_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.
[Illustration: Marble outcrop]
Thus mixed deposits of volcanic and ocean sediments continued to collect
for several million years. Apparently this steady transfer of material
from one part of the earth’s crust to another created a crustal
imbalance. The edge of the continent was under a strain. Then, like an
accordion, a tremendous folding of the lands along the Pacific Coast
occurred.
The floor of the sea was lifted above the ocean’s surface to form a new
coast line in this vicinity. Violent stresses in the earth’s crust
created intense heat and pressure which changed, or metamorphosed, the
rocks. Shales were altered to slate. Sandstone became quartzite.
Limestone became the marble so important to Oregon Caves. Even the
volcanic materials were altered considerably from their original form.
The resultant geological belt composed of inter-bedded layers of slate,
quartzite, marble, and metamorphosed volcanics is known as the Applegate
Group.
After the uplift, there was a long period of crustal stability. The
folded mountains were eroded away and the area became a flattened plain
near sea level. As a result, its streams were sluggish and meandered
slowly to the ocean. Then, in various stages, the plain was uplifted in
another period of crustal adjustments which produced a flat-topped
plateau, so to speak, known as the ancient Klamath Peneplane. This
restored the vigor of the stream erosion, which helped at times by
glacier sculpture, dissected the plateau into the mountains we know
today. The Siskiyou Range surrounding Oregon Caves National Monument is
part of the Klamath Mountain System.
Let us focus on one of the ancient marble layers of the Applegate Group,
for this is the rock strata in which Oregon Caves were formed. It is
actually a narrow, tilted belt, varying in thickness up to 400 feet. It
dips eastward into the earth at an angle of about 60° and can be
followed in a southwest-northeast direction for about 4 miles along the
west shoulder of Mt. Elijah (see illustrations on page 4). Examination
of the marble layers inside the exit tunnel or the outcrop at the
beginning of Cliff Nature Trail reveals many fractures caused by the
stresses of upheaval. Some are vertical cracks, but there are also many
cross fractures at varying angles.
[Illustration: Oregon Caves marble—8 inches wide]
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_.
[Illustration: _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.
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.
[Illustration: Solution Rills in Ghost Room ceiling]
[Illustration: Crack widened by solution—Watson’s Grotto]
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_ (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.
[Illustration: ]
_SOLUTION_
RAIN + CARBON DIOXIDE = MILD CARBONIC ACID
MOLECULES
(H₂O) (CO₂) (H₂CO₃)
CARBONIC ACID + CALCIUM = CALCIUM BICARBONATE SOLUTION
CARBONATE
MOLECULES FROM
MARBLE STRATA
(H₂CO₃) (CaCO₃) (CaH₂(CO₃)₂)
_DEPOSITION_
CALCIUM CARBON DIOXIDE, FOR EACH CARBON DIOXIDE MOLECULE
BICARBONATE CALCIUM CARBONATE ESCAPING INTO CAVE AIR, AN
REACHING CAVE EQUIVALENT MOLECULE OF CALCIUM
AIR CARBONATE MUST BE REDEPOSITED AS
SOLID DRIPSTONE, FLOWSTONE, ETC.
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 surfaces—sideward, downward, and upward. (The
solution rills in the original Ghost Room ceiling reveal the upward
dissolving of water-filled cavities, see illustration page 9). 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_, (CaH₂(CO₃)₂). The removal of solid calcium carbonate in a
liquid is the key cave forming process and is called _solution_ (see
illustration page 10). In Watson’s Grotto we find several examples of
early crack-widening by phreatic solution (see illustration page 9).
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 strata 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.
[Illustration: Smooth-walled erosion of River Styx]
_Decoration_
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 evaporated
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 10).
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.
[Illustration: Cave deposits in Joaquin Miller’s Chapel]
[Illustration: Drapery in Ghost Room]
Cave deposits are collectively termed _speleothems_. Their variety is
infinite: Those left by dripping water are called _dripstone_, and take
on two basic forms—if 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.
[Illustration: Formations on page 16]
Contrasted with dripstone is _flowstone_—smooth 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 19). 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 17). 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.
[Illustration: Calcite crystals in layer of flowstone, 8 inches
high.]
[Illustration: ]
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.
[Illustration: Helictite formed on soda straw]
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.
[Illustration: Popcorn in Adam’s Tomb]
[Illustration: Rimstone]
At the “Devil’s Washboard,” and at the foot of Paradise Lost, we find
still another type of speleothem—_rimstone_—which 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 below). 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.
[Illustration: Blade formation]
So far we have discussed cave features—_speleothems_—created 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.
[Illustration: Detail in Paradise Lost]
[Illustration: Speleogen above River Styx]
_The Cave’s Age_
The many variables make it difficult to accurately estimate the age of
cave deposits. We have no way of determining past conditions which have
influenced the rate of development. Oh, we know the formations are many
thousands of years old, beyond that we can only guess. Some crude
examples of known age are the tiny, one-half-inch stalactites formed in
the exit tunnel since it was completed some 38 years ago (see
illustration below). Bathed in warm, dry air from the outside, they have
probably developed much faster than those inside the main cave. Other
active formations deep in the cave show very little visible depositing
over names written on them by early explorers in the 1880’s. So the true
secret of the age of the formations must rest with the cave itself.
Perhaps this is best.
[Illustration: Stalactites formed since exit tunnel was completed in
1933]
_Other Cave Features_
An obvious feature you may see in Neptune’s Grotto is the brown lacework
on the walls. They are lines of clay. Molecular attraction causes the
clay particles to cling together into what we call _clay worms_. (See
illustrations on page 27). Where does the clay come from? Some of it may
be washed in from the ground above. The rest of it is the remnant of
marble solution. Oregon Caves marble is 93 percent calcium carbonate.
The remaining 7 percent is non-soluble clay and remains in the cave
after the calcium carbonate is carried away in fluid state. Clay worms
are temporary features; vadose water or the touch of a careless hand can
easily remove them.
In the Ghost Room we find an interesting object. Projecting from the
ceiling is an 8-inch thick slab of angular brown rock. Its edges have
been broken, rather than dissolved. This is a _clastic dike_. Evidence
is lacking to definitely state its origin or manner of emplacement. But
at some time in the past, there was an extensive crack in the marble
which was filled by a mudlike material made up of bits of quartz,
plagioclase, horneblende, epidote, clay, and other minor ingredients. It
may have been washed in from the surface, or it could have been injected
from below by earthquake shocks which cracked the marble and forced the
pliable material into the cracks.
Eventually it hardened into rock. Due to its non-soluble ingredients,
the dike was not dissolved when the Ghost Room was formed. Like the
“blades” we discussed previously, it remained as a projection into the
room while the marble walls receded under solution activity. Being
brittle, it has apparently been broken off periodically by the jar of
earthquakes or cave collapse.
Another obvious discrepancy in the marble framework of the caves is the
thin layer of slate found in the 65-foot tunnel. This reveals an
interruption in the limestone sedimentation of the Triassic sea. A thin
layer of shale was deposited between limestone layers. Later, when the
limestone became marble, the shale became slate. It should be mentioned
here that limestone and shale vary greatly in their contents and often
interblend with each other. “Pure” limestone is white. Different shades
occur with different amounts of claylike impurities. When the impurities
overshadow the limestone, then the rock may be called shale. The whitest
marble in Oregon Caves came from the purest limestone. The darker,
blue-banded marble is rich in slate impurities.
[Illustration: The lacework effect of clay “worms”]