Regional Geology of Southeastern New
York State for Teachers and Travelers
Field Guide 2008
by
J Bret Bennington
Department of Geology And The IDEAS Institute
Geologic Provinces of the Hudson River Valley
Description of Field Trip Route with Notes on Geology and Geologic History
This field trip guide is organized according to the major geomorphic and
geological regions of New York State. In most cases, these regions correspond to the
landforms map found in the NYS Regents Earth Science Reference Tables.
Atlantic Coastal Plain (Late Pleistocene glacial moraine and outwash deposits)
Depart Hofstra, travel north across the outwash plain to the Northern State
Parkway. Parkway climbs the Harbor Hills moraine south of Roslyn and follows the
crest of the moraine. Exit to Cross Island Parkway north, descend slope of moraine and
follow west side of Little Neck Bay (drowned subglacial tunnel valley) to the Throgs
Neck Bridge.
Manhattan Prong (Proterozoic to Late Ordovician, metamorphic)
Cross the entrance to the East River at the western end of Long Island sound via
bridge to Throgs Neck. Note view of Manhattan to the west and view of north shore of
Long Island to the east with the ridge of the Harbor Hills moraine visible in the distance.
Follow signs for I95 south, Cross Bronx Expressway. Exposures of micaceous
Manhattan Schist (Upper Ordovician) can be seen in low road cuts along the north side of
the highway. Exit onto the Bronx River Parkway north, which runs up the center of the
Manhattan Prong, becoming the Sprain Brook Parkway in Yonkers. Exit to I287 west
and note extensive road cuts exposing the Proterozoic Fordham Gneiss between I287,
I87, and the Tappan Zee Bridge.
Newark Lowlands (Late Triassic to Early Jurassic, sedimentary and igneous)
Crossing the Tappan Zee Bridge over the Hudson River affords a magnificent
view of the Palisades Cliffs on the west side of the river. Less apparent is the fact that
prior to the mid-span of the bridge you cross a major regional nonconformity, jumping
from the Proterozoic and Paleozoic metamorphic rock of the Manhattan Prong onto
Mesozoic sedimentary and igneous rock of the Newark Basin (the actual unconformity
itself is buried beneath the bed of the Hudson River). The Newark Basin is a large rift
that developed in the continental crust of New Jersey and New York during the Late
Triassic and Early Jurassic as the Atlantic Ocean began to open, splitting Pangea into the
modern continents of North America and Africa. The crust beneath the basin subsided
along an extensive normal fault to the west as it hinged downward to the east. During
subsidence of the basin rivers carried water and sediments toward the center of the basin
from the uplands to the west, depositing them in a variety of terrestrial environments,
from marginal alluvial fans to braided streams to large freshwater lakes. These sediments
were deposited over millions of years to form a vertical sequence of layers over 5 miles
deep. Magma derived from melting of the Earth’s mantle beneath the rift zone intruded
the basin in the Early Jurassic, moving horizontally between sediment layers at several
miles depth and vertically to erupt as flood basalts on the surface. As the rift basin
continued to subside through the Jurassic the entire sequence of rock layers became tilted
toward the west. Erosion has cut into the tilted stack of layers, exposing the deeper layers
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east along the Hudson River and the shallower layers west along the basin-bounding
fault.
The Palisades Cliffs are formed by the tilted edge of a thick sheet of igneous rock
called the Palisades intrusive sheet that was emplaced at depth between layers of
sedimentary rock. The magma of the Palisades cooled slowly enough to form a finegrained igneous rock called a diabase (similar to basalt, only with larger crystals) but
rapidly enough to develop vertical fractures (called columnar joints) that give the cliffs
their distinctive palisade-fence appearance. The Palisades rock is hard and relatively
erosion resistant, forming a north-south strike-ridge on the modern landscape. At the far
side of the bridge I87 turns north to follow the front of the ridge before turning west and
cutting across the ridge. A large road cut on I87 exposes the diabase – a dull, black rock
that weathers into thick vertical columns.
I87 Mile 18: Roadcut in Palisades diabase.
Continuing west I87 descends down the dip slope of the Palisades intrusive sheet
into the Newark lowlands. Here the bedrock consists of layers of siltstone, sandstone and
conglomerate that are dipping gently to the west. Although mostly hidden beneath soil
and grass, the sedimentary strata are exposed in a series of road cuts west of the toll plaza
(unfortunately, we will not see these because we exit north onto the Palisades Parkway).
Exit 13N to head north on the Palisades Parkway. Two miles south of Thiells the
Palisades Parkway crosses the northern hook of the Palisades ridge and shortly thereafter
the bounding fault of the Newark Basin, entering the Hudson Highlands and Harriman
State Park. Exit 19 takes you to Perkins Memorial Drive, which winds its way up to the
summit of Bear Mountain.
