Seas have covered parts of what is now North America for many hundreds of millions of years. To trace the ebb and flow of these seas across the central United States, or Missouri, would require some fancy computer graphics. Keep in mind that to speak of North America or Missouri around, say, 500 million years ago (early Paleozoic Era; see Figure 1), is to speak of places that did not exist in the shape or at the latitude and longitude they do today. To say "Missouri" in the Paleozoic Era describes a particular spot on the North American plate between 600 and 230 million years ago (mya). Nevertheless, that particular spot will undergo many changes over those 600 million years and become what we now see in Missouri.
Within the Paleozoic Era--during, more specifically, the Cambrian, Ordovician, and Silurian periods (from about 600 to 400 mya)--seas spread to the North American interior. Missouri lay east of the "Transcontinental Arch" (see Figures 2 & 3), which was to be the foundation of the continent, and which was generally dry land on the western shore of these interior seas. At times, a proto-Appalachian range formed the eastern shore.
The current position of Missouri in the Mississippi River drainage is considered to be a southward extension of the Canadian Shield, but covered by sediments from these shallow inland seas. The seas came in about 520 mya, and more or less stayed around for a couple of hundred million years. It's hard to be precise: again keep in mind that sea levels, when considered over such a long stretch of geologic time, oscillated constantly. The St. Francois Mountains are remnants of a much older time of volcanism, around 2 billion years ago (bya) in the Precambrian Era. The seas left about 4000 ft. of sedimentary rock over what are now the Ozark Mountains. The Ozarks (or Ozark Dome) have been periodically uplifting over the last 300 million years.
What did this Midwest sea look like, and what lived in it? First, it moved and receded over time. Early on, it had an outlet to the Atlantic, and later to a southern sea. About 400 mya it stretched at various times from the Appalachians to the Transcontinental Arch (at this time it is too soon for the rise of the Rocky Mountains). Marine invertebrates and simple plants started appearing in great numbers, and left behind the fossilized record of their existence. The great age of trilobites, bottom feeders with light skeletons, was in the Cambrian Period ending about 500 mya. We don't see trilobites in eastern Ozark fossils, but we do see animals from later times:
1. foraminifera, a type of protozoa that forms shells of lime or sand glued together by a sticky secretion. 2. gastropods (see Figure 4), simple snails with calcium carbonate shells. At this early time they had not yet developed much variety. The fossils themselves are molds rather than the shell itself. 3. brachiopods (see Figure 5), which resemble clams. Early ones had no hinge, but later ones were articulated. Most were stationary and attached to the sea floor by a fleshy appendage. 4. bryozoans, (see Figure 6), small animals growing in colonies that strained food from the water. 5. cephalopods, tentacled, large-eyed marine creatures that could expel water for propulsion. The forerunners of modern squid and octopus. 6. crinoids (see Figures 7 & 8), echinoderms with parts resembling a root, stem, and flower-like crown. The stems allowed these animals to collect food above the sea floor. Usually the fossils we find are parts of the disc-like stem. 7. coral, both solitary and colonial.
Some of these animals are fossilized in the large rock in the visitor center museum. Their shells are made either of calcium carbonate (calcite) or phosphatic material. Possibly sea creatures increasingly used calcium as the seas warmed, since calcite is deposited and secreted better in warm water. These creatures are considered primitive because they were energy-inefficient and not highly mobile (if they were able to move at all). They were also vulnerable, sensitive to waves, currents, sediment, and temperature changes.
The plants at this time were abundant, but left little fossil record. We know there was blue-green algae; in fact, it left several types of evidence in Fisher Cave, mainly in the form of stromatolites, the mineralized remains of the cylindrical algae.
In later periods--the Devonian, Mississippian, Pennsylvanian, and Permian--plant and animal composition in the seas changed, becoming more advanced. Eventually animals moved more efficiently, and increasingly preyed on other animals. It is in these later periods that ancestral fish and true forests appeared. These are not in the fossil record around Meramec State Park, however.
From the dead remains of these calcite-shelled animals, from calcite that precipitated out of the marine water, and from limy mud and other sediments, time and pressure have produced limestone. Now much of the Ozarks limestones are dolomite (more on that transformation later). So we have dolomite, with its fossil record of gastropods, monoplacophorans, cephalopods, trilobites, and other shelled animals. But we are still a long way from having a cave.
