When tested for chemical composition, Oregon Caves marble samples have averaged 93 percent pure calcium carbonate (CaCO₃). Its bluish color is derived from the remaining percentage of impurities. Without these, it would be white. A good example of nearly pure calcium carbonate is the white chalk used on blackboards.
Without this belt of soluble marble, and without the fractures within it, natural processes could not have produced the “Marble Halls of Oregon.” It is the foundation, the framework, and the raw material of the caves.
Underground Erosion
The first requirement in the genesis of Oregon Caves—the right kind of rock—was met. Next came the erosive agent which was to carve it into caverns. This was the flow of underground water.
The present rainfall in this area averages 50 inches a year. During the many thousands of years the caves were forming, the climate may have varied from wetter to drier many times, but it is safe to assume this has always been an area of relatively heavy precipitation. The steep, mountainous terrain and deep-cut valleys of southwestern Oregon are characteristic of aggressive stream erosion that goes hand-in-hand with a healthy supply of rainfall.
Some of the rain evaporates and returns to the air. Some of it soon runs into streams and is carried rapidly to the ocean. The rest of it seeps into the ground where it is delayed for a time in its inevitable return to the sea. Under the force of gravity, it trickles downward rather steeply through joints and cracks in the rocks, or seeps between particles of sand, gravel, or clay. Below the ground surface it joins a zone of saturation, or phreatic zone.
GROUND WATER
RAIN FALL } Vadose zone: water moves from surface to water table (vertically) —Water table } Phreatic zone: total saturation, water moves slowly and nearly (horizontally)
Here cracks, pores, and all spaces within the rock are completely filled with water. There are no airspaces. Water movement within the phreatic zone is comparatively slow, varying from a few inches a year to a few feet a day, depending upon the permeability of the rock structure. And the movement is usually horizontal, following the contours of the land in the same direction as surface streams. Eventually this water will find its way back to the surface at a lower elevation where it usually emerges as a spring. It is phreatic water which feeds the mountain streams and rivers many weeks or months after the last rainfall. It might also be pumped from a well for human use. A large portion of the earth’s population depends upon well water from these great underground reservoirs.