ORIGIN OF LIMESTONE CAVERNS
Water from rain and melting snow is disposed of in three ways. Part of it sinks into the ground, where it remains for an indefinite time before re-appearing at the surface as springs. Some of it runs off to join surface streams and lakes, and the third part is returned to the atmosphere by evaporation and plant transpiration. The amount that is absorbed into the ground depends upon soil depth and texture, steepness, type and quantity of vegetation, rate of downfall, cracks and fissures in the rocks, and several other factors. The character of the soil and plant cover in the drainage basin above Crystal Cave favors the retention of water in the soil despite steep slopes, consequently there is a steady source of underground water to feed clear mountain streams throughout the long, dry summers. The absence of grazing and logging in National Parks encourages delayed runoff and the conservation of water.
Ground water descends slowly under the influence of gravity until it reappears at the surface as springs or reaches a level below which all spaces are filled with water. This level is called the water table. Springs appear on valley sides at this level and deep wells become filled to a height even with the water table. The water table conforms roughly to the surface of the land, although it is much deeper beneath divides and comes closer to the surface in valleys to join streams and lakes. It rises following wet periods and drops after droughts, and as valleys are deepened it descends to successively lower levels. The movement of ground water is said to be principally downward until it joins the water table, when lateral movement is reported to be dominant. There is also abundant evidence that considerable movement occurs below the water table in sub-water table streams where passages have been dissolved out. Water is forced through such passages by hydrostatic pressure. (2) Lateral movement is encouraged by the existence of caves which have openings and suitable grades near the level of surface streams. Such streams sometimes are called water table streams (8), and they are considered by some authorities to be the principal agents which excavate caves. (26 and 27) Some geologists explain the origin of caves by simply stating that openings gradually are dissolved large enough to permit free flowing water, thus adding abrasion, or mechanical wear, to the work accomplished by solution (6), while others have contended that such caverns as the Mammoth Cave of Kentucky were entirely "worn out" by running water. (15)
Until recent years, it had been generally assumed that practically all solution by ground water occurred above the water table, sometimes called the zone of solution, (20) and that as the descending water joined the reservoir below the level of permanent ground water it had become so burdened with dissolved minerals that it could no longer change and dissolve limestone. Recent investigations have disproved this theory, as will be shown later.
The term "vadose" is applied to ground water above the level of the water table, while "phreatic," which in Greek means a well, applies to that below the water table.
After an exhaustive review of the literature on caves, William Morris Davis in 1930 wrote a masterly thesis on the "Origin of Limestone Caverns," (8) which had the effect of stimulating a renewed interest in caves, some geologists supporting and some opposing his theory that caves are first formed below the surface by phreatic water, and that due to uplift with a resultant lowering of the water table the caves were drained and air admitted for the first time, thus permitting the growth of stalactites and other deposits which decorate most limestone caves. His deductions are supported by the evidence found in the caves of Sequoia National Park. The outstanding field investigations and conclusions of J. Harland Bretz in 1942 largely confirmed the ideas of Davis. He concluded that most of the caverns he had seen were largely of phreatic origin, and that of 107 caves he studied only three simple ones were entirely vadose in origin. (2) Many of the features he describes are applicable to Crystal Cave.
That sub-water table caverns may exist is suggested by huge, deep-seated springs or rivers that rise upward to the surface of valley bottoms in many parts of the country, as well as from the ocean floor off the coast of Florida. (2 and 8) These streams must traverse large water-filled caverns. The Mammoth Hot Springs Terraces in Yellowstone National Park suggest another method of cave origin by solution at considerable depth. There the hot, rising water heavily charged with carbon dioxide has deposited a mountain of travertine about two miles long and one-half mile wide on the surface. This material was dissolved from a formation of limestone which has a thickness of about 1,600 feet. It is possible that caverns filled with hot water exist in this limestone in such proportions as to dwarf some of our best known caves. In 1941, Berlin C. Money maker described drilling operations and deep cut excavation in various parts of the Tennessee Valley which revealed the presence of hundreds of cavities at depths exceeding 100 feet below the water table, some of which had the dimensions of large caverns. (19) Similar drillings in other localities indicate such caverns are common geologic features. (2)
Last Updated: 31-Jan-2007