"Formation" is the proper word for a geologic layer and should not be used to refer to cave speleothems. In Meramec State Park four formations are present, but remember the Eminence and Gasconade formations (see Figure 9). Here is what Missouri Geology says of the Eminence Formation:
The topmost part of the Cambrian rocks is a sandy dolomite that reaches a thickness of 350 feet and is called the Eminence Formation. It is abundantly supplied with chert, which may suggest a clearing and warming of the sea. The existence of sand suggests the possible uprising of an area to the west or north, which exposed older, clean sandstones. Many of the boulderlike masses of chert seem to have been formed as algal masses. The Eminence Formation is the host rock for many of the caves and springs that add so much to Missouri's popularity as a place for camping and other recreation.
Let's recap how limestone and dolomite form. The limestone-producing sediments contained the carbonate shells of creatures and carbonate precipitates from seawater. In deeper parts of the sea, there also tended to be calcium, magnesium, and dissolved silicates. In shallower areas and shorelines, we find more silt, mud, and sand.
Most dolomite is recrystallized limestone. The marine saltwater had magnesium in it from the weathering of substrate rocks. The magnesium entered at microscopic levels and warped the limestone lattice into rhombohedrons, reshaping it and making it stronger. In another process leading to the type of dolomite seen in the park today, chert (which is addressed in more detail below) chemically replaced part of the carbonate host, or it may have been an original precipitate.
Stromatolites were one of the first life forms. A blue-green algae, they appeared as curtains in the sea, and were browsed by mollusks. Once the laying down of sediment beds began, their cylindrical voids were invaded by chert in the form of SiO2 (quartz) and CaSiO2.
The Eminence Formation is dolomite with lots of chert. The chert often appears on the surface and as stream gravel because it is so hard that it is not eroded easily. Above the Eminence is the Gasconade, another cherty dolomite. Between the two formations (technically at the bottom of the Gasconade) is a layer of sandstone called the Gunter. The Gunter sandstone is a result of a seacoast beach that accumulated during a time when the sea became shallower and exposed areas that had earlier been underwater.
The Gunter, then, has dolomite above (the Gasconade) and dolomite below (the Eminence). Another sandstone present in the park, the Roubidoux (younger and thus higher in the geologic column), also formed as lowering sea levels built up new coastal beaches. To simplify some facts:
|cherty dolomite||sandy, cherty dolomite||cherty limestone||sandy dolomite|
|Cambrian, 550-495 mya||late Cambrian||early Ordovician, 495-437 mya||Ordovician|
|70-300 ft. thick||150-350 ft. thick||250-300 ft. thick||125-200 ft. thick|
|has abundance of quartz-filled vugs (cavities) in red clay||top of formation occurs at Fisher Cave ballroom ceiling||calcareous Gunter sandstone from time when seas returned||some sand and gravel, sandstones probably from Wisconsin Dome|
Dolomite, as we learned, formed when magnesium penetrated the limestone. Metal ores such as copper, zinc, and lead likewise appeared in the limestone, carried in by aqueous solution. The lead, copper, zinc, and so on originally got to the earth's surface via magma. Another possible mineral source was sea life, such as primitive bacteria and algae, which utilized iron and copper in their cells. Iron ore, on the other hand, derives from volcanic activity in the Ozark Mountains. Magnetite and other iron-bearing minerals weathered and oxidized to form hematite and limonite, the two common iron ores we find in Meramec State Park.
Besides the large Hamilton Ironworks in Hamilton Hollow, miners sought copper and lead in Meramec State Park as well. Large and small pits are scattered throughout the park, notably around Chinkapin Ridge and Cactus Ridge glades.
In this county, Franklin, as well as in surrounding counties, rocks from the Silurian Period (which followed the Ordivician) are missing because an uplift left the area above sea level and thus erodable.
Chert still poses something of a geologic mystery. We know it appears in nodules (round lumps), lenses (a lens shape), and layered beds, and that silica-rich water carried it into the limestone. The presence of chert suggests a warming of the sea, since minerals are more soluble in warm water. (The silica came before the magnesium, and hence before the transformation to dolomite.) Silica originated from the weathering of granite, rhyolite, and other rocks. But why does chert form where it does, and by what mechanism?
Shawn Williams, a geology major and former naturalist here, constructed a theory. To begin with, the dolomite at Meramec State Park is classified as wackestone, a carbonate rock that contains lime mud as the matrix, with a significant amount of carbonate grains. The magnesium changed the crystal structure of the existing limestone by alternating magnesium and calcium layers. This new structure is a twisted rhombohedron, characteristic of dolomite.
