Before going on, let us define the word _cave_ as we consider it here. True caves are formed in soluble rock—limestone, marble, gypsum or dolomite. They usually contain some redeposited mineral in the form of stalactites, etc. As most caves occur in limestone, the term _limestone cave_ is often used to describe any true cave, even though it may actually occur in dolomite or marble. The cave-forming process will be basically the same for either type of soluble rock. We exclude from this definition mines, lava tubes, ice caves, and sandstone depressions such as those used by cliff-dwellers in the Southwest. While they are “holes in the ground,” they are formed in different ways than are the limestone type caves, and therefore are not usually referred to in geological discussions of caves. HOW OREGON CAVES WERE FORMED _The Raw Material—Rock_ If we could turn back some 180 million years into geologic time, we would find the North American continent a much different place. This was the Triassic Period. Early dinosaurs thrived in primitive forests over much of the United States. The area around southwestern Oregon was not yet part of the continent; it was a shallow arm of the sea. Smoldering volcanoes jutted out as cone-shaped islands or poured forth fumes and lava from the distant mainland. During quieter centuries the age-old process of life and death went on within the sea waters. Fish, clams, coral—even tiny one-celled creatures too small to be seen—extracted a mineral called calcium carbonate from the water. With it they built bones, shells and skeletons. When these animals died, their hard parts settled to the ocean bottom. Gradually, layers of calcium carbonate were built up. At the same time certain chemical functions of ocean plants extracted carbon dioxide from the water and caused still more calcium carbonate to precipitate and add to the sediments. The layers deepened. Eventually the weight of overlying sediments and the ocean above compressed them into a rock called limestone. In different parts of the sea, and under varied conditions, other ocean sediments were deposited. Near the shore, wave-swept sand accumulated and eventually became sandstone. Fine silt and clay carried to the sea by rivers settled in bluish layers which were to become shale. Near rocky headlands, course gravel deposits became cemented into a hard mass called conglomerate. This steady formation of sedimentary layers was periodically interrupted by volcanic activity. Heavy clouds of volcanic ash and fragments settled into the sea. Molten lava poured into shallow bays or welled up from subsurface volcanoes to mix with calcium carbonate muds. When volcanism subsided, the seas went back to the quiet deposition of limestone. Today at Oregon Caves we find evidence of this interbedding of sedimentary and volcanic materials. An example of such interbedding can be seen above the parking area near the Chateau.