HOW OREGON CAVES WERE FORMED (continued)
Surface erosion continued to tear away at the mountains. Streams cut their valleys deeper. In response, the water table gradually sank below the level of the caverns and they, in turn, were drained. Air entered the rooms. The basic excavation process was completed except for a few minor changes. In some places, vadose water continued to dissolve away portions of the cave ceilings into dome shapes. In other rooms previously drained, water re-flooded certain portions during wet cycles. And some rooms were filled with clay and gravel brought in from the surface, then washed clean again in later stages.
Most important is the entrance of air, which ushered in the second major stage in cave formation. The unadorned grottoes were now to be decorated. Nature, through the process of deposition, next created the eerie beauty which delights today's cave visitors. In fact the process continues even now, for Oregon Caves are "live" caves, meaning they are still being decorated by natural deposition.
The weak carbonic acid in vadose water kept eating away the roof marble above the caves. Reaching the caverns, drops of vadose water exaporated into the air and left their load of calcium carbonate as thin layers of solid mineral. The amount left by each drop was infinitesimal, yet millions of drops eventually left thick deposits coated on the walls, ceilings and floors of the cave. The crusty white deposits in the Beehive Room are fine examples of deposition by evaporation. They were left there in much the same way as the coating in the bottom of a teakettle or steam iron.
However, evaporation is important only near the surface. Deeper inside Oregon Caves the relative humidity averages 98 percent. Evaporation here is almost non-existent. Instead, loss of carbon dioxide becomes the chief agent of deposition. We have learned that vadose water contains 25 to 90 times the normal amount of carbon dioxide found in the atmosphere. Much of it, of course, unites with calcium carbonate to form calcium bicarbonate solution. When this mineralized water reaches the caverns, large quantities of carbon dioxide are able to escape into the air due to the difference in carbon dioxide amounts in the water and air. The chemical balance is upset. For each molecule of escaping carbon dioxide, an equivalent molecule of solid mineral is deposited (see illustrations page 9).
An interesting side effect of the loss of carbon dioxide is experienced by the cave visitor. Although cave air is constantly replenished by outside air through natural exchange, it has a rather high carbon dioxide content due to release of this gas by vadose waters. This partly explains the heavy breathing you find necessary inside the cave, because the nerve centers which control our breathing are stimulated by a high percentage of carbon dioxide in the air we breathe. It also explains the odd "peroxide" odor many people notice when they reach the exit. The odor is oxygen. We notice it because our senses have become adjusted to slightly lower oxygen percentages inside the cave.
Cave deposits are collectively termed speleothems. Their variety is infinite: Those left by dripping water are called dripstone, and take on two basic formsif they hang down from the ceiling they are called stalactites, if they grow up from the floor they are stalagmites. The two may join together to form a column. Where the water drips rapidly and the loss of carbon dioxide is slow, stalagmite growth is favored because little deposition can take place on the ceiling. If the drip rate is slow and loss of carbon dioxide is rapid, stalactite formation is favored.
Contrasted with dripstone is flowstonesmooth layered deposits left along walls and floors by flowing water. In Joaquin Miller's Chapel, flowstone deposits are many inches thick. (See illustration on page 16). A close look at the structure of dripstone and flowstone reveals six-sided crystals called calcite, which is merely the crystalline form of calcium carbonate. (See illustration on page 18). Banded crystal layers in cave deposits are often called alabaster, or cave onyx. These can be easily seen at the "wishing post."
Other shapes and forms accrue. Flowstone forming on backsloping walls tends to produce graceful sheets called drapery. (See illustration on page 19). Reddish bands may develop in drapery where iron oxide is imbedded with the calcite. Contrasted with the pure white layers of calcium. carbonate, this gives the appearance of bacon. Good examples of "bacon" can be seen in the Ghost Room.
Most shapes and forms of dripstone and flowstone are occasionally duplicated by freezing water. Stalactites form on the edges of roofs, stalagmites form on the sidewalk beneath them, etc. But one of several cave formations which can't be thus copied is the soda straw (see illustration page 20). Deposition begins as a ring of calcium carbonate around a water drop. The ring has the unique feature of being a single crystal. As more drops leave their deposits, the ringed crystals form one on the other to create a tube. The water continues to seep through the inside of the tube, eventually producing the fragile, crystalline pipe with the obvious name.
The diameter of a soda straw is apparently determined by the specific gravity and surface tension of water, for they are all nearly the same diameter, about one-quarter inch. In a cave in western Australia one soda straw has reached a length of 20 feet, 6 inches, yet is still only one-quarter inch in diameter. If the drip rate decreases, the tip of the soda straw may sometimes seal itself closed.
Some speleothems apparently defy gravity. Now and then internal hydrostatic pressure causes secondary formations to project out from others in unusual directions. These are helictites (see illustration on page 21). A related form is popcorn (see illustration on page 22), the mat of small nodules which coat the "beehive" and other objects in the cave. Also called "cave coral," popcorn can form under water or along wet walls in response to air currents.
Soft, fibrous mats of calcium carbonate deposited near the surface at Oregon Caves are termed moonmilk. In time moonmilk may harden into popcorn mats. Its manner of formation is not fully understood.
At the "Devil's Washboard," and at the foot of Paradise Lost, we find still another type of speleothemrimstonewhich forms in pools of water. Agitated by dripping or flowing water, some of the carbon dioxide in the pool escapes. The resultant deposition of mineral takes place on irregularities in the bottom of the pool, or creates stone wavelets where the pool spills over. Ridges and dams subsequently build up, often constricting the pool surface (see illustration on page 22). Another type of rimstone, or cave ice, develops when flowstone builds up around the edge of a pool and gradually closes across it. The pool may be completely sealed over, just like a pond in winter, except that "cave ice" can never melt.
Cave students are often confused by another deposition found in the form of thin blades jutting out of the wall (see illustration on this page). They have a woven, crystalline texture. Prior to removal by solution, some of the marble cracks were filled with calcium carbonate. Being less soluble than marble, the sheets of calcite crystals remain for a time after the surrounding rock is dissolved away.
So far we have discussed cave featuresspeleothemscreated by deposition of mineral from the solid to liquid, and back to solid state. But certain objects in the caves are simply what remains of a piece of marble after some of it is dissolved away. These are speleogens, cave features created by the dissolving of mineral (see illustration page 24). They can be striking, but primarily it is the speleothems which make Oregon Caves a thing of wonder and beauty. The zenith of such spectacular development as seen at Paradise Lost leaves little doubt of this.
We have followed the mineral calcium carbonate through many forms: from sea creatures to ocean mud, to limestone and then marble, next to a liquid solution called calcium bicarbonate, and lastly as calcite crystals in cave formations. The size, shape and variety of cave deposits are determined by many factors which seem to prevent any two being exactly alike. Changes in temperature, relative humidity, available carbon dioxide, amounts of vadose water, air circulation, surface tension, permeability of roof rock, vegetation above the cave, bacteria action, and the amount and kind of impurities in vadose water may all combine to vary the nature of cave formations.
Last Updated: 10-May-2006