The next two ages, the Silurian and the Devonian, which brought our earth down to a scant 250,000,000 years ago, did not see the deposit of any silting in the Black Hills region. No doubt the waters which covered the locality dried up gradually. The Mississippian period, however, was a time of great depositional activity. A layer of limestone between five and six hundred feet thick was set down over the entire section. In later periods this limestone underwent much decay and water erosion, which formed the amazing caverns for which the Black Hills are known. Wind Cave, now the site of a National Park, Crystal Cave, and Jewel Cave are the best-known tourist attractions among the many, although there are a number of lesser ones, some even today only partially explored. The chemical activity which accomplished this erosion was caused by the seeping of rain water down through later accumulations of sediment on top of the layer of limestone. As it seeped through rotting vegetation and timber the water collected carbonic acid gases which, when it reached the level of the Mississippian limestone, eroded the structure and ate out huge hollows in it. The thickness of the Mississippian deposit indicated that at this time the earth had again sunk beneath the waters to a considerable depth. The shallow sea which had not offered sediment to a greater depth than a few feet was replaced by active currents which carried heavier sedimentary materials from great distances, laying them down on the floor of the sea in various strata to a depth of several hundred feet. Finally, after an unknown number of millions of years, but perhaps during the Triassic period, the land again rose above the level of the waters. A red shale suggests a time of great aridity when the region must have been a near desert, and certain discernible patterns in the shales suggest periods of rapid evaporation and a consequent change in chemical activity. Finally the land subsided again, for the last time to date. At times salt water covered the region, and at other times fresh water left its chemical mark. At some levels in this last layer of sedimentary rocks an abundance of fossils can be found, indicating deep water, and at others ripple-marked rock indicates very shallow water. It remains a period of great mystery. How long this final submersion continued we do not know; but in all probability it lasted nearly a hundred million years, and then was terminated by the vast upending of North America which created the Rocky Mountains. This upheaval did not take place suddenly, as a volcanic eruption or a series of earthquakes, but apparently commenced about sixty million years ago and lasted, as a continuous series of shiftings and slow upheavals, for about twenty million years.