Plate 19.—General View in the Appalachian Mountains Along New River, Virginia. This is a typical portion of the great area which, during Mesozoic time, was reduced by erosion to the condition of a low-lying plain (“peneplain”). Since early Cenozoic time the peneplain has been upraised and New River has carved out its V-shaped valley to its present depth, while tributary streams have carved out a series of valleys along belts of weak rocks nearly at right angles to the main valley. The remarkably even sky line marks approximately the old peneplain surface. (Photo by Hillers, U. S. Geological Survey.)
Plate 20.—(a) A Big Glacial Bowlder of Plutonic Igneous Rock Carried Miles from Its Parent Ledge by the Ice Sheet Which Passed Over the Adirondack Mountains During the Ice Age. (Photo by the author.)
Plate 20.—(b) A Long, Winding Ridge of Sand and Gravel (Called an “Esker”) Deposited by a Stream in a Channel in the Ice Near the Margin of the Great Glacier During Its Retreat from the Adirondack Mountains. (Photo by the author.)
“If the rocks are dry, then the chief points of accumulation of the oil will be at or near the bottom of the syncline (downfold), or lowest portion of the porous bed. If the rocks are partially saturated with water, then the oil accumulates at the upper level of saturation. In a tilted bed, which is locally porous, and not so throughout, the oil, gas, and water may arrange themselves according to their gravity in this porous part.” (Ries.)
Although the term “oil pool” is commonly used, there is really no actual pool or underground lake of oil, but rather porous rock saturated with oil. It has been estimated that in an oil field of average productiveness a cubic foot of the porous rock contains from six to twelve pints of oil. The life of a well drilled into an “oil pool” varies from a few months to twenty or thirty years, or sometimes even more, but a heavy producer (especially a “gusher”) almost invariably falls off very notably in production in a few months, or at most a few years. The typical Pennsylvanian oil well is said to last about seven years. The fact that the United States is still able to increase oil output is because new fields are found and developed, the most recent being in the interior and northern parts of Texas. It is practically certain, however, that the climax of oil production in the United States will be reached before many years—long before that of bituminous coal.