Geological Survey Water-Supply Paper 1766 The Water Supply of El Morro National Monument

From rocks we obtain many valuable products for our everyday use. Near El Morro some of the rocks contain fluoride minerals, coal, oil, natural gas, and uranium. Many of the barren rocks cannot be excelled for natural splendor—even artists may fail to capture their beauty in the changing light of day. As far as the monument is concerned, however, the most important product of the rocks is ground water.

Ground water is not found in underground lakes or streams, as many people think. The water clings to the rocks and between them. When it rains, part of the water is absorbed by the soil; some runs off the soil into rivers and lakes, and the rest sinks deeper into the ground until it eventually reaches and wets the rocks beneath the soil cover. The rocks at some depth below the earth's surface nearly everywhere are wet, but geologists looking for water have to find a supply that is free to come into a well. Certain rocks lack adequate openings between the rock grains, through which water can move. There may be water in these rocks, but it cannot move fast enough to be of much use. In other rocks the openings are larger and the water will move into wells.

Each of the rock units in the vicinity of El Morro was carefully examined to determine whether or not it was capable of yielding adequate supplies of water to a well. Some rocks, such as shale, can be ruled out immediately because the spaces between the grains of rock are too small to permit movement of much water. Also, shale generally contains a variety of minerals that are easily dissolved. Only small supplies of water containing much dissolved material are generally found in shale.

Sand and sandstone differ widely in their capacity to store and transmit water. Uniformly coarse sand generally yields large supplies of good water to wells. Beds containing mixtures of different sizes of sand grains yield less than those containing sand grains of uniform size. Beds containing mixtures of sand and clay yield even less than those of pure sand. Sandstone is made of grains of sand cemented together by various minerals. Because the spaces between the grains are partly sealed up, sandstone yields less water than sand. The pores in a very fine grained sandstone or one in which grain sizes are mixed may be completely sealed and yield little or no water to wells.

Limestone may be so dense that water does not move easily through it, or it may contain many openings which permit easy movement of water. The openings in limestone are caused by solution of the rock along natural breaks. These openings become larger as water circulates through them, and eventually may become caverns. Wells of large yield may be drilled in beds of cavernous limestone, but the water generally is hard because of a high content of calcium and magnesium carbonate that may have been dissolved from the rock.

Metamorphic rocks and some igneous rocks are generally too dense to store and transmit water. Volcanic-flow rocks (igneous rocks) may be very dense, or they may contain many openings caused by contraction of the rock during cooling. Intensely broken volcanic-flow rocks are generally capable of yielding large supplies of water to wells.

Superimposed on the igneous and metamorphic rocks underlying El Morro is an accumulation of sedimentary rocks. Most of them are not very permeable. However, the San Andres Limestone, of Permian age, and the Chinle Formation (sandstone and limestone), of Triassic age, 4 to 7 miles north of El Morro, yield potable water in ample quantities.

A nearer source of water is the alluvium of Quaternary age. Alluvium is the name given to sand and gravel washed away and deposited elsewhere by streams. The Quaternary alluvium is the only formation within 2 miles of the monument capable of yielding adequate supplies of potable water to wells. The water in the alluvium is of good quality, according to chemical analyses. Alluvium is discontinuous in the area, and test drilling is necessary to locate favorable occurrences of it and to determine its water-bearing properties. The prospects of adequate water supplies are best in the deeper buried valleys; that is, in areas where the alluvium has been covered by lava flows.

Much of El Morro lies in a valley which widens to the northeast between Inscription Rock and a high cliff to the east. This valley, which has been partly filled by alluvium and basalt, is tributary to a larger valley that trends westward of the monument.

Five test wells were drilled in the northeast valley on the monument land. Alluvium was found below the lava flows in only one of the five test wells. The yield of the alluvium in the well was only 3 gallons per minute, which is far from adequate to supply the monument. The test drilling indicates that the buried valley that crosses the monument is very narrow and that an adequate supply of water probably cannot be obtained from wells on monument land. However, the alluvium in a larger buried valley just outside the north boundary of the monument seems to contain a larger supply of potable water.

The Importance of Water

El Morro is a dramatic record of the unique mixture of cultures in the old Southwest, as well as an interesting display of chapters in the geologic history of our earth. Its story is a perfect illustration of the importance of water to any civilization. The Indians settled on top of El Morro because of the pool at the base of the rock. Spanish and American travelers planned their journeys to stop at Inscription Rock because of the pool.

Now we have completed a full circle. People used to come to the rock because of the water; now water must be brought to the rock because of the number of people. So many visitors come to see the inscriptions left by those who stopped to take advantage of the pool that an additional supply of water had to be located. We know from news stories that the distribution of our national water supply may pose serious problems by the year 2000. Yet it is hard to grasp this huge problem and understand it on a national scale. Here at El Morro we can see it demonstrated as though under a microscope. The more numerous the population, the more rich and complicated our civilization becomes, the more we need ample water supplies. Water is an indispensable resource, even for a National Monument.