But before the magnesium came the silica. In Williams's theory, the chert formed at a contact zone where the saltwater-saturated rock under the sea met the freshwater-saturated rock on land. The silica was carried by a solution of marine water and deposited in the "mixing zone." Here the silica-rich marine phreatic water, slightly basic, met the phreatic freshwater, which was slightly acidic (acidic because water plus carbon dioxide equals carbonic acid). In the neutralization process, the silica was deposited as nodules. Thin layers of chert also occur, possibly from the deposition of silica-shelled organisms such as diatoms. Also, stromatolites underwent silica replacement, a form of mineralization.
For other points of geologic reference (we need all the help we can get!), dinosaurs appeared more recently, about 245 mya in the Triassic Period. Soon after came the first mammals. The super-continent we call Pangea appeared about 200 mya. It was made up of a pair of semi-super-continents, the northern mass of Laurasia (the North American plate and Eurasia), and the southern mass of Gondwana (South America, Africa, Antarctica, Australia, and India). Flowering plants made an appearance 160 mya in the Cretaceous Period.
Jumping forward, the Pleistocene Era (the "Ice Age") of the last two million years consists of four glacial periods, with "interglacials" in between. In fact, our current era, the Holocene (the last 10,000 years) may be an interglacial. The Missouri River is the rough boundary of glacial advance in Missouri.
Missouri is a karst landscape par excellence. Karst is a German word that comes from Krs, a limestone area in Slovenia and northeastern Italy where this kind of terrain was studied for the first time. Karst is present where rock is more soluble. Under that precondition, voids become enlarged, and surface water can descend to underground passages quickly (see Figure 10). Two carbonate rocks, limestone and dolomite, are typical foundations for karst, and several soluble evaporites--halite, gypsum, and anhydrite--can also produce karst.
Caves, springs, and sinkholes characterize Missouri's karst areas. Sinkhole karst, in fact, is the karst type in our region. If a cave roof collapses, we may see a sinkhole on the surface, or more dramatically, a natural bridge. Natural bridges are the uncollapsed remains of a cave passage that collapsed to each side; one good place to see one is at Rock Bridge Memorial State Park near Columbia (see Figure 11). Karst is an important environmental issue in the state because it is a landscape highly sensitive to urban development. Septic systems are vulnerable in karst, and surface dumps and water pollution are constant threats to groundwater and cave systems. Since there is minimal water filtering in karst, pollutants can move downward quickly. A statement from the Speleology Workshop of the Missouri Department of Natural Resources shows how sensitive a karst landscape is:
While our caves, springs, and sinkholes are certainly amongst the most beautiful and most interesting features of our state, they are also responsible for some of our most serious problems. Immediately obvious are the hazards of constructing building in karst terranes; cavern collapse may result in weakening or even failure and destruction of poorly located structures. Potential failure can be minimized by carefully selecting each building site (using exploratory testhole drilling) and by designing structures that can withstand a certain amount of slumping or collapse. Both procedures are costly and result in a more expensive structure. Perhaps the most significant problem we have to face in karst areas today, is that of maintaining the quality of our groundwater systems. The dolomite and limestone terranes of Missouri are like giant sponges . . . they readily accept water. Likewise, they will readily accept sewage, chemical wastes, bacteria, and other pollutants. Great care must be exercised to properly locate and regulate sewage lagoons, sewage treatment plants, solid waste disposal facilities, landfills, and other possible sources of contaminants. I hasten to point out that this problem is not a hypothetical one in Missouri . . . it is a real problem that exists right now. Urban refuse has been used as landfill material to "reclaim" sinkhole plains-with resultant pollution of the local groundwater system; giant sprawling suburban communities utilizing individual septic tanks for waste treatment have been built in upland karst areas-with disastrous results; and poorly located sewage lagoons have collapsed into cave systems which readily swallowed up the waste products of entire towns in a few days-millions of gallons of raw sewage which polluted the local groundwater system and resulted in illness in hundreds of people depending on the system for their water supply. Fortunately, laws passed in recent years (administered by the Missouri Department of Natural Resources) require permits for construction and operation of the various public waste disposal systems. Sites located in karst areas are rejected as unacceptable. This is certainly a step in the right direction, but we must always be mindful of the potential pollution problems associated with karst areas, and exercise whatever action may be required to avoid contamination of this great natural resource of Missouri-her groundwater system.
