Camping and woodcraft - Scoutmastercg.com

342 CAMPING AND WOODCRAFTAlong the sea-coast are many interesting butshallow grottoes that have been pounded out byrocks hurled by the incoming waves, and worn intocurious forms by the restless waters themselves.Neither these nor any of the preceding kinds ofnatural excavations or fissures are extensive enoughto rank with the caverns that abound in limestoneformations throughout the earth.Vast subterranean passages and chambers areFig. 188.—Cross section of cavernA B, upper gallery (ancient); g g, sink-holes r C D,lower gallery (modern); h, stream; A, old mouth ofcavern; f f f, limestone; C, present mouth of cavern;e e e, hard rockformed in limestone by an agency far gentler thanany cf those mentioned above.First, there must have been at some time a de^ciduous forest,shedding, each autumn, a thick layei'of leaves. Upon these leaves the rains fall, andtheir waters absorb from the decaying vegetationcharges of carbonic acid (the same gas that is usedin soda fountains). This acidulated water, seep^ing Into crevices in the limestone, dissolves outmuch of the lime and leaves only a shell of theoriginal rock. Thus the cracks in the surfacegradually widen and deepen.When the rainwater reaches an underlying bedof sandstone, or of some other rock that is noteasily dissolved by carbonic acid. Its downwardcourse Is stayed. Then, under pressure from above,it begins eating and cutting a more or less horizontalcourse along the underground drainage plane. Thismining process is hastened by erosion. Wheneverthere Is a crack or fault large enough to admit aconsiderable riii of water, sand and gravel are car-