PP Mile 19.3: Roadcut in Palisades diabase.
Mile 26-27: Highway crosses northern margin of diabase intrusive sheet.
Cross section across the Newark Basin. From “Roadside Geology of New York” by
Bradford Van Diver
2
Geology of the Palisades Parkway region.
From “Roadside Geology of New York” by
Bradford Van Diver
3
Hudson Highlands (Middle to Late Proterozoic, metamorphic)
The Hudson Highlands are an extension of the Ramapo Highlands to the south
that trends northeast across the Hudson River. The metamorphic rock of the Hudson
Highlands formed from sedimentary and igneous formations that were caught up in a
continental collision and mountain building event called the Grenville Orogeny over one
billion years ago. The Grenville Orogeny was a major event in the growth of North
America and Grenville bedrock underlies younger strata across much of the eastern
margin of the continent. Later collisions and orogenies in the Paleozoic have remetamorphosed the Grenville rock to the east and thrust large regions of it upward to be
exposed by erosion at the surface. Grenville rock equivalent in age to the Hudson
Highlands strata is also found in the Manhattan Prong (the Fordham Gneiss) and in the
Adirondack region.
Stop 1: Bear Mountain in Harriman State Park
The landscape and geology of the Hudson Highlands is magnificently revealed at
the summit of Bear Mountain. From the parking area a short walk leads to glacially
polished bedrock that descends in a series of slabs along the southern slope. The pink
bedrock is a K-feldspar rich metagranite called the Storm King granitic gneiss. Although
the rock may appear at first glance to be no different from granite, careful inspection of
several blocks will reveal a distinct alignment or foliation of the some of the mineral
crystals – a telltale signature of metamorphism. The Storm King gneiss is the dominant
rock type forming peaks in the central Hudson Highlands. The hard, crystalline, granitic
gneiss is erosion-resistant relative to the metasedimentary gneisses of the bordering
regions, creating higher peaks. The summit of Bear Mountain also reveals abundant
evidence for the flow of glacial ice over the mountain during the peak of the last ice age
(Wisconsinian – 20,000 ka). Glacial striations and chatter marks are visible on most of
the bedrock surfaces and indicate a flow of ice from the N-NW to the S-SE. Large
boulders and cobbles at the summit are glacial erratics displaying a variety of rock types
found to the north of Bear Mountain. The most distinctive of these are red coarse cobble
conglomerates (sometimes called “puddingstones”) found at the top of Skunnemunk
Mountain 30 miles to the north. Also visible from the summit are the glacially rounded
peaks of the surrounding highlands. Some of these peaks show a distinct asymmetry,
with more gentle slopes to the north and steeper slopes to the south, the result of glacial
carving and plucking.
In describing the geology of the Bear Mountain region, Alexander Gates, a geologist at
Rutgers University, writes:
“The geologic history of the Bear Mountain area is long and varied. It is part of the
Hudson Highlands, the northern segment of the Reading Prong. This geologicphysiographic province contains some of the oldest rocks in the Appalachians. The now
crystalline rocks were originally formed in a volcanic arc as volcanic and sedimentary
rocks about 1.2-1.3 billion years ago. The area was probably similar to modern Japan at
that time. As a result of the collision between North America and South America about 1
billion years ago all rocks were converted into the gneisses that we see today and
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intruded by magma that formed granite plutons. This collision is known as the Grenville
Orogeny and helped to build the supercontinent Rodinia. At the end of the Grenville
Orogeny, a large fault system similar to the San Andreas of California developed in the
area. In the late stages of faulting, fluids deposited extensive magnetite deposits within
the faults. This magnetite supplied iron ore to America from colonial times to the late
19th century. Besides several times of magma intrusion and minor faulting, the area
remained fairly quiet until it was glaciated during the last ice age which ended about
12,000 years ago. Active research projects are further refining this history.”
Stop 1a: Bear Mountain Bridge Overlook
Continuing through the parking area on Perkins Memorial Drive leads to the east
side of the summit and an overlook of the Hudson River and the Bear Mountain Bridge.
The Hudson River is an extensive tidal estuary, with high and low tides that extend 160
miles from the mouth of the river at the Verrazanno Narrows to Troy, New York (where
the level of the river is only 10 feet above sea level). Across the Hudson at the Bear
Mountain Bridge is a prominent peak called Anthonys Nose. The dark rock of Anthonys
Nose is amphibole gneiss, which is intruded in places by large granite migmatites of
uncertain age.
