Pleistocene surficial deposits of the Grand Mesa area, Colorado
— Rex D. Cole and John L. Sexton

Grand Mesa, with an average surface elevation of about 3050 m, is a basalt-capped plateau that forms one of the most prominent physiographic features in west-central Colorado (fig. 1). Basalt flows capping Grand Mesa have protected the underlying weaker sedimentary rocks from erosion. Progressive erosion of Grand Mesa since the outpouring of basalt 10 million years ago (K/Ar date of 9.7 ± 0.5; Marvin and others, 1966) has produced an impressive array of pediments, alluvial fans, glacial outwash fans, landslides, and colluvial deposits which now flank the mesa on all sides.

 

The Grand Mesa area has been a topic of study since the Hayden Surveys (Hayden, 1876). Most recent studies have focused on the glacial geology of the area (Henderson, 1923; Nygren, 1935; Retzer, 1954; Yeend, 1969). Sinnock (1978; this guidebook) ad-dressed the geomorphology of the western front of Grand Mesa in his study of the Uncompahgre Plateau and Grand Valley. General treatment of the geology of Grand Mesa is given by Young and Young (1968, 1977). Detailed mapping of parts of the Grand Mesa area has been done by Yeend (1969), Donnell and Yeend (1968a, b, c, d, e), Yeend and Donnell (1968), Hail (1972a, 1972b), and Sin-nock (1978).

 

In this paper we briefly address the general geology of the Grand Mesa area, and then we provide a more detailed description of the Pleistocene surficial deposits. Our intent is to illustrate the complexity and diversity of the surficial deposits, and to bring together for the first time an overview of the depositional processes that were active during their formation.