A recent issue of the “Friends of Thacher Park” newsletter featured an essay by state geologist Dr. Chuck Ver Straeten describing some of the rock layers that form the Helderberg Plateau. Reading it got me thinking about my many trips to Thacher Park over the years and my slowly dawning awareness of the geologic history locked within it.
The Irish novelist James Joyce defined the word “epiphany” as a moment of sudden insight into oneself or some situation — a moment when disparate facts or events come together to bring about a revelation, and since I was a kid I have had more than one such revelation there.
My memory of my first trip to Thacher Park is lost in the mists of my early years on this planet, but by the time I was old enough to understand the word “fossil” I had come to marvel at the fact that the craggy, forest-draped plateau to the west of Albany contained many traces of past life on Earth.
I can remember even as a small child being excited that it seemed that every rock I picked up in the park contained curious shapes — tubes, rings, and frequently whole or fragmented seashells. My father told me that they were there because “a long time ago” — I doubt I could have comprehended the phrase “hundreds of millions of years ago” — the whole Helderberg area was at the bottom of the sea.
By the time I entered high school, on my trips there I would sometimes search the rock outcrops for dinosaur tracks or bones — unaware that the rocks of Thacher Park were laid down around 180 million years before the first dinosaurs walked the Earth.
Early in my college years, I had become involved in the sport and science of caving (adherents consider the term “spelunking” to be rather politically incorrect!). As my friends and I crawled and climbed our way through Knox Cave, the Clarksville Caves, gloomy Onesquethaw Cave, and others beneath the hills, we had all learned enough to comprehend the term “Devonian Period” to classify the time in which the rock layers had formed.
The fact that this made them something like 400 million years old was — and remains — exceedingly difficult to comprehend for humans accustomed to think in terms of years, decades, and centuries. I also had become aware that caves tended to form in certain types of rock — most commonly limestone — but do not recall when I first heard the term “strata” to describe the rock layers that make up the Helderberg Plateau or to wonder why they looked different from one another and weathered differently.
But when I began my first teaching assignment — English and American literature and language — I was fortunate to make the acquaintance of the high school’s Earth Science teacher who coincidentally had done some caving. I assisted him on some field trips into the Helderbergs and began to learn as did his students about the fact that every rock layer is of an age different from those above or below it and formed under sometimes wildly different conditions.
I became aware that the rocks making up the plateau vary in age from the Ordovician Period — prior to 420 million years ago — to the Devonian Period — roughly 400 million years in the past — and contain within them stories of rising and falling sea levels, the building and erosion of mighty mountain ranges, and the emergence of strange forms of life, many long gone extinct.
A “geo-epiphany” to coin a term!
Layers of history
The strata of the Helderberg Plateau consist mainly of sandstone, shale, and limestone. Each of these rocks types forms under different physical and climatological conditions and it must be understood that, during the times in which they formed, the landmass that would become North America lay far to the south, with the Equator running through the middle of what would become New York State.
Through much of this time — the late Ordovician Period through the later Devonian — roughly 500 million to 360 million years ago — this area was under warm, shallow waters much like the sea surrounding today’s Bahamas.
A view of the Helderberg escarpment at Minelot Falls offers a cross-section of many of the major “strata” — or “formations” — that make up the plateau and which form the cliffs that tower above the Indian Ladder Trail. Rising up from the valley below in the talus slopes are the Ordovician Period Indian Ladder beds atop the Schenectady Beds.
The latter are difficult to see because in the valley the sandstone and shale strata of the Schenectady Beds are covered with glacially-deposited soils and rocks; the shaly Indian ladder beds rising up to the base of the escarpment are in many places obscured by gigantic boulders and sediment deposits that have crashed down from the cliffs.
Both of these strata contain marine fossils, but the sand and clay content indicate that they formed in muddy waters containing large quantities of sediment, likely shed from ancient nearby mountains.
Atop them are two relatively thin layers often regarded as having formed in the Silurian period, some 440 million years ago. The lower of the two is the Brayman shale/sandstone, which has not been found to exhibit fossils though the shale layers contain tiny, gleaming crystals of iron pyrite — “fool’s gold.”
Like the two strata below it, the Brayman seems to have formed from the petrification of sediments eroded from nearby highlands, perhaps in an environment that was hostile to life.
Immediately above it is a layer of limy shale called the Rondout. Like the Brayman, it does not display fossils in the Helderberg area but some of its thin layers show mud cracks, indicating that from time to time the sediments that formed it were exposed to the drying effects of the sun.