To the south the river exits the highlands and widens into the Tappan Zee (Tappan
Sea) as it crosses a wide belt of easily eroded shales of the Newark Basin. To the north
the river valley is steeply eroded into the hard rock of the highlands following a zigzag
path through the peaks. The path of the river is controlled by a series of northeast
trending faults that cut across the highlands. The rock along these faults is easily eroded,
forming valleys that captured the flow of the river along short segments. The Hudson
River valley in the highlands was carved to great depth by flowing glacial ice to form a
fjord – a glacial valley flooded by the sea. Borings into the river bottom show that the
depth of the river valley extends to over 1000 feet below present river level, although the
present channel is only 80 feet deep. Most of the depth of the Hudson Fjord was
backfilled with glacial sediments during deglaciation. Also visible along the river are flat
terraces that are remnants of the bed and course of the pre-glacial Hudson River. One
such terrace is the site of the Bear Mountain Lodge, which is visible at the bottom of the
mountain beneath the overlook, sitting 120 feet above the modern river. This is where
we will stop next for lunch.
5
Hudson Highlands, Greenpond Outlier, and the Wallkill Valley
From our picnic lunch on the shores of Hessian Lake, we take Rt. 6 west across
the Hudson Highlands. Note the rounded hills and deep valleys, many with ponds and
lakes, typical of the highlands. Nearing Harriman, Rt. 6 descends a long hill into a valley
floored by Cambro-Ordovician carbonates (limestones and dolostones). This valley is
associated with an elongate region
of Paleozoic sedimentary strata in
the western part of the highlands
called the Greenpond Outlier. An
outlier is a region of rock that has
been preserved in a local area in
spite of having been eroded
everywhere else. The Greenpond
Outlier is a folded sequence of
Silurian through Devonian age
strata that are equivalent to
formations exposed farther west in
the Shawangunk and Catskill
mountains, but that were deposited
in environments farther east, closer
to the region of uplift. How the
rocks of the outlier came to be
preserved is not completely
understood. One possibility is that
the outlier formed as a graben,
faulting the younger Paleozoic
strata downward into the
Proterozoic strata. Another
possibility is that the Proterozoic
http://3dparks.wr.usgs.gov/nyc/valleyandridge/greenpond.htm
strata of the highlands were thrust
on top of the strata of the outlier during the Alleghenian Orogeny and that erosion has
subsequently uncovered the buried sequence of Paleozoic rock. From a highway
overlook on Rt. 6 the northern end of the Greenpond Outlier is visible in the form of
Skunnemunk Mountain, a prominent ridge west of the NYS Thruway. The ridge exists
because of the Skunnemunk Conglomerate, a Late Devonian deposit of quartz pebbles
and red, hematite-cemented sands that is very resistant to erosion (we saw glacial erratics
composed of this rock at the top of Bear Mountain). Beneath the Skunnemunk
Conglomerate are older Devonian and Silurian formations totaling over 3000 ft in
thickness. Some of these older formations are visible in roadcuts along the NYS
Thruway (I87).
6
Geology of the Wallkill Valley
region. From “Roadside Geology
of New York” by Bradford Van
Diver
7
I87 Mile 47-48: Roadcut in Devonian Esopus Formation (sandstone, siltstone, and shale).
Mile 49.5: Skunnemunk Mountain on west side of highway.
Mile 53-55: Storm King Arts Center – highway exits the highlands and enters the
Wallkill Valley.
Passing the Storm King Arts Center, the landscape opens up into a broad valley
with gentle hills. This is the Wallkill Valley, which is floored by Upper Ordovician
shales, greywackes, and slates of the Normanskill Formation. Many of the small hills of
the Wallkill Valley are glacial drumlins, composed of compacted glacial till deposited
beneath the advancing continental ice sheet. Seen from the air, drumlins have a distinct
teardrop shape that is difficult to discern close up at ground level. However, the welldrained till of drumlins makes them ideal places to grow apple trees, so many apple
orchards are planted on drumlins.
I87 Mile 68.6: Roadcut in Normanskill slate. Note orchard planted on drumlin.
Mile 69.3: Roadcut in Normanskill slate.
Mile 71: View of Shawangunk Ridge to the northwest.
Mile 72.8 View of the “Gunks” at the crest of a hill.
