand. Looking ~NNW.