Mammoth Cave National Park is situated in an area of west-central Kentucky famous for its textbook karst topography. This landscape is characterized by sinkholes, springs, rivers that disappear below ground, and extensive caves. The formation of these features requires a little explanation.
Just below the surface here there are massive beds of limestone, deposited around 350 million years ago when this area was a shallow sea. Limestone is made up of the remains of shelled marine organisms, predominantly composed of the mineral calcite.
Over the last few million years this limestone has been eroded by water. Rain water picks up carbon dioxide from the atmosphere and soil as it falls, forming weak carbonic acid. Over a great deal of time, this acid dissolves limestone. The combination of this chemical weathering along with with the physical weathering of moving water has carved the massive caves and sinkholes in this area.
This park encompasses the largest cave system on earth with nearly 400 mapped miles. Geologists estimate there could be about 600 more miles yet to be explored, and there are over 200 other caves disconnected from the main system. While the caves are the real draw here, the surface features are no less important to understanding the geology and ecology of this area.
I had reserved a spot on the 8:45 Historic Cave Tour but wanted to get there at dawn to hike some surface trails. My first stop was at Sloans Crossing Pond:
This pond is somewhat unusual in this karst region. Where limestone sits near the surface, water wears it down. Here, however, there are layers of impermeable sandstone and shale cap rocks above the limestone. In some places depressions in the sandstone collect water, creating these havens for aquatic organisms.
At the early hour of 7am I was spared the chatter of people and was deafened only by the sounds of numerous frogs and songbirds. More on that in a future post.
My second stop was at the Cedar Sink Trail, also delightfully devoid of people. In the nearly two-mile hike I was free of any sight of civilization, save for the modestly-groomed trail and intermittent stairs.
After descending a couple of hundred feet, I reached Cedar Sink. Here the Hawkins-Logsdon river system emerges briefly from the limestone caverns below on its way to the Green River.
First it flows out from beneath the sandstone cap rock:
Then it covers a short distance above ground:
It disappears again:
And finally pops up again briefly before disappearing for good until it emerges at the Green River:
This short foray into daylight introduces organic matter into the river, which in turn helps feed the subterranean ecosystem farther downstream. In the complete darkness of the underground stretches, this river hosts several blind fish, crayfish, and shrimp, among other organisms.
In this area the sandstone cap rock exhibits variable resistance to weathering, leading to the formation of recess caves like those found at Hocking Hills State Park in Ohio:
After exploring some of the surface I entered Mammoth Cave itself. The heavy rain from the previous few days kept crowds to a minimum, and my group only contained about 20 people. During peak season in the summer, groups can have up to around 140.
The Historical Tour begins at the Historical Entrance, as do several other tours:
This is only one of many entrances, and some of the other tours enter through other points. Although there are nearly 400 known miles in this cave system, the tours altogether only cover about 12 of them. I can only imagine the remaining miles are too tight, too dangerous, or too sensitive to heavy traffic to be open to the public.
We soon entered a large chamber known as the Rotunda. The feeble electric lights and my camera couldn’t begin to capture the grandeur of this location:
A staircase led down to The Church, another chamber where a local Methodist church sometimes held service in the heat of summer during the 1800s. The caves are normally around a cool and constant 54F. In one spot the wall was blackened by the candles used during the services.
Saltpeter mining was economically important here during the War of 1812. With the British navy blockading nascent American shipping, our nation had to turn to domestic sources for war materiel like the saltpeter used in gunpowder. Here slaves were exploited to extract thousands of pounds of the mineral with crude tools and their bare hands for the war effort.
Slaves were also used to give tours later in the nineteenth century. Stephen Bishop was probably the most famous slave guide. He discovered more mileage of the cave system than any other person, and was the first person to traverse the Bottomless Pit:
After Niagara Falls, Mammoth Cave was the second-best known tourist destination in early America. Nineteenth-century visitors often left their names and dates on the cave walls, created by the soot from candles. This historical graffiti is treasured for its record of early Americana. Modern etched graffiti is not so treasured, earning any would-be vandals a hefty fine.
After descending about 310 feet, the final ascent back to the surface was through Mammoth Dome via the Ruins of Karnak:
These columnar structures extended upward toward the surface, creating an impressive sight. After snaking back towards the Rotunda in what amounted to a big loop, we exited back out of the Historical Entrance:
It’s interesting how little dripstone is present in this section of the cave. Since most of the area is capped by water-impermeable sandstone and shale, there’s little water available to drip through the cave and create stalactites, stalagmites, columns, curtains, and other formations. At the same time, the lack of water flow is what has helped make this the largest cave system on earth. The large caverns of much of the cave were formed by the underground rivers that once flowed through them, until they found lower elevations to run through. If water continued flowing through these caves, they would be destroyed by erosion and dripstone fill. The perfect storm of limestone capped by sandstone and shale is what has helped create this most impressive wonder of nature.
My modest photographs (or even professional photographs) really do this place no justice; you have to see it for yourself. I plan on visiting here again to see more of the caves. At least one section lacks the water-impermeable cap rock, allowing dripstone to develop. The Frozen Niagara formation near the New Entrance will be my first stop when I come back.