Exit 18 on the Thruway leads to the town of New Paltz, gateway to the Shawangunk
Mountains. Rt. 299 west passes through the center of town and crosses over the Wallkill
River and its broad floodplain eroded into the Normanskill Shales. These shales were
deposited in the Late Ordovician during the Taconic Orogeny as high mountains to the
east eroded and the sediments were swept into a deep foreland basin that subsided
adjacent to the uplift. By the end of the Ordovician thousands of feet of sediments filling
the basin were themselves caught up in the Taconic collision and uplifted. Exposures of
dark Normanskill shale can be seen along Rt. 299 approaching the base of the
Shawangunk Ridge. At the intersection of Rt. 299 and Rt. 44/55 the road begins its climb
up the face of the ridge. Midway up the ridge front is a hairpin turn in front of a large
exposure of Normanskill shale. The bedding of the shale appears to be horizontal at this
spot, but keep looking as we continue to climb the hill. Not far uphill from the turn the
shale beds are steeply tilted and overlain by beds of white conglomerate outcropping
farther up the hill. Unfortunately, the contact between the Normanskill shale and
Shawangunk conglomerate (an angular unconformity) is not visible at road level.
Diagram illustrating the arrangement of tectonic plates during the Taconic Orogeny.
http://csmres.jmu.edu/geollab/vageol/vahist/images/h-midlo.gif
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Shawangunk Mountains
ew
as
in
n
H
u
R dso
iv n
er
ke
La
W
R allk
iv il
er l
2000 ft.
Wallkill Valley
M
Rondout Valley
R
o
C nd
re ou
ek t
H Sh
ig ok
h a
Po n
in
t
4000 ft.
E
ka
id
Sl
W
e
M
t.
Catskill Mountains
sea level
5 miles
5 kilometers
Cross section showing orientation of strata from Hudson River west to the Catskill
Mountains.
Appalachian Fold Belt
The Appalachian Fold Belt is a region of folded and thrust faulted sedimentary
rock that extends from Georgia to New York. Paleozoic strata deposited from the
Cambrian through the Mississippian were caught up in the massive continental collision
of the Alleghenian Orogeny (Pennsylvanian) and deformed into syncline and anticline
folds. In southeastern New York the Appalachian Fold Belt consists of a relatively
narrow belt of Silurian and Lower Devonian rocks that have been upturned so that the
strata dip to the northwest and strike northeast to southwest. This structure results in a
prominent strike ridge called the Shawangunk Ridge, which forms a steep eastern cliff
that rises abruptly from the Wallkill Valley. The ridge resists erosion relative to the
underlying shales and overlying limestones because it is composed of the Shawangunk
Formation, a very hard, almost pure quartz pebble conglomerate.
STOP 2: Minnewaska State Park Preserve (Beacon Hill Overlook, Lake
Minnewaska, Awosting Falls).
Minnewaska State Park Preserve encompasses over 10,600 acres of woodlands,
including Lake Winnewaska, Lake Awosting, the upper Peters Creek Kill and its several
large waterfalls. As well as providing many exposures of the Shawangunk
conglomerates and sandstones, the numerous natural ledges surrounding the park provide
beautiful vistas across the Shawangunks, Catskills, and landscapes to the east.
Beacon Hill Overlook
From here are afforded views up and down the spine of the Shawangunk ridge.
To the east you can see the Smiley Memorial Tower at Skytop perched on the main ridge
in the Lake Mohonk preserve. Beyond the ridge on a clear day you can see the Wallkill
Valley and distant Hudson Highlands off to the southeast. Take a moment to examine the
beds of conglomerate outcropping along the ledge. Note the large, well rounded pebbles
of translucent quartz and the hard, pure quartz composition of the rock. Fossils are not
9
present in these rocks, which were deposited by extensive braided streams flowing across
the Silurian coastal plain. The source of the pure quartz sediments making up the
conglomerate is not known and it is interesting to note that this particular unit of rock
extends southwestward through New Jersey (where the Shawangunks become Kittatinny
Mountain), Pennsylvania, Maryland, Virginia, and North Carolina. Although the exact
facies represented in the Shawangunks appears to change from terrestrial to marine to the
south (as does the name of the formation, from Shawangunk in the north to Tuscarora in
the south) the lithology remains the same - a ridge forming, white quartz-pebble
conglomerate.
Map detail of Minnewaska State Park
showing locations described below.
Glacial striations and
chatter marks on bedrock.
Lake Minnewaska
One of several pristine, clear water lakes perched along the Shawangunk ridge,
Lake Minnewaska is .5 miles long and 72 feet deep. Like Lake Awosting to the west,
Lake Minnewaska does not support fish (it’s not clear why it shouldn’t) although it has a
large population of aquatic newts.
Lake Minnewaska formed during the last ice age as glacial ice flowed over the
ridge and gouged out depressions in the bedrock. Evidence for the movement of glacial
ice over the mountain top can be seen in the exposures of polished bedrock between the
parking lot and the lake. Prominent chatter marks are also evident as crescent-shaped
gouges in the rock and when the sun is low in the sky glacial striations can also be seen.
