Clarksville Cave: a geology course free for the taking

Most New York State caves are closed to visitors from Oct. 1 to May 1. For information about Clarksville Cave and other area caves, visit www.northeatserncaveconservancy.org. I thank David Wallingford and his son Owen Tobias-Wallingford for their assistance in photographing the Clarksville Cave.
 

For generations, the Clarksville Cave system has drawn sport cavers and scientists alike.

Lying under a preserve owned and managed by the Northeastern Cave Conservancy, the cavern is half-an-hour drive from downtown Albany, just off Route 443. Histories of the village record visits to the cave in the mid-1800s, and graffiti from the Civil War Era and before are carved in sometimes elegant characters on its walls. (The precise moment at which graffiti go from being vandalism to history has never been determined, but carving on cave walls today is — to put it mildly — strongly discouraged!) 

There are forms of vandalism other than carving, of course — spray-painting and littering of the passages with trash are sadly not unknown; some forms such as muddying formations by climbing on them may be unintentional but are no less damaging.

Regrettably, most of the Clarksville Cave’s delicate formations such as stalagmites, “soda-straw” stalactites, and the translucent “draperies” were broken off long ago, though in hard-to-reach areas of the cave some flowstone deposits and the unusual natural dams called “rimstone pools” have managed to escape vandalism.

In years’ past, visitors were invited to scratch their names into the walls; two of the most prominent were left by one “D.C. Gould” whose name was carved in neat letters on Aug. 12, 1864, and one “E. Brinley” whose name was incised in 1839 on a wall above a pool.

Yet, in spite of visits by untold thousands of people over the decades, the Clarksville Cave and its preserve remain iconic examples of geologic processes, and specifically of cave geology — known as speleology.  Teachers of Earth science and geology in both secondary schools and colleges have used it as a resource for many years and, in the summertime, camps and environmental groups from all over the Northeast run field trips to the cave.

The preserve covers some 19.4 wooded acres, and within the forest are trails that lead not only to the cave’s multiple entrances but past classic examples of karst geology features: exposures of the bedrock called the Onondaga Limestone in which the cave has formed; mossy sinkholes and vertical shafts; and long solutionally-widened fractures in the bedrock called grikes.

In very wet weather, one of the cave entrances becomes an artesian spring and hourly thousands of gallons of turbulent water under pressure bubble upward against gravity and flow into the nearby Onesquethaw Creek.

The Onondaga limestone formed in a warm, shallow sea during what geologists call the Devonian Period, some 400 million years ago, when the landmass that would become North America lay much farther south than it is today and the Equator ran through the section that would become New York State. 

It is a very clean limestone — almost pure calcium carbonate — with little or no clay or sand within it.  This indicates that there were no high mountains near where it was forming that would have shed sediments into the water. It is densely packed with fossils such as crinoids (sea lilies), trilobites, clam-like brachiopods, and several species of coral.

The corals in particular are indicative of an environment of clear, relatively shallow, water. In some sections, the limestone is studded with nodules and shelves of the silicate rock called chert — or flint — which has precipitated there through processes still not completely understood.

The ages of caves are often difficult to determine. Except for lava caves such as those in Hawaii that form in real time as lava flows cool, the age of a cave has nothing to do with the age of the bedrock from which it has formed.

But one of the obvious features of the Clarksville Cave system is that it contains enormous deposits of rounded pebbles and cobbles in a clay matrix: remnants of debris left when glaciers retreated at the end of the last ice age some 12,000 years ago and their meltwaters roared through the cave carrying massive quantities of sediments.

These deposits can be found in even the loftiest parts of the cave. They indicate that the cave passages were there before deglaciation and for a time were choked with tightly-packed sediment; subsequently post-glacial streams found routes into the cave through the limestone bedrock layers that border Stovepipe Road west of the hamlet and elsewhere and began to flush out the sediments.