If you have spent time in the Ozark Mountains, you know that the hills are deeply dissected by streams and rivers (locals may call these steep valleys "hollows"). How did this come to be? Ozark rivers follow "entrenched meanders." This is a result of the rivers initially meandering over a low, flat, featureless peneplain. In any flat landscape, rivers tend to meander. The Ozark uplifts came later: a major uplift took place about 80 mya, with the last uplift beginning about 25 mya. There were perhaps three uplifts in all (possibly four--the number and timing of uplifts is still under debate). With each uplift, the abundant streams cut and dissected this Ozark Dome (domed because the middle of the uplifted landmass was higher).
Even today, remnants of the flat peneplain can be seen in the Salem (also called Rolla) and Springfield plateaus (see Figure 12), which lie at the same elevation. In general, the Ozarks continuously move through four phases: submergence (by seas), deposition (of sediments), uplift (of the Ozark dome), and erosion (by streams). Erosion is the current phase. The Ozarks are not much, as mountains go. Relief is often 300-400 ft. If the Ozarks are a dartboard, the bullseye is in the St. Francois Mountains (where relief is as much as 800 ft). The St. Francois Mountains have a very different origin.
The nearby St. Francois Mountains are important in Missouri geology and have greatly influenced what we now see in Meramec State Park. Here are Precambrian landforms (see Figure 13), the oldest in the state, and in fact the oldest exposed rock in the midcontinent. Most of the rock was originally magma, which cooled underground and has now been exposed through erosion. The rest came from volcanic eruptions, in the form of ash deposits and lava flow. These eruptions began between 2.0 and 1.5 bya.
In the St. Francois Mountains we see mainly granite, rhyolite, and felsite. Granite is an intrusive (or plutonic) igneous rock with a large crystal matrix. Rhyolite (glassy, hard, and brittle, with a color range from gray to pink to purple) and felsite (light-colored, glassy to fine-textured) are both extrusive igneous rocks that cooled and solidified quickly, with a small crystal matrix. (Two other well-known extrusive rocks are basalt and obsidian.)
State parks in the area showcasing this ancient volcanic activity include Elephant Rocks, with exposed granite boulders, and Johnson's Shut-Ins. There, very hard rhyolite forms a narrow river gorge because it is highly unerodable. A shut-in is an Ozarkian word for a gorge. Taum Sauk, Missouri's highest point at 1772 ft., is a knob of igneous rock. One reason the St. Francois Mountains are important to Meramec State Park is that the iron, lead, and copper so prevalent in the park--and so persistently mined--arrived here in solution during an uplift of the St. Francois area. The mountains at times may have been as high as 10,000 ft.
The New Madrid fault in the Mississippi River valley also has some bearing on Fisher Cave and Meramec State Park. Most visitors from Missouri will know a little about the fault, and naturalists should be prepared to answer questions about it, especially because almost everyone wonders what would happen to Fisher Cave in an earthquake. If there is to be an earthquake here, most likely New Madrid will be the source. Fisher Cave has survived the massive quakes of 1811 and 1812: witness the giant stalactiflats in the cave that were not disturbed by that event.
The first shock of the largest historical earthquake happened on December 16, 1811. Based on news reports and personal journals, the magnitude is estimated to be at least 8.0. The quake was felt up and down the eastern seaboard, with street damage in Washington, D.C. The other two large quake events took place on January 23, 1812, and February 7, 1812, but there were hundreds and perhaps thousands of smaller shocks in about a year's time. One researcher estimates these three largest quakes had magnitudes of 8.6, 8.4, and 8.7--in other words, some of the most powerful in recorded history.