Both the chatter marks and the striations indicate the direction of ice flow that appears to
have been from north to south down the Hudson Valley.
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Awosting Falls
These are the first set of cascades encountered hiking along Peters Kill (creek)
and are the largest falls in the Shawangunks. Note the prominent plunge pool formed by
the erosion of bedrock at the base of the falls. Descending the trail from the top of the
falls to the plunge pool, we pass outcrops of sandstone that show distinct, angled layers
within the thicker beds. These smaller, angled layers are called cross beds and they
formed as the sand in the bed was being washed along the bottom of an ancient river,
forming small underwater ripples and dunes as it moved.
Trail to Peters Kill Parking Area
Time permitting, we may hike down the front of the escarpment (cliff-like drop in
the landscape) formed by the erosion-resistant layers of conglomerate and sandstone. As
we descend the front of the escarpment, you can see that the beds of strata are not
horizontal, rather they are tilted to the west, with a distinct strike (trend of the upturned
layers: north to south) and dip (direction of tilting: west). Hiking down the front of the
escarpment keep an eye out for massive old rock slides – jumbled piles of slabs and
boulders that collapsed off of the eroding front of the Shawangunk ridge.
Shale Quarry
Our last stop in the park is a small quarry for mining the crushed shale that is
spread on the trails in the park. The black shale in the quarry is the MartinsburgNormanskill Formation and was deposited in a deep, oxygen-poor sea during the Middle
Ordovician. It was uplifted during the Late Ordovician (notice how crumbly the shale is
– a result of being tectonically stressed) and eroded prior to the deposition of the
overlying Shawangunk sandstone and conglomerate in the Silurian. Although the exact
horizon is not visible in this part of the park, somewhere between the sandstone ridge and
the shale quarry we have descended through an unconformity between the Ordovician
rocks below and the Silurian rocks above.
Roundout Valley
Leaving Minnewaska State Park we will continue west over the Shawangunk
ridge. Traveling down the back of the ridge (the dip slope of the tilted strata) note the
layers of quartz conglomerate and sandstone. There are several pulloffs along the road
where one can stop for a view westward across the Rondout Valley to the Catskill
Mountains. Rt. 44/55 crosses over Rondout Creek just before the junction with Rt. 209.
Taking Rt. 209 north we travel along the valley for about 8 miles to Rt. 213. Turning
east on Rt. 213 we cut across the Roundout Valley. At the village of High Falls there is a
small dam and waterfall on the Rondout Creek. Although we will not likely have time to
stop here, there is a parking area and path along the creek where one can walk down a
section of the Late Silurian strata overlying the Shawangunk conglomerate. In
descending order, the formations exposed are the Rondout dolostone, Binnewater
sandstone, and High Falls shale. Continuing east, Rt. 213 passes through the town of
Rosendale, famous for its Nineteenth Century cement industry. Abandoned and flooded
cement mine adits are visible along the road in several places near the town. Turning
11
right on Rt. 32 south we cross Rondout Creek. Near the town of Tillson we will stop to
visit one of the abandoned cement mine adits, located in the wood on the east side of Rt.
32, just past the “Welcome to Rosendale” sign.
Optional Stop - Rosendale Quarry at Tillson
Walk to the south end of the metal guard rail. Continue along trail to exposure in
woods and beware of poison ivy. The entrance to a former cement mine, now flooded, is
just off the trail. The lowermost rock unit exposed is the Rosendale Dolostone, the
brownish-weathering rock found in the bottoms of the pillars. Cementstones were
quarried because the dolomitic layers contain just the right amount of magnesium and
clay to make a cement when roasted. These formed the basis of the Rosendale cement
business of the 19th century (see historical marker in village of Rosendale).
The thickness of the Rosendale Dolostone here is about 4 m and it is overlain by
about 1 m of Glasco Limestone with corals, followed by the Whiteport Dolostone (about
2 m thick), up to the roof of the mine. Above the roof of the mine, the cliff continues,
and within it one can see vertical strata at the nose of a recumbent fold. The rock above
the mine roof is the Devonian Manlius Formation (Thacher member), the next unit
overlying the Whiteport Dolomite member of the Rondout Formation.