However, clues to a cave’s age are the features known as speleothems: stalactites, stalagmites, and flowstone that form from mineral-saturated water seeping into the cave. The old rule-of-thumb that every cubic inch of such features requires 100 years to grow has been shown to vary tremendously from one cave to another, but a massive flowstone feature such as that in the accompanying photo (Figure 3) undoubtedly required tens of thousands if not hundreds of thousands of years to form.

To put it simply — Clarksville Cave is old!

It has formed along a tectonic fault that shows itself in many places in the cave passages and at the surface where scratches called slickensides and folds in the bedrock appear.  One particularly prominent display is in the bed of the Onesquethaw Creek just a few yards east of the stone bridge on Plank Road. 

Within the cave, the ceilings of many passages show a slight tilt from west to east as a result of the fault movement. This tilt has allowed great quantities of mineral-saturated water to enter the cave over millennia, with the result that masses of flowstone accumulated on the western sides of the cave passages — much of it now damaged — and in some places the waters have acted as a natural cement, turning large piles of glacial debris into the sedimentary rock called conglomerate.

The cave was frequently visited in the 1800s — at least for a time its owner offered guided tours by lantern light and the cave was written up in Harper’s Weekly.

Until the early 1960s, the system was regarded as two separate caves, with one entrance in the village close to Route 443 called “Gregory’s Cave” after its then-owner. It consists of a series of lofty chambers connected by a wide tunnel carrying a meandering stream and terminating after around 450 feet in a pool rising nearly to the ceiling — a feature which cave explorers call a “sump.”

A longer section entered through a sinkhole in the woods bordering the hamlet trends north and was known as “Ward’s Cave.” Around 1,200 feet long, this section resembles a subway tunnel through which flows the upper stretch of the same gurgling stream, rising in a deep artesian pool near the north end, known rather grandly as The Lake.

But for years after the 1948 publication of “Underground Empire” by Clay Perry, which dealt with the caves of New York State, explorers were intrigued by Perry’s statement that the two caves were connected by what he described as a half-mile long tunnel lined with calcite crystals.

Then, during the summer of 1962, an extended drought hit New York State and the water level in the sump in the Gregory section dropped almost three feet. Following the cave explorer’s directive — “Follow the water!” — within a couple of days of each other, one group of explorers from Albany and another from the Boston area waded through the sump and found that the cave continued in the direction of the Ward section.

To this day, no one is certain which group was first; cavers are notoriously — some would say obsessively — reluctant to give details about their discoveries. But within weeks the word was out that a major breakthrough had been made in Clarksville and that the two caves were now connected.

To everyone’s amazement, in lamplight the then-pristine formations in the connection section did indeed sparkle in the form of calcite crystals that seemed to cover every surface. This discovery obviously raised the intriguing possibility that some intrepid explorers in the distant past had also made the connection yet somehow left no trace of their passage.

Alas, most of the crystalline surfaces are today obscured under a coating of mud left by the boots and clothing of more recent visitors.

Yet even so, the connection is not without its wonders: mysterious side passages that sometimes loop back upon themselves; the artificial-looking rimstone pools; bedrock twisted and distorted under pressure of the fault movements; great slabs of rock coated with slickensides; a high, funereal chamber where lengthy roots from trees in the forest above hang like black veils in the gloom; a 40-foot wide waterfall whose currents of crystal-clear water send echoes throughout this section of the cave, sounding eerily like energetic conversation; and on almost every exposed surface the marvelous, mysterious Devonian fossils.

One can easily understand why the Clarksville Cave is considered a veritable textbook of geologic phenomena.

Today, being under the protection of the Northeastern Cave Conservancy and its appointed cave managers and lying so close to so many colleges and primary and secondary schools, the Clarksville Cave will continue to attract sport cavers, adventurers, and students for generations to come.

And, while walking or crawling through its inky-black recesses, one can only wonder what secrets the cave yet holds in the unknown passages from which its waters come, and the mysterious conduits into which they flow.