HOW OREGON CAVES WERE FORMED (continued)
The first requirement in the genesis of Oregon Cavesthe right kind of rockwas 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.
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.
How deep does phreatic water exist? This depends upon the porosity of the rock, or its ability to contain water. Most mines that penetrate many hundreds of feet encounter little water at great depths. Pressure of the overlying rocks is so great that open spaces capable of holding water cannot exist. So the earth's crust contains available well water only in restricted zones not far from the surface. The top of the zone of saturation is called the water table. It is here that the most rapid flow of phreatic water occurs, for the joints and interspaces are wider nearest the surface. Between the water table and the surface is the vadose zone, in which the spaces are partly filled with water and partly filled with air. (See illustrations on page 7). Vadose water content varies greatly with weather conditions. As rainfall is scant in southwest Oregon during the summer, visitors find the caves relatively dry at that time. In the winter, however, the passages will be veritably "raining" vadose water within a few days after snow or rain.
The water table itself is more stable, but varies somewhat from winter to summer, or during extended periods of unusually wet or dry seasons. Its lowest possible level is ultimately controlled by the elevation of the largest nearby surface stream or lake, which acts as a base level. When the streams and lakes are lowered by erosion, the water table of a given locality keeps pace by slowly sinking until eventually it lies scarcely above sea level.
Rain falling on the mountains above the cave seeps into the surface cover of vegetation and humus. Here it absorbs carbon dioxide released from the process of organic decay. Seeping further through the vadose zone and down to the water table, this water carries many times the normal amount of carbon dioxide found in the atmosphere. In fact, it becomes acid. For water (H2O) and carbon dioxide (CO2) unite to form a mild solution of carbonic acid (H2O + CO2 H2CO3). In this manner, phreatic water is constantly charged with mild acids. Not the kind that harm us, of course. The fountain water by the chalet, and probably that in your home, is actually mild carbonic acid. So is bottled pop.
It was thus that phreatic water, charged with soil acids, percolated century after century through cracks in the marble. The acids ate away at all exposed rock surfacessideward, downward, and upward. (The solution rills in the original Ghost Room ceiling reveal the upward dissolving of water-filled cavities, see illustration page 10). To fully understand this, we must recall that the marble is 93 percent calcium carbonate (CaCO3). To dissolve it, carbonic acid mixes with the calcium carbonate to form an unstable liquid compound called calcium bicarbonate, H2CO3 ± CaCO3)2. ??? The removal of solid calcium carbonate in a liquid is the key cave forming process and is called solution (see illustration page 9). In Watson's Grotto we find several examples of early crack-widening by phreatic solution (see illustration page 10).
The enlarged cracks allowed faster movement of water against an increased surface area, and a subsequent increase in solution activity. Partitions between them fell apart and were dissolved. A series of water-filled passages evolved deep underground. Their pattern and orientation followed the pre-cave network of joints and cracks in the original strata. Gradually the openings were further enlarged into the cave system we know today.
There is more to it than that, of course. You may ask, "Why aren't there caves continuously throughout the belt of marble? The joints and cracks are everywhere. And certainly all the marble near the surface has been subjected to ground water action at some time or another. Why are Oregon Caves limited to one particular part of the marble belt?"
The answer to this involves several considerations. To begin with, we do find small cavities and solution cracks throughout the exposed marble. So there has been varying degrees of solution activity nearly everywhere, although not sufficient to produce caverns comparable to Oregon Caves.
Secondly, we must reconsider the mineralized water, calcium bicarbonate. We called it an unstable compound, meaning it will alter readily with slight changes in conditions. Once the amount of carbon dioxide dissolved in the water has united with an equivalent amount of calcium carbonate (marble), the solution is saturated. No more marble can be dissolved until additional carbon dioxide is absorbed by the water. If the solution loses some of its carbon dioxide into the air, then an equivalent amount of calcium carbonate must be redeposited as solid stone. The balance can be delicate. In an underground pool of mineralized water, solution may be going on at one end of the pool and deposition at the other.