Time permitting, we may examine the strata beneath the Rondout dolostone
exposed just to the south along Rt. 32. Upper Ordovician Normanskill Shale of the
Wallkill Valley is unconformably overlain by the Shawangunk Conglomerate, although
the unconformity is not exposed. The thickness of the Shawangunk Conglomerate here is
about 4 meters, compared to over 300 meters at Minnewaska State Park! Just to the north
in the town of Rosendale the Shawangunk Conglomerate is missing completely, and you
will have noticed that we did not cross a high ridge on our way back east. Geologists
refer to this as a “pinch out”. North of New Paltz the environment of deposition that
produced the gravels of the Shawangunk Conglomerate came to an end, and so does the
formation. The Shawangunk Conglomerate is overlain by the High Falls Shale and the
Binnewater Sandstone – the same stratigraphic sequence visible in the village of High
Falls.
Whiteport Dolostone
Glasco Limestone
Rosendale Dolostone
Rondout
Formation
Silurian
Binnewater Sandstone
Dev.
Thatcher Member
Manlius Formation
High Falls Shale
Shawangunk Conglomerate
Taconic Unconformity
Martinsburg Shale
Rosendale Cement Mine
Ord.
Stratigraphic section at Tillson and diagrammatic sketch of strata exposed in the cement
mine.
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Natural Cement: The dolostones exposed in the Rosendale region formed the basis of a
lucrative natural cement industry that peaked in the 19th century but continued well into
the 20th century. Natural cement was produced by quarrying the bulk carbonate rock,
crushing it, and cooking it in kilns (many of which can still be seen in and around
Rosendale) until it formed klinkers of calcium, silicate, and aluminum oxides through a
chemical process called calcination. The klinkers were then ground to a fine cement
powder and shipped to market in wooden barrels or canvas bags. Rosendale cements are
strong and long-lasting and they were used in the construction of many U.S. landmarks,
including the pedestal of the Statue of Liberty, the Brooklyn Bridge, Federal Hall, and
one wing of the United States Capital. The natural cement industry began to decline at
the end of the 1800’s as cheaper Portland cement became available. However, prior to
WWII it was discovered that natural cement could be combined with Portland cement to
speed production and add strength. This led to a brief resurgence of the regional cement
industry, including the use of Rosendale cement in the construction of nearby sections of
the New York State Thruway.
Continuing south on Rt. 32 we cross the Wallkill River and turn left on Rt. 213, heading
northeast. Passing Perrine’s Bridge on the left (an 1844 covered bridge across the
Wallkill) and under the NYS Thruway we come to a dam and hydroelectric plant on the
Wallkill River.
Optional Stop – Ordovician Section at the Dashville Hydroelectric Plant
Exposures along the river and on the south side of Rt. 213 show syncline and
anticline folds in the Upper Ordovician Normanskill Formation. The Normanskill
consists of thick beds of greywacke (a muddy sandstone, often containing pebbles at the
base of layers) alternating with thinner beds of siltstone and shale. These sediments were
eroded off of the rising Taconic orogen (mountain belt) to the east and deposited
westward into a subsiding foreland tectonic basin. Close inspection of the Normanskill
sedimentary layers reveals that they form a repeating sequence of rock types. The thick
layers of greywacke often have coarse pebbles and large rip-up clasts of shale at their
base, with the sand grains becoming finer upward through the layer. Directly above the
greywacke layers lie layers of quartz silt topped by clay shale, which are usually
truncated above by the base of another greywacke layer. Sedimentologists call these
distinctive packages of sedimentary layers turbidites and interpret them to be the product
of turbid flows of gravel, sand, and mud that rapidly washed downslope into the basin
after being set in motion by storms and earthquakes.
As the plate tectonic collision that produced the Taconic Orogeny progressed, the
Normanskill shale and greywacke layers were themselves eventually folded as the region
of uplift migrated eastward. Thus, these Normanskill layers provide two observations in
support of mountain building in the Late Ordovician, the thick sequence of detrital
sediments deposited in a rapidly subsiding basin provides evidence of the uplift and
erosion of high mountains and the folding of the layers is direct evidence for tectonic
deformation associated with a plate collision.
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Hudson Valley Lowlands to the Helderberg Escarpment
Departing New Paltz, we take I87 north toward Albany, traveling up the west side of the
Hudson Valley. The Thruway skirts the edge of the Catskill Mountains, crossing Upper
Ordovician, Lower Devonian and Middle Devonian rock formations. Note that the Upper
Ordovician strata are usually tilted at a steep angle, the result of uplift late in the Taconic
Orogeny, and that the Lower Devonian carbonates are often folded into a series of
synclines and anticlines, most likely the result of the Alleghenian Orogeny.
I87 Mile 79-81: Roadcuts in Upper Ordovician Normanskill shales and greywacke.
Mile 85-90: Roadcuts in Lower Devonian carbonate rock.
