Tower karst of the Onesquethaw Valley: a bit of classic Chinese landscpe in Albany County

When I started composing this article in my head, this thought came to me:  “tower  karst” and “Onesquethaw” — two rather obscure terms (to most people) — in one headline.  Might be an attention-grabber!

The Onesquethaw is the stream that originates in Helderberg Lake and flows southeast  — sometimes through impressive canyons and over surging rapids — on its way to the Hudson River.  It passes photogenically through a number of diverse natural preserves, some of which I have written on in the past in this column.

“Karst” is the geologic term for an area of carbonate bedrock — most commonly limestone, but occurring in marble, dolostone, and gypsum as well — with resulting characteristic landscape features.  Each of these rock types dissolves in slightly acidic water, and, given the amount of moisture in Earth’s atmosphere and the carbon dioxide emitted by decaying vegetation on the floors of forests and fields, there is no lack of mild carbonic acid on our planet.

When the acid meets the limestone, sinkholes, caves, underground streams, and bubbling springs are frequently the result.  But, even in the same general areas of the Earth’s surface, outcrops of the same rock type may weather and erode at different rates and leave behind odd and even fantastic remnants of the bedrock — a phenomenon known as differential weathering and erosion.  Think of the great buttes and pinnacles of Monument Valley beloved by Western filmmaker John Ford.

Famous Asian landscape

The landscape of Guilin Province in China — commonly pronounced “Guy-lin” — is famous the world over, figuring in thousands of classic Chinese paintings and ubiquitous on the walls and menus of Chinese restaurants: giant limestone towers wreathed in morning mist, often featuring diminutive rice farmers or fishermen dwarfed by the magnificence of the rocky scene.

These towers are the last remnants of ancient limestone layers hundreds of feet thick.  They are laced with enormous cave systems and in some cases have been used by the inhabitants of Guilin as dwelling places and temples for thousands of years.

They have a mystical, otherworldly look to them and their attraction for artist and visitor as well is obvious.  And, though the towers of Guilin are perhaps the best known to travelers, there are many other such sites in Southeast Asia and elsewhere in the world.

Helderberg discovery

As often as I have hiked the hills and forests of the Helderbergs, I am still astounded to find fascinating geologic phenomena in places that — for one reason or another — I have managed to miss in my travels.

Though I have followed the valley of the Onesquethaw Creek through many of its twists, turns, and drops, there is one area off Rarick Road south of Route 32 that had escaped my notice until a long-time friend and fellow caver Thom Engel asked me if I knew that there was a display of tower karst in a natural preserve owned by the Mohawk Hudson Land Conservancy.  It is called the Onesquethaw Creek Preserve; it is a bit tricky to find in the thick forest that borders Rarick Road and there is no clearly defined parking area.

But Thom was able to get his feisty little Honda Fit off the road on what might once have been the beginning of an old logging trail, and, after a short hike, we found the fairly well-defined path that leads into the preserve.  Like many of the forested areas of the Helderbergs, the trees grow thickly, forming a green canopy that keeps the ground relatively free of large, dense shrubs, which obscure the view. 

The surface undulates and the relatively thin ground sediment is a mixture of soil and glacially deposited pebbles, cobbles, and boulders known as glacial drift.  Tree roots tend to spread out horizontally due to the thinness of the sediment, and walking is a bit precarious.

We had gone only a few hundred feet when we found ourselves confronted by the first of at least a dozen pinnacles of the Coeymans limestone, this one featuring three rounded peaks.  It was highly weathered and covered with mosses, ferns, and other shade-loving plants.

Diminutive compared to the lofty towers of Guilin — 20 feet in height compared to the Chinese towers that soar hundreds of feet — it was definitely an example of tower karst, and scattered around it were others of equal or lesser height.

Exploring the mystery

As it happened, the sky was overcast that day, making the woods rather gloomy, and so I returned on a sunny day a week later with my research assistant, Devin Delevan, to take more photographs and to see if I could come up with some explanation of why these features had formed here.  One tower with a double peak was a particularly striking example, and Devin climbed part way to the top to show scale.

The pinnacles stand close to the edge of the Onesquethaw Creek, which in this stretch usually appears as a dry bed buried in rounded boulders and soil, showing flow only in times of exceptionally heavy spring melt or a storm event such as a tropical storm or hurricane.

