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Page 11 <br /> GEOLOGY <br /> GENERAL <br /> Ground water supply in the study area is related primarily to <br /> three aquifers. These aquifers are the Ogallala Formation, Dakota Sand- <br /> stone and Cheyenne Sandstone. All three of the aquifers crop out in the <br /> study area. The Ogallala lies on the surface over the major portion of the <br /> region. The Dakota outcrops to the west and is exposed by erosion in some <br /> of the creeks and canyons. The Cheyenne is exposed only in the severely <br /> eroded canyons to the west. <br /> Intermediate formations, consisting primarily of shales and lime- <br /> stones separate the aquifers, making them essentially isolated from each <br /> other. The geologic formations in the area and their water bearing pro- <br /> perties are summarized in Table 2 on the following pages, described in <br /> sequence from youngest to oldest. The aquifers, although varying in <br /> thickness, are distributed rather evenly over the study area except in the <br /> southeastern part of Baca County where the Cheyenne and Dakota forma- <br /> tions are pinched out. <br /> STRUCTURE <br /> The regional structure of the area is simple, with all of the beds <br /> of the sedimentary sequence lying fairly flat with a gentle easterly dip. <br /> Two features have an effect on structure locally. They are the Las Animas <br /> arch and the Two Buttes intrusion. The Las Animas arch runs generally <br /> north-south and passes through the extreme western part of the study area <br /> in Las Animas County. The Two Buttes intrusion is along the boundary of <br /> the area north of Springfield. Both of these structures, together with the <br /> resulting erosion in the canyons to the west, tend to geologically isolate <br /> the aquifers, making the hydraulic limits of the subsurface formations con- <br /> form generally to the surface drainage area limits, which were established <br /> as the boundaries of this study. <br /> In the study area there are numerous small faults which are gen- <br /> erally not extensive and possess only minor displacement so consequently <br /> have only a limited effect upon the flow of ground water. The quality of the <br /> data used for this study was insufficient to define the faults adequately. In- <br /> formation from drillers logs were often quite inconsistent and indications of <br /> variation in the elevation of a formation in many instances could be the result <br /> of erroneous correlations or incorrect elevations rather than being evidence <br /> of faulting. <br />