Mile 106-109: Exposures of Lower Devonian carbonate rock.
Mile 109.4: Large roadcut showing banded carbonate rock overlying tilted shales and
greywackes. This is the Taconic Unconformity.
Mile 110-111: Lower Devonian carbonates.
Mile 112-113: Normanskill Shales overlain by Lower Devonian carbonates.
Exit 21
Catskill
Mile 115-116: Lower Devonian carbonates.
Mile 118-119: Normanskill shales and greywackes.
Mile 120-121: Lower Devonian carbonates showing folding.
Mile 123-130: Normanskill shales and greywackes.
Exit 21A
Mass. Pike: Landscape flattens out as the Thruway enters the glacial
Lake Albany plain.
Exit 22
Selkirk
At Selkirk we begin travel westward toward the Helderberg Escarpment, an
abrupt rise in the level of the land formed along the erosional edge of the Allegheny
Plateau and Devonian outcrop belt. Near Albany the escarpment forms an impressive
series of high cliffs that rise hundreds of feet above the Albany plain. The route to John
Boyd Thacher State Park at the top of the escarpment is somewhat complicated – there is
no direct route! From the Thruway, turn right on Rt. 144 south toward Rt. 396. Turn
west on Rt. 396 and continue, crossing Rt. 9W and going through the town of South
Bethlehem. Rt. 396 becomes Rt. 301. Cross Rt. 32 and bear right on Rt. 301/443. When
301 ends go left on Rt. 443, then right on Rt. 85 east. Finally, bear left on Rt. 157 west
and continue to park entrance.
14
John Boyd Thatcher
State Park
15
Geology of the mid-Hudson Valley
region. From “Roadside Geology of
New York” by Bradford Van Diver
STOP 3: Helderberg Escarpment at John Boyd Thacher State
One of most spectacular exposures of geologic strata in New York State can be
seen by hiking the Indian Ladder Trail at John Boyd Thacher State Park. From the
parking area at the top of the escarpment, the trail descends the face of the cliff to the
base of the Devonian limestones and then climbs back up, affording the opportunity to
examine the rock units in great detail. Of course, the views from the top and along the
trail are also breathtaking!
The best look at the rock formations exposed along the Indian Ladder Trail begins
about midway at Minelot Falls. Here, the trail curves along a broad reentrant in the cliff
face developed at a stream valley formed by the water flowing over the falls. Just below
the trail, thick beds of greywacke with thin shale interbeds can be seen. These are the
Schenectady beds – Upper Ordovician clastics deposited during the Taconic Orogeny.
Standing just off of the trail, one can see that these beds are tilted and are in angular
contact with the horizontal strata above (although the angular unconformity is obscured
by the trail). At trail level a thinly bedded, brownish interval can be seen that is
weathering into the cliff face. Several springs emerge from this unit from small cavities
in the rock. This is the Upper Silurian Rondout Dolostone – the same formation we saw
at Rosendale in the cement mines. Above the Rondout Formation, the cliff face exposes
two more limestone formations. The Manlius Limestone appears thin bedded relative to
the more massive Coeymans Limestone that projects outward from the cliff face above
the Manlius. Continuing along the trail one can inspect the Manlius closely. At several
levels it shows very thin, wavy beds characteristic of deposition of carbonate mud
trapped by algal or bacterial mats in an intertidal environment. The Manlius also
contains beds with shelly fossils such as brachiopods, ostracods, and tentaculitids. Only a
few species are found and these often occur in great abundance, indicating ecologically
stressful conditions such as high salinity (the few species that can tolerate these
conditions are not kept in check by competition or predation). At the top of the Manlius
beds there is an interval containing abundant stromatoporoids – a mound-like calcified
sponge that formed low reefs in the Devonian. These fossils indicate deepening sea level
(shallow subtidal) and the transition to the Coeymans Limestone, which is a highly
fossiliferous skeletal limestone containing abundant and diverse brachiopods, corals, and
crinoids. The Coeymans fossils are best seen in weathering bedding plane exposures
along the trails at the top of the escarpment.
Block diagram of the
Helderberg Escarpment
at John Boyd Thatcher
State Park. Goldring,
1933.
16
Diagram illustrating depositional environments preserved in Lower Devonian formations
in New York State. From Isachsen et al., 2000.
Stratigraphic section at Minelot Falls. Photo
and fossil illustrations from Goldring, 1933.