Damage was severe, both to existing towns and to the landscape. Islands in the Mississippi River sank, and two large rapids were created near the town of New Madrid. On land, sand and lignite (intermediate between peat and coal) were blown from underground; sand patches can still be seen in the Bootheel. Hundreds of thousands of acres of forest were destroyed. Few people died, however, since the Bootheel population was only about 3000, and most were scattered on farms. And by the time of the second and third earthquakes, many had left the area seeking safer ground
The New Madrid fault may actually be several or more faults in the same area. The faultlines are not at plate boundaries as is usually the case, and in fact are practically in the middle of the North American plate. There are probably about 150-200 earthquakes per year at the New Madrid fault complex with magnitudes greater than 1.0 (see Figure 14). Very few are felt, however (generally, quakes under 3.5 are not felt). According to Missouri Geology:
Johnston (1982) has postulated the presence of three major, deeply buried faults in the region. One strikes southeast from near New Madrid into northwestern Tennessee. Another, longer one extends about one hundred kilometers southwest out of Missouri into northeastern Arkansas. A third, which is less well developed, extends northward from the first one to the vicinity of Cairo, Illinois. Johnstons's discussion of the geology of the New Madrid area raises the question of why these earthquakes occur in the Midcontinent region instead of near the edges of colliding plates. The answer seems to be that this area was, in Mesozoic time, covered by a northward extension of the Gulf of Mexico in which thick layers of fine-grained sediments accumulated and that they are still not well consolidated.
If you've been in Missouri for long, you know that this is the Cave State. Missouri's largest cave is Crevice Cave in Perry County, with--as of 1986--28.2 miles of passage. Cathedral Cave in Onondaga Cave State Park ranked 10th at 2.99 miles. As for show caves, Meramec Caverns was 3rd at 2.06, Onondaga 4th at 1.72, and Fisher Cave 9th at 1.18. Fisher Cave is actually longer than this 1986 figure, with around 2 miles of passage (other caves may be longer as well).
The longest cave in the world, by a good margin, is Mammoth in Kentucky at 563 km (or 350 miles). The deepest cave is Jean Bernard in France, 1602 m (or 5255 ft.). Fisher Cave is not so long or deep, but it's like these two in a basic way. Most caves, and all really big caves, form by water dissolving limestone (or in our case, a variant of limestone called dolomite). Missouri, Kentucky, and France are all covered with deposits of limestone. These places also all have abundant surface water. First there's limestone, then water acting on it.
Fisher Cave and other caves in the area were created from soluble dolomite. Where did that rock come from? The short answer (we learned the long answer above) is the sediment of seas containing calcite-shelled creatures.
Caves are formed in a surprising number of ways. Besides "solutional" caves, caves may be created by the action of lava, waves, and other forces. Let's briefly look at these types.
Lava tubes are formed when flowing lava solidifies on the surface while it continues to flow underneath. Eventually this interior liquid lava drains out, resulting in a hollow tube, usually accessible from a collapsed ceiling section.
Sea caves form at the base of sea cliffs. For various reasons, parts of the rock may be more easily eroded, and strong wave action attacks this weakness and carves out a cave at the cliff base.
Shallow caves are also created in sandstone by erosion and the freeze-thaw cycle. These are more cliff shelters than caves, however. The American Southwest has many spectacular examples that were inhabited by Native Americans. Caves of a sort may also form in talus, glaciers, or as a result of earthquakes.
Our Missouri solutional caves may be further categorized. The Terrestrial Natural Communities of Missouri describes five kinds of caves: effluent cave (stream flowing out), influent cave (water flow into cave via swallow hole), wet pit cave (contains permanent water), dry pit cave (sinkhole shaft or open-joint fissure), and dry cave (generally, elevated cave without significant water).
In Missouri, we have a karst system with limestone or dolomite caves (see Figure 15). When there is pressure or stress upon limestone, it can fracture in three ways:
1. partings, which are parallel to the bedding 2. joints, which cut across the bedding 3. faults, which are joints that have been displaced
Joints (see Figure 16) formed during the drying and compaction of sediments and during Ozark uplifts (250-300 mya). Partings and joints may occur as often as every meter in a limestone bed. Now we add water to the equation. Water moves from the surface downward through cracks to the watertable. It collects in these partings and joints and moves horizontally through the limestone. Inevitably some of the partings and joints become larger than others. As they do, even more water flows through these channels. They continue to enlarge, take on more water, enlarge further, and so on. Water flow can become turbulent in a channel only 5 mm in diameter, and this turbulence helps dissolve the limestone. An enlarged channel (whether parting or joint) is a potential future cave passage. At this early stage, though, the channel may end at a narrow exit in a stream valley. To our eyes, this spring may be the first sign of the cave formation underway below. Another way to look at it: totally saturated passages leading to springs are young, developing cave systems.
Cave passages seem to form just below the line of the watertable, regardless of which way the limestone beds may slope (see Figures 17 & 18). Two factors may explain this preference for the horizontal. There may be more carbon dioxide in this zone, and water is in contact with the limestone long enough to make a carbonate solution.