These layers are poorly resistant to the effects of weathering and erosion and have formed a shallow shelter at the base of the cliffs known for years as “Paint Mine Cave.” The name apparently derives from the presence of rusty iron deposits found in the strata that supposedly in the 19th Century were mined and mixed with sour milk to produce a reddish paint.
These layers also feature streams issuing from a number of small caves and one larger one, known traditionally as “Fool’s Crawl.”
Forming at the boundary between these lower layers and the towering limestone cliff above them, they represent the resurgence points of underground streams that have worked their way down through the limestone — composed of the mineral calcium carbonate — and encountered layers that their mildly acidic waters cannot dissolve.
This boundary has also been regarded by some paleontologists as representing the transition between the Silurian and Devonian periods of geologic time — an interpretation that has been sometimes energetically disputed.
The great escarpment is formed mainly of two limestone layers, the Manlius and Coeymans, named for the localities in which they were first studied. Each of these is a nearly pure limestone containing little or no sand or clay, indicating they formed hundreds of millions of years ago in clear water with no mountains around to shed muddy sediments into them.
Resembling nothing so much as stacks of manuscripts, the thin layers of the Manlius show evidence of having formed in a coastal environment during a time of rising sea levels. Some fragments that spill from the cliff show mud cracks or ripple marks. These tell us that the limy sediments that formed the rock were from time to time exposed to the drying sun, characteristic of a tidal environment.
The Manlius is sometimes known as the limestone of the “tentaculites” — tiny, needle-like fossil shells that coat the surface of slabs that weather from the bedrock. The exact nature of these marine creatures is still being debated but they are found throughout the layers of the Manlius in outcrops that stretch from the escarpment at Thacher Park west to Gallupville and beyond.
They frequently line up on rock samples in parallel displays consisting of thousands of individuals, much as boats in ocean bays form parallel patterns from receding tidal waters.
Due to its being almost pure calcium carbonate, the Manlius dissolves readily in acidic water and many caves in the Helderberg area have formed in it. There are disputes as to whether the Manlius represents a late stage of the Silurian Period or as having formed in the early Devonian period — known to paleontologists as “the age of fishes.”
However, fish fossils have not been found in local exposures of the Manlius. The upper layers of the formation, visible in exposures on the Indian Ladder Trail, are marked by the presence of thick masses of stromatopora — extinct coral-like fossils.
Above the Manlius is the massively-formed Coeymans formation, described as a semi-crystalline limestone that contains great numbers of a rich variety of fossils: trilobites, crinoids (sea lilies), corals, and clam-like brachiopods, among others.
Interestingly, some years ago, two geologists from Brown University rappelled down the cliff face and noted that there seemed to be a cycle in the fossils as they descended: deeper-water fauna alternating with shallower-water creatures — a pattern perhaps indicating the rise and fall of sea levels during a Devonian ice age.
Like the Manlius, the Coeymans is a clean, pure limestone with little mud content — indicating, once again, that it formed in waters that were far from any mountains that were shedding sediments. Many area caves such as Howe Caverns are formed through both of these formations, and their thickness has allowed high passageways and chambers to have developed within them.
Higher and later
Moving higher in the geologic cross-section and into the later Devonian period are the strata called the Kalkberg and New Scotland limestones. Their fossil fauna are similar to those in the Coeymans, showing the continuing marine environment in outcrops visible in the valleys descending from higher areas in the Thacher Park area carrying streams in wet weather that produce impressive waterfalls as they cascade from the cliffs.
Along the lower section of the Beaverdam Road that ascends from Route 157, the Becraft Limestone forms a flat stretch featuring numerous grikes — cracks in the bedrock that have been enlarged and deepened through solution by mildly acidic surface water.
The Becraft appears to be made almost entirely of fragments of shells and crinoids, similar to coquina — the rock formed from naturally cemented such fragments found in the waters off Florida and elsewhere in the Caribbean. The stone can be cut and polished to a high sheen, producing an attractive surface for counters and tabletops.
The presence of the fragmented fossils indicates that the Becraft formed in a near-shore environment in an ancient sea in which powerful waves transported and smashed fragments of sea creatures.
Atop the Becraft lies the Oriskany Sandstone, which in most areas of the Helderbergs has a thickness of little more than three feet. The Oriskany is what geologists call a “calcareous sandstone,” meaning that, although its matrix is common silicate sand, it is packed with shell fragments made of calcium carbonate.
It apparently formed in the environment of an ancient locality termed a carbonate beach. A famous modern example is Sand Beach in Acadia National Park in Maine. The presence of so many fossils makes the Oriskany highly sought after as a decorative stone, appearing in many places in the Capital District in exterior walls and fireplaces.
