When karst waters flow, the landscape reverberates as sinkholes, grikes, springs, and disappearing streams gurgle

Rainy weather is the bane of most geologic field trips; perhaps the only weather worse is wet falling snow. A geology field trip in our part of the country often involves tramping through rough and sometimes wet terrain in search of locales in which our ubiquitous forests and ground cover do not obscure the bedrock; hence it may reveal fossils or minerals or tectonic structures contained therein and precipitation in any form can make for a genuinely miserable learning experience.

But the exceptions are field trips illustrating karst features. In times of snowmelt or heavy rainfall, a karst landscape reverberates as its sinkholes, grikes, springs, and disappearing streams gurgle with the sudden flow of turbulent waters. 

At such times, it is well worth the effort to put on a rain slick and waterproof boots and head off into one of the many karst preserves of the Helderberg area and appreciate the dynamic features of these landscapes and of the hidden mysteries that lie beneath them.

However romantic the notion of cave geology may seem, the understanding of karst begins with a singularly mundane fact: Caves are in essence natural storm drains.

The term karst itself is derived from “Kras,” which is a plateau bordering on Slovenia and Italy. In the 19th Century, geologists recognized that its limestone bedrock laced with caverns and featuring sinkholes, underground rivers and streams, and springs represented a whole relatively unexplored branch of geologic studies.

The storm drains found in city streets, which are often miscalled “sewers,” transport runoff beneath the streets to the nearest river or stream without filtering it.

Likewise, in a karst region, rainfall and surface waters that are naturally mildly acidic can dissolve carbonate bedrock such as limestone or marble and produce subterranean streams that can flow for miles before they reach a place where they return to the surface in springs that may be either gravity-fed or artesian defying gravity. And in doing so they often carry volumes of sediments — both organic and non — and spew them back into the world.

Much of the Helderberg Plateau contains limestone surface bedrock. In the area of Thacher Park, two limestone layers named the Coeymans and the Manlius form a cliff 100 feet high in places featuring numerous small caves. A couple of hundred feet higher is a broad terrace formed on top of the Onondaga limestone cliff and traversed by the Beaverdam Road.

There are on the surface great numbers of sinkholes and fractures called grikes that can take water in wet times, and the caves in these limestone strata will then produce springs that burst from the cliffs under the pull of gravity and flow down into the valley below.

A similar gravity spring emerges from Barton Hill in Schoharie above Route 146. These springs frequently produce micro-environments conducive to the growth of mosses, ferns, algae, and watercress.  At times — often seasonal — the waters emerging from gravity springs may be saturated with calcium carbonate in solution and will coat the rocks and organic materials in their paths with the mineral producing a spongy-looking rock known as tufa.

In dry times, water may be flowing just beneath the surface of dry-looking stream beds but will overflow to the surface during periods of snowmelt or high precipitation. One such example can be found in the upper reaches of the Onesquethaw Creek where it flows beneath a bridge near the intersection of Routes 443 and 85.

What for much of the time is a flat, arid limestone pavement featuring numerous grikes and other fractures becomes a series of plunge pools and rapids when excess precipitation renders the conduit just beneath the surface inefficient to carry the excess water.

Similar weather conditions can also produce temporary artesian springs in which water under pressure flows upward against gravity. The volume of water in a sizable cave passage overwhelms the ability of the cave to carry it, just as storm drains blast water to the surface when the volume of water exceeds the ability of the conduit to transport it.

An impressive example occurs several times each year near what is called the Gregory Entrance to Clarksville Cave. A fracture at the base of the surrounding cliff exhibits an impressive artesian spring: a cloudy pool with a surging hump of water in the middle as turbulent waters flow upward out of the cave and down an otherwise dry streambed and under Route 443.

This spring not only flows during times of snowmelt but may form and be gone within 24 hours in warmer weather following sudden heavy rainfall.

Disappearing streams

But perhaps the most impressive feature of a karst landscape during periods of exceptionally wet weather or spring thaw is the presence of disappearing streams. Pockmarking the terrain will be dozens — sometimes hundreds — of sinkholes that form when surface bedrock collapses into a void below.

At such times, flowing liquids will head toward the lowest topographic point and will produce streams that may flow for hundreds or thousands of feet over bedrock that does not dissolve as limestone does or tightly-packed glacial debris, both of which are common in the Helderberg area.

Suddenly reaching a sinkhole, a raging stream may vanish abruptly into the darkness below, leaving the surrounding landscape quiet except for bird calls. A well-known example is the sinkhole entrance to the Onesquethaw Cave system south of the village of Clarksville.

Sport cavers have long had great respect for Onesquethaw, which is capable of sudden flooding with rapidly-moving water following short periods of intense precipitation. The sinkhole entrance is in a very low area surrounded on its west side by steep shale hills, the runoff of which can cause huge volumes of water to cascade into the entrance. The flow resurges in a gravity spring a mile or so away off Route 32.

Geologists are known for braving dangerous topography and unpleasant weather conditions in pursuit of knowledge and mild, dry weather will ordinarily be most welcome on expeditions. But to truly appreciate a karst landscape it must be seen in weather conditions that would keep the less adventurous indoors.

For when the heavy rains fall or the snows melt, the landscape comes alive, with otherwise dry fractures in cliffs and other bedrock exposures suddenly blasting great volumes of sediment-laden water. These waters form rushing, meandering streams that seem to have come from nowhere; fields and forests full of gaping sinkholes that swallow those temporary streams then conduct the waters through dark chambers to often unknown destinations.

It is a landscape filled with mysterious sounds and sights that are well worth the temporary discomfort in experiencing them.