Several things can happen next. If the watertable is lowered, the passage will drain, though it may continue forming at the lower level where the watertable has moved. Or, if the passage develops an outlet because of erosion at the surface (usually in a stream valley), it will be opened to the outside and stop forming. In a larger sense, this is the dissection of the Ozark peneplain: surface stream erosion cutting downward into the ancient peneplain.
The game changes now. With outside air ventilating it, there is no concentration of carbon dioxide in the passage. The stage of calcite deposition begins. Water picks up carbon dioxide from the atmosphere and surface organic matter, becoming acidic in the process (carbonic acid, in fact: H2O + CO2 = H2CO3). Acidity helps dissolve calcium carbonate from the limestone or dolomite, and the water carries these disassociated mineral elements (Ca, HCO2, and possibly other minerals such as iron and manganese) into the air passage of the new cave, where it precipitates out (or reassociates) upon the release of the carbon dioxide.
As long as water continues to seep into the passage from above, the familiar speleothems--stalactites, stalagmites, flowstone, and so on--start to grow. This stage, like the first stage of channel enlargement, lasts an indefinite period of time. In geologic terms, however, the lifetime of a channel-passage-cave is short. After several million years, the roof is liable to partially collapse, forming a natural bridge or tunnel. In time, a complete collapse of the cave roof may leave nothing but a large open fossa (or trench) on the landscape. See Grand Gulf, Ha Ha Tonka, or Rock Bridge Memorial state parks for examples of collapsed caves.
In the speleothem-building stage, other things can happen. The cave is now highly influenced by what is called the "perched" groundwater. Water from rain or slow melt is able to work its way down from above the cave into the system. Generally, if the discharge is discrete, it can create a vadose stream (see again Figure 17) throughout the cave. Alternatively, a stream may enter the cave from the surface. In either case, the stream, as with any surface stream, laps at its edges, and if conditions are right, the retreating stream can leave a record on the cave floor of ripple marks. We can see these marks on the south side of Fisher Cave's main passage (though not on the other side). Also, depending on events at the surface, a domepit may develop below a sinkhole--this did not happen in Fisher Cave, however.
Attempts have been made to mine Missouri caves for gold, silver, copper, lead, saltpeter, and the calcite deposits themselves. The more translucent calcite is sometimes called cave onyx (true onyx is banded SiO2, or quartz). The mining of cave onyx was promoted heavily in the 1890s and first decade of the 1900s. Great demand for marble as decoration drove this market, even though cave onyx is not true marble (which is metamorphic). Marble, for building purposes, is any calcareous rock that can be worked and polished. Bedrock limestone or dolomite can fit the bill, but onyx (especially Mexican onyx, which is travertine, or surface deposits) is also prized. In the late 1880s, experts found three good cave onyx sources in Missouri, two in Pulaski County and one in Crawford County.
Onondaga Cave was also targeted, but nothing came of it. Removal costs were prohibitive, and the onyx was not of high quality. Onyx from Onyx Mountain Caverns in Pulaski County was mined, beginning in 1892, but found to be commercially unusable. Fissures along the cleavage plane caused the onyx to break apart during cutting and shaping. Blasting during removal probably contributed to the weakness, and the onyx was also flawed due to impurities and interruptions in its depositional history, which resulted in voids or clays. Fortunately for cave-lovers, this assessment proved true for almost all Missouri cave onyx, and mines in Arizona and California easily outcompeted Missouri ventures after 1915.
The entrance of Fisher Cave is in the Meramec River floodplain at about 585 feet above sea level. The entrance passage is in Eminence dolomite. Unlike many caves in karst regions, no surface features coincide with the cave below (see Figures 19 & 20). The depth under the surface ranges from about 60 to 140 ft. (excepting the entrance, of course). The rimstone dam is 80 ft. underground, the bear claw marks 68 ft., and the end of the Hugh Dill Room is 154 ft. underground.
Fisher Cave was once part of a larger network of passages. This statement should not be surprising, since it is the nature of cave passages to become separated by collapse or dissection from erosion. Certainly Indian Cave was the former main entrance of Fisher Cave. The cave stream in the main chamber (containing the ballroom, breakdown room, calcite forest, and vandalism room) once continued in a line through the blocked passage behind the large column in the vandalism room, and out at the Indian Cave entrance. After that blockage occurred, however, the cave stream was forced over time to turn back and head downward through the most erodable dolomite it could find, and out the current entrance. The new entrance passage is probably on the order of several hundred thousand years old.