But its appearance is misleading, for, in karst lands, streams often are flowing underground.  In many stretches of the Helderbergs, one can drive for considerable distances without seeing a major stream; however, under the surface, extensive cave systems such as the eponymous Onesquethaw Cave near Clarksville as well as Albany County’s Knox Cave and Skull Cave, and Howe Caverns, Secret Caverns, and Ball’s Cave in Schoharie County feature streams that eventually resurge from valley walls or artesian springs to find their ways to the Hudson or Mohawk rivers.

We hiked some distance upstream of the karst towers to the higher area near the edge of the preserve — beyond is private property and farmed land.  Here the ground flattens out for a long distance into a level stretch, which geologists call a bench, likely planed off by the glaciers. We soon heard the sound of flowing water, incongruous given the exceptional dryness of the downstream bed.

Shortly, we came to a portion of the stream bed that was radically different from its downstream character  It is a wide, flat exposure of bedrock limestone, which is obscured by the glacial deposits in lower parts of the preserve:  dimpled with numerous potholes and deeply fractured, with some rifts several feet deep. 

Slightly farther upstream, hundreds of gallons of water were pouring over the exposure, forming pools full of clear or froth-laden water, many of which were inhabited by seeming thousands of inch-long tadpoles.  A startled great blue heron interrupted his lunch and took wing when he spotted us but undoubtedly returned after we had left, unwilling to abandon such a fruitful feeding ground.

We then discovered that the ponded areas covered in froth also exhibited small whirlpools, and the rotating water could be heard gurgling as it was sucked downward.  Here the Onesquethaw goes underground much of the time, just as it does in the stretch near Clarksville that parallels Route 443 where Route 85 joins it, only to reappear farther downstream.

And this explained the dryness of the stream in the lower portion of its bed: The water is moving through a subterranean conduit — in common terms, a cave.  The fact that so much water was simply vanishing from the surface hints of the size of that conduit — yet we searched in vain to find an entrance that would admit a human being.

Joints across the universe

But that open expanse of deeply fractured rock offered a tantalizing clue to the existence of the tower karst features.  An examination of the fractures reveals that they occur in straight lines and frequently intersect at something close to ninety-degree angles.  These fractures are referred to as “joints” and they occur everywhere in bedrock.

The Mars Rovers Spirit, Opportunity, and Curiosity have shown joints to be common in Martian bedrock as well.  There are a number of ways in that they may form.  When erosion removes upper layers of rock — or massive glaciers melt at the end of an ice age — the lower layers, free of all that immense weight, are allowed to expand, and their uneven rate of expansion causes the rock to crack. 

 Earth, the violent clashing of the tectonic plates sends shockwaves for many miles beyond their collision boundaries, fracturing the rock often to great depths.  Mars is not believed to have active tectonic plates, but a third possible method of formation is the uneven expansion and contraction of the bedrock due to heating and cooling at different times of day and the year, a process which certainly occurs on Mars and other planets.

Once the joints form, weathering agents such as water and ice can enter them and begin the process of widening and deepening them.  Particularly in a carbonate rock such as limestone, the solution process can go down as far as the water table.  Then, as over the centuries the water table drops, the joints in the rock can continue to widen and deepen.

Due to differential weathering and erosion, some areas will break down faster than others.  In the photograph of Devin on the flat outcrop, note that some stretches are more highly fractured than others, and consequently will eventually disappear faster than the more massive, unfractured stretches.  In centuries to come, these more massive areas could emerge as examples of tower karst.

The limestone strata (layers) in this part of Albany County are around 100 feet thick, so there is a limit to the heights tower karst features can reach — unlike in Geilin and other areas of Southeast Asia where the limestone can be a thousand or more feet in thickness.  But it should be noted that anywhere they appear, the towers have emerged from rock that formed in seas that covered the landscape scores or hundreds of millions of years ago.

And it should not be a surprise that, beneath the mosses, lichens, ferns, and other hardy plants that can grow in the thin soils atop the karst towers of the Onesquethaw Creek Preserve, one might find fossils of trilobites, brachiopods, gastropods, and other shelled creatures that dwelt in the warm and shallow sea that covered this part of New York State a couple of hundred million years before the first dinosaurs ever walked the Earth.