Water Supply and Irrigation Papers of the United States Geological Survey . outcrop zones in the Black Hills and flows eastwardthrough the interstices of the rock in its extension under the lowerlands. Some of it finally escapes in springs in the outcrops in easternSouth Dakota and Nebraska. In such water-bearing strata as theDakota and underlying sandstones, which are overlain by a thickmass of impermeable deposits, the waters are under great pressure,for the intake zone has an altitude of about 4,000 feet and the regionof-outflow is only 1,000 to 1,200 feet above sea level. The evidence ofth
Water Supply and Irrigation Papers of the United States Geological Survey . outcrop zones in the Black Hills and flows eastwardthrough the interstices of the rock in its extension under the lowerlands. Some of it finally escapes in springs in the outcrops in easternSouth Dakota and Nebraska. In such water-bearing strata as theDakota and underlying sandstones, which are overlain by a thickmass of impermeable deposits, the waters are under great pressure,for the intake zone has an altitude of about 4,000 feet and the regionof-outflow is only 1,000 to 1,200 feet above sea level. The evidence ofthis pressure, as found in many wells in eastern South Dakota, isconclusive that the water flows underground for many hundreds ofmiles. Several wells show surf ace pressures of 175 to 200 pounds tothe square inch, the latter indicating a pressure of 780 pounds to theinch at the bottom of the well. Such pressures can be explained onlyby the hydrostatic influence of a column of water extending to a highaltitude on the west. If it were not for the outflow of the water to the. Fig. 2.—Diagram of apparatus for illustrating the declivity of head of liquids flowing from a reservoir. The shaded portions are water. east and south the initial head which the waters derive from the highlands of the intake zone would continue under the entire region, butowing to this leakage the head is not maintained, and there is agradual diminution toward the east known as hydraulic grade, aslope sustained by the friction of the water in its passage through thestrata. Figure 2 shows a simple apparatus which illustrates thegeneral conditions. Another factor which undoubtedly somewhat influences the hy-draulic grade in the Great Plains region is a certain but unknownamount of general leakage through the so-called impermeable strata,all of which permit the passage of an appreciable proportion of water,especially when under great pressure. The conditions shown infigure 2 are closely similar to those found in
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