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(0 C'J r- fC" ~.:,.-: { '- 4 o Precambrian Complex--this unit consists of schist, gneiss, granite, and pegmatite dikes, this group of rocks is separated from the overlying sediments of Upper Cretaceous to Upper Triassic age by an erosional unconformity representing the loss of more than a billion years from the geologic record (Lohman, 1981). (l The structure of the sedimentary bedrock units is affected by the Uncompahgre arch, a northwest plunging anticline. According to Lohman (1965), there is evidence for uplift of the Uncompahgre arch as recently as the late Pliocence or early Pleistocene (approximately 3 million years ago), and perhaps even more recently. The influence of this arch on the structure of the bedrock in the area can best be seen()n Figure 3. The red lines are structural contours showing the elevation of the base of the Dakota sandstone. In the site area, the Dakota is structurally arched, however, the possibility that this arching may influence the integrity of the Mancos shale and the possibility that recent pediment deposits (CGS, 1982) are also arched was not apparent from the investigations. The Mancos shale,which is the primary bedrock unit at the site, is typically characterized by fissility in the plane of bedding, plus frequent, persistent vertical joints. These can be observed in road cuts and excavations in the Grand Junction area. The joints often contain gypsum_and occasionally calcite. If not filled with mineral matter, the joint surfaces are often iron stained. Where weathered, the Mancos either loses its well indurated strength and decomposes to clayey or silty soils or remains relatively strong and forms a matrix of relatively permeable chips. Where decomposition to clayey soils occur, most of the joints appear to close. ( Underlying the Mancos, the crystalline rocks of the Precambrian complex and the well cemented beds in the Kayenta, Summerville, Morrison, Burro Canyon, and Dakota Formations are cut irregularly by predominantly vertical joints. The Wingate sandstone contains vertical joints in many places, but can contain very few in exposures along cliff faces. The Slick Rock Member of the Entrada sandstone is remarkable for an almost total absence of jointing. See Lohman (1965) for a more extensive discussion of jointing in these units. The mineral resources in the site vicinity include natural gas, oil, coal, and sand and gravel. The following discussion of the mineral resources is taken from Schwochow (1978). One-third of the natural gas fields in Mesa County are located in units which underlie the site. These are the Mancos, Dakota, Morrison, and Entrada Formations. Oil has been produced in small quantities only in recent years. C\