17
Optional Stop – Taconic Unconformity at Thruway Interchange near Leeds, NY
The Rt. 23 eastbound thruway toll-plaza approach road at exit 21 (Catskill-Cairo)
has a roadcut that exposes the angular unconformity between Upper Ordovician
Normanskill greywacke and shale below and Siluro-Devonian carbonates above. This
exposure is remarkable in that it reveals evidence for a prolonged sequence of geologic
events spanning Late Ordovician to Late Devonian time: 1. Deposition of Normanskill
sediments during the Late Ordovician. 2. Uplift and folding of Normanskill sedimentary
layers at the end of the Taconic Orogeny, followed by erosion to a level surface through
the Early and Middle Silurian. 3. Flooding of the erosion surface and deposition of
carbonates in the Late Silurian through Early Devonian. Uplift and folding of the entire
sequence of rock during the Late Devonian Acadian Orogeny.
Taconic Unconformity exposed near Leeds, New York
References
Goldring, W., 1933, Guide to the Geology of John Boyd Thacher Park (Indian Ladder Region) and
Vicinity, New York State Handbook No. 14. (Special Reprint published 1997, updated by
Landing, E. and Skiba, J.B.)
Isachsen, Y.W., Landing, E., Lauber, J.M., Rickard, L.V., and Rogers, W.B., eds., 2000, Geology of New
York, 2nd ed., New York State Museum Educational Leaflet 28.
Merguerian, Charles; and Sanders, J. E., 1994e, Geology of the Little Appalachians and the Catskills:
Guidebook for On-The-Rocks 1994 Fieldtrip Series, Trip 32, 24+25 September 1994, Section of
Geological Sciences, New York Academy of Sciences, 103 p.
Snyder, Bradley, 1981, The Shawangunk Mountains - A History of Nature and Man. Mohonk Preserve Inc.,
New Paltz, NY.
Snyder Estate Natural Cement Historic District, Wikipedia. Available Online:
http://en.wikipedia.org/wiki/Snyder_Estate_Natural_Cement_Historic_District
Stoffer, P., 2003, Geology of the New York City Region, United States Geological Survey. Available
Online: http://3dparks.wr.usgs.gov/nyc/common/contents.htm.
Van Diver, B.B., 1992, Roadside Geology of New York, Mountain Press Publishing Company.
Weinman, Steve, 1995, A Rock with a View - Trails of the Shawangunk Mountains. A Rock with a View,
PO Box 178, New Paltz, NY 12561
18
Geologic History of New York
Tectonic
Setting
Tectonic Events
Ma
Opening of the modern Atlantic
Ocean, formationof a passive margin
with deposition of cratonic clastics
and carbonates.
Holocene Cretaceous
146
Rifting of Africa from N.A.
Faulting and formation of Triassic
basins with Jurassic basalt intrusions.
Jurassic Triassic
245
Permian
290
Effect on the Geology
of New York
Glacial sculpting of the modern
landscape. Uplift of the
Adirondack Dome and
Allegheny Plateau.
Deposition of Coastal Plain strata.
Formation and filling of
Triassic basins,
emplacement of Palisades Sill.
No geologic record exists for the Permian in New York
Alleghanian Orogeny, oblique
collision of N.A. and Gondwanaland,
proto-Atlantic completely closes.
Pennsylvanian
323
Mississippian
Southern proto-Atlantic continues
to close.
360
Acadian Orogeny, probably
oblique collision of N.A.with Europe
and West Africa.
Northern proto-Atlantic closes.
Devonian
408
Proto-Atlantic continues
to close.
Silurian
439
490 - 440 Ma Taconic Orogeny
Collision of N.A. with volcanic island
arc.
Ordovician
510
Proto-Atlantic begins to close.
Cambrian
570 - 490 Ma Opening
of the proto-Atlantic Ocean
and passive margin formation.
540
Deformation of Appalachian
fold belt and strata to the
east.
Erosion of the Acadian
mountains produces the
clastic strata of the
Allegheny Plateau.
Deposition of carbonates seen
along the Catskill and
Helderberg escarpments.
Deposition of carbonates and
evaporite deposits of
Ontario Lowlands and
clastics of the Appalachian
fold belt.
Deposition and metamorphism of
Taconic Klippe and Manhattan
Prong rocks.
Deposition of Wallkill Valley and
Hudson-Mohawk clastics.
Deposition of thick carbonate
deposits following sand in strata
of Hudson-Mohawk, St.
Lawrence, and Ontario
Lowlands, and Tug Hill Plateau.
690 - 570 Ma Rifting and
faulting of Precambrian
rock.
Proterozoic
(Precambrian)
1.1-1.0 Ga Grenville
Orogeny, collision of
eastern N.A. with ?
Metamorphism of Adirondack
rocks and rocks of the Hudson
Highlands.
Key
Rifting
Diverging
Converging
19
Colliding
20