So a state of chemical balance tends to develop in normal phreatic drift through the marble. Water saturated with minerals might easily move through many hundreds of feet of marble stratta without further enlarging the openings. Instead, it might even deposit some of the dissolved minerals, filling small cracks and veins, possibly even blocking its own passage during dry cycles when phreatic flow is at a low ebb. The "dry" room in the cave is an example of vein filling. Clay, gravel, and other surface sediments can also be washed into the openings, plugging them up and halting further solution for a time. All these factors lead toward a stabilization of the solution process. Openings and small passages continue to be formed, yet normal phreatic movement at Oregon Caves seems to lack the force for large scale cave sculpture.
This opens the door to our third consideration; we know the greatest amount of solution occurs in the water table zone. Therefore, to gain the impetus needed to carve out a cave system, some local condition must have increased the water table flow in the immediate vicinity of Oregon Caves. The solution process was magnified as larger quantities of freshly acidic phreatic waters were channeled into a restricted zone. Surging on, they scoured through the marble, dissolving larger volumes of calcium carbonate and sweeping it away. The early solution pattern of enlarged cracks had set the stage for the onset of this swift phreatic erosion. But some geologically sudden event was necessary to trigger the forces which completed the act.
We do not know exactly what the triggering action was. We know that the water table either received a sudden increase in supply from surface drainage, or found a larger or lower outlet downslope which tapped phreatic water over a widespread zone and channeled it through a localized area. There are several possibilities.
1. A perched water table may have been held in the cave zone by a lower and impervious layer of rock. This barrier may have been suddenly cut through by erosion, as if the plug were pulled in a bathtub. The perched water would now pass through the barrier, rather than over it, evacuating parts of the former phreatic zone, and inducing surface streams to channel underground through the same route. With such a subterranean diversion of water from a higher to a lower drainage pattern, the water table flow would increase considerably. A cave-forming condition would exist.
Several small streams lose their identity and sink into the ground a few hundred yards above the caves. Doubtless, they join the water table inside the caverns to emerge at the entrance as the River Styx (called Cave Creek outside). Possibly they aided in the early stages of cave formation in a manner described above.
2. It is difficult to imagine what the surface topography was like when the cave was forming, yet we know it hasn't always been the same. The mountains were higher. The streams occupied higher positions in the valleys. The ridges lay in a somewhat different pattern. Now and then stream piracy, or drainage rearrangement, took place when a rapidly eroding stream cut away the ridge separating it from a less active stream. Suddenly the slower stream was diverted into the drainage system of its captor. Both surface and phreatic waters of the aggressive drainage were increased. The flow at the water table speeded up in response.
If stream piracy occurred in the drainage overlying the caves, it might have played an important part in cave carving.
3. Nor can we omit the conditions that occurred here during periods of glaciation. Shifting masses of ice and glacial debris characteristically cause damming and rechanneling of water in minor stream valleys. The temporary results are similar to stream piracy. Coupled with this is the great volume of water which drains from melting glaciers. Evidence of partial glaciation in the Siskiyou Mountains lends serious consideration to its effect on early cave development.
The whole process might have involved all three of the above situations in varying degrees, for a "geologically sudden" event may take several thousand years. Several distinct levels of cave erosion indicate that the water table moved along at a certain level for a time, then rapidly dropped to a lower course where it was stable for another extended period. This was repeated until it now stands near the level of the River Styx.
Successively, the caverns at higher levels were drained and left empty. So as your tour climbs from the cave entrance to the highly developed sections near the Ghost Room, you encounter galleries that are progressively older. The first room inside the entrance, Watson's Grotto, is the best example we have of a cavern "recently" drained.
A word about the River Styx. Above it in several places you can see very smooth walls left by the familiar erosive action of a stream. (See illustration page 14). Most of the cave walls show the more pitted, concave surface left by the acidic dissolving action of phreatic water. The water which produced the main cave system moved much slower than the River Styx, and over a wider area. The stream as we see it did not produce the cave. Rather, the caverns, when drained, left a free flowing course for ground water to channel into. The only true underground streams occur in caves. They are a by-product of the cave-forming process.
Last Updated: 10-May-2006