RESULTS OF THE WORK OF GROUND-WATER.
Fig. 203.—Ground-plan of Wyandotte Cave. The unshaded areas represent the passageways. (21st Ann. Rept., Ind. Geol. Surv.)
Weathering.—Where the solution effected by ground-water in any locality is slight and equally distributed, the result is to make the rock porous. If, for example, some of the cement of sandstone is dissolved, the texture of the rock becomes more open; but if all the cement be removed the rock is changed from sandstone to sand. If a complex crystalline rock contains among its many minerals some one which is more soluble than the others, that one may be dissolved. This has the effect of breaking up the rock, since each mineral acts as a binder for the rest. It might happen that no one of the minerals is dissolved completely, but that some one of them is decomposed by water, and certain of its constituents removed. Such change would be likely to cause the mineral so affected to crumble, and with its crumbling, if it be an important constituent of the rock, the integrity of the rock is destroyed. Where considerable chemical changes, especially subtractions, are going on, the rock is likely to crumble. The increase in volume attendant on hydration, etc., sometimes leads to the disruption of rock. These are phases of weathering. (For other phases of weathering see pp. [54] and [110].)
Caverns.[106]—Where local solution is very great results of another sort may be effected. In formations like limestone, which are relatively soluble, considerable quantities of material are frequently dissolved from a given place. Instead of making the rock porous, in the usual sense of the term, large caverns may be developed ([Fig. 202]). In their production, solution may be abetted by the mechanical action of the water passing through the openings which solution has developed. Considerable caves are found chiefly in limestone. They were probably developed when the surface relief was slight, and surface drainage therefore poor. Regions where caves were developed under these conditions may subsequently acquire relief, so that caves are not now confined to flat regions.
One of the best known regions of caves is in the basin of the Ohio in Kentucky and southern Indiana, where the number of caves is large, and the size of some of them, such as Mammoth and Wyandotte, very great. A ground-plan of Wyandotte (Ind.) Cave is shown in [Fig. 203]. The aggregate length of the passageways is about 23½ miles.
Fig. 204.—Deposits in Wyandotte (Ind.) Cave. (Hains.)
Fig. 205.—Deposits in Wyandotte Cave. (Hains.)