Fisher Cave may also have been connected to Bear Cave, and it's even possible that still today a small lead from the Hugh Dill Room goes toward Bear Cave. This passage remains unexplored. Lone Hill Onyx Cave, directly across the river, may have been joined with Fisher before the Meramec River cut down to its present level. Meramec Caverns may also have been part of a larger system that included Fisher Cave. (Though it is a 10-plus mile drive to Meramec Caverns, that cave is not far from the park boundary, and just downstream from Cane Bottom.)
In fact, the area is so riddled with caves that many large passages may lie undiscovered. Until geologists can perform the equivalent of an X-ray on the park landscape, we probably won't know what we're missing.
Some more facts and figures: the stabilized temperature is between 56 and 57 degrees. The stabilization point changes seasonally. In the summer, it's about 400 ft. from the entrance. In winter, it's about 1350 ft. inside the cave. Relative humidity is around 97 or 98 percent, though in rare situations it may push 100 percent when spring rains cause the rivers and creeks outside to flood. In that case, the oversaturated cave air produces fog.
The clay on the cave floor is currently in removal stage by the cave stream. Clay from the Grand Canyon section appears to be an older, red clay. The Fisher Cave clay was first described by the eminent geologist J. Harlen Bretz, but geologist Tom Aley termed the clay "silty clay loam sized particles" typical of flood deposits. A recent observer, Lang Brod, contends that it is actually tan silt, perhaps a remnant of backflooding.
Speleothems in Fisher Cave have not been precisely dated, and so the "age" of Fisher Cave (and of its speleothems) can only be roughly estimated. The void of Fisher Cave (first water-filled, then air-filled) dates from between two and 80 million years ago--that is the most precise we can be--while some speleothems (the giant columns in the ballroom, for example) are half a million year old at the most. Strontium, thorium and uranium isotopes can be used to date deposits. Whether a deposit can be dated with radiocarbon techniques (i.e., using the decay of Carbon-14 as a marker), is open to question.
In the main chamber of Fisher Cave, you occasionally see narrow bands of green clay along walls. This is glauconite clay, and in not so simple terms it is a potassium iron magnesium aluminum silicate. The chemical formula is even more daunting: K15(Fe,Mg,Al)4(Si,Al)3 O20(OH)4. The parentheses in the formula mean that some of the elements may or may not be present in a particular glauconite clay. In Fisher Cave, all elements are present.
The basic makeup of clay is aluminum silicate. The other elements in glauconite come from weathering of igneous rock or bedrock, and were present (in differing amounts over time) in the early seas that spread across the midcontinent. The Fisher Cave glauconite is found mainly in the Gasconade formation, which is in the higher elevations of the cave. Traces of glauconite are also in the Eminence formation. Many caves in the Meramec basin have glauconite. The glauconite settled by slow sedimentation in a marine environment. The glauconite can be dated because the potassium (K40) is radioactive.
Saltpeter is found in the silt of cave floors. Especially during the War of 1812 and the Civil War, it was mined for gunpowder. Doug Plemons has documented 684 caves in the U.S. where saltpeter was mined, with 23 in Missouri. The major saltpeter-mining caves were probably all gray bat colonies. Water and potash added to the "petre dirt" and boiled resulted in niter (KNO3), which was used for gunpowder. A negligible amount of saltpeter was mined from Fisher Cave, but nearby Meramec Caverns was known as Saltpetre Cave until Lester Dill renamed it for the tourist trade.
The main nitrate source is nitrogenous bat guano, but saltpeter is found in caves that seem to have no guano. Another saltpeter source seems to be cave rat urine, broken down by the bacterium Nitrobacter agilis.
Another geologic curiosity in Fisher Cave is the reflective bacteria, which is also called chemoautolithotrophic bacteria. An excellent example is the powdery silver band that runs low along the wall in the entrance passageway. This band shows up well under a black light. This bacteria is not well understood. The "litho" in the name tells us that it works chemically ("chemo") on rock. In other words, the bacteria are not growing on anything organic. In some areas of Fisher Cave these bacteria may appear gold. They repel water, which forms a sphere on the bacteria and enhances the reflection under a light.
But in Fisher Cave, not all that glitters is bacteria. A careful inspection will show that other sparkling patches (on either dolomite or chert) are plain water droplets.