Above the Oriskany is a gritty shale layer known as the Esopus, named for its type locality, a small settlement near Kingston. It spreads over a large section of New York State, having its greatest thickness — nearly 300 feet — in the southeast part of the state. In the Helderbergs, it is around 100 feet thick and appears as a gray or tan rock that weathers to fragments easily.
Exposures of the Esopus such as those along the Beaver Dam Road usually feature piles of dusty, crumbly gravel at their bases. Rock such as this forms in extremely muddy marine environments, indicating the presence of nearby highlands from which torrents of sediments are being shed and deposited in the ocean.
Such an environment is not conducive to the sustaining of much life, and the only fossils common in the Esopus are those of a humble marine worm known by biologists as zoophycos. Existing side by side in the thousands on exposed surfaces of the Esopus, such as in the Onesqethaw stream bed in the village of Clarksville, the worms’ fossils confused some early paleontologists who mistook them for patterns in the ancient mud caused by spinning currents in the water and termed them “rooster tails.”
Each worm anchored itself on the seafloor and formed a series of curving tubes radiating out from the anchor point in search of food. The great thickness of the Esopus indicates that this muddy, low-oxygen environment persisted for many millions of years.
Above the Esopus is another massive layer known as the Onondaga, indicating an abrupt change to conditions once more conducive to the formation of limestone. A pure, heavily fossiliferous stone again indicating the presence of clear, warm, sunlit waters, with no high mountains nearby to shed muddy sediments into the sea, the Onondaga is rich in the fossils of corals, sea lilies, and brachiopods whose pearly remains stand out clearly against the clean, gray limestone.
Towering outcrops of the Onondaga are visible near Clarksville in the gorge of the Onesquethaw Creek on the south side of the village and in the arc-shaped fold of the bedrock known as an anticline that borders Route 443 just west of Clarksville. The popular Clarksville Cave system that stretches nearly a mile through a preserve in the village and is well-known for its fossils, underground stream, and classic geologic features is dissolved from the Onondaga.
Major change as plates shift
Following the formation of the Onondaga, a major change was occurring involving the tectonic plates that would eventually become North America and Europe. The plates were moving together, resulting in the great mountain-building episode known as the Acadian Orogeny, and in the area that is today northeast of the Helderberg region an extensive range of lofty mountains of Himalayan grandeur was rising.
But, as a land mass rises, the forces of weathering and erosion immediately begin to attack it and its ultimate elevation depends on whether the processes of elevation or of leveling predominate. For millions of years, as the mountains rose, they shed immense masses of sand, silt, and clay that spilled down through the valleys that separated the summits and into the shallow sea in which the Onondaga had formed.
These sediments buried the limestone and eventually filled up the sea, forming the series of interlocking deposits known as the Catskill Delta from which our present-day Catskills and much of the Appalachian Plateau evolved after the delta was elevated far above sea level in the continuing plate collision.
A dramatic illustration of these great geologic events is visible in one of the many sinkholes that border the south side of the Beaver Dam Road above Thacher Park. The surface bedrock of the broad, flat terrain on which the road lies is the top of the Onondaga limestone layer and directly above it are slopes composed of the shales and sandstones of the non-carbonate rocks known as the Hamilton strata.
These layers do not dissolve in the mildly acidic waters that fall from the skies or collect in pools on the surface; therefore, those waters flow down from the heights in small permanent or seasonal streams. But, as they meet the soluble limestone, they are able to infiltrate it through sinkholes and produce underground streams in caves.
The sinkhole called “TV Tower Cave” with its mossy natural bridge lies precisely at the boundary between the two rock units, illustrating the beginning of the formation of the Catskill Delta as the Acadian Mountains rose those hundreds of millions of years ago.
In higher elevations of the delta’s layers near Rensselaerville, the rocks begin to show ripple marks indicating the shallowing of the ancient seas and near Gilboa are the petrified trunks of giant ancient fern trees, some of Earth’s oldest large land plants.
The section of the Appalachian Plateau known locally as the Helderbergs once stretched to the base of the Adirondacks and, over millennia, erosion has caused it to retreat far to the south. Viewed from any distance, its strata resemble a series of stacked books.
And metaphorically — that is what they are: the record of stretches of Earth’s history so far back in time that the mind boggles trying to comprehend their age and contents.
But they tell of great movements in the planet’s surface, climatic changes, and the rise and erosion of great mountain ranges, and they preserve traces of the emergence and sometimes extinction of strange creatures from the past — all of which constitute a mind-expanding epiphany in the Helderbergs.
