Field Trip to the Western Hudson Highlands, New York

Long Island Geologists Field trip on June 28, 2003

Led by Prof. Alexander E. Gates
Department of Earth and Environmental Sciences
Rutgers University
Newark, NJ 07102

Download the field guide as a pdf file

Bret Bennington's photos for this field trip

Virtual Reality Field Trip to Harriman State Park

The following descriptions of the stops are from the "Field Trip to the Western Hudson Highlands, New York" 

We did not have time to visit Stops  5 and 7.

The following photos were taken by Gil Hanson unless otherwise noted. If you have better photos or photos that show features not shown here please e-mail them to me at

Photos courtesy of Gates et al, 2003 are from the Guide "Field Trip to the Western Hudson Highlands, New York"

The rocks in Harriman and Bear Mountain State Parks record a long and complex history that began 1.3 billion years ago and continues today. Most of the rocks were deposited as sediments and volcanics about 1.3 billion years ago in a setting similar to Japan today. About 1.0 to 1.1 billion years ago, a huge mountain building event called the Grenville Orogeny resulted from a continental collision similar to that which built the Himalayas. This event turned all of the rocks into the gneisses that we see today. The rocks were buried to about 25-30 km depth during this event and heated to over 700C. A second phase of the Grenville orogeny was the formation of large strike-slip faults similar to the San Andreas Fault, CA of today. Magnetite (iron ore) in veins several miles in length was mined for iron for two centuries in early American history. The Grenville Orogeny was one of several that built the supercontinent Rodinia. There was a period of tectonic quiescence for over 200 million years until Rodinia broke-up in a worldwide rifting event.

STOP 1: Metapelites and Graphite-sulfide Gneiss

(Seven Lakes Drive at Lake Sebago Dam)

 The rocks at this stop include sillimanite-garnet gneiss, cordierite-sillimanite gneiss, garnet-biotite gneiss, all of these are locally migmatitic, garnet-quartz granofels, graphite-pyrite or marcasite gneiss, and quartzofeldspathic gneiss. The sulfide bearing rocks weather to a red rust color on the surface. The deformation state ranges from somewhat randomly oriented grains to mylonitic. Late warping to gentle folding can be seen on the layer surfaces. They have shallowly northeast-plunging fold axes that parallel the mineral lineation.

These rocks are representative of the low energy deposits of the sequence. They are interpreted to have formed in a restricted marine basin that was likely euxinic and with a significant volcanic input. In other areas, these rocks can contain biotite gneiss with 55% garnets, thin marble lenses, and layers of pyroxene-plagioclase gneiss that are interpreted to be of volcanic origin.  

The following photos are thumbnail. Click on them to get larger images.

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Stop 1 Metapelites and graphite-sulfide gneiss. Notice the iron staining on the outcrop due to iron oxide resulting form oxidation of iron sulfide. Photo courtesy of  Joe Malave.

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Stop 1 Nearly vertical "Bedding" in metasedimentary rocks

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Stop 1 Close up of "bedding"

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Stop 1 Garnet-rich facies.

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Stop 1 Quartz-graphite "vein


STOP 2: Nappe-Stage Folds

Tight to locally isoclinal recumbent folds are displayed within biotite and hornblende quartzofeldspathic gneiss. The folds trend shallowly to the northeast and have a sheared out lower limb. Parasitic folds are common. The gneiss is interlayered and is dominated by biotite quartzofeldspathic gneiss which locally contains garnet. One outcrop shows a progressive change from biotite-poor to biotite-rich gneiss over a 2 m interval with an abrupt contact to biotite-poor quartzofeldspathic gneiss. There are local granitic veins which are also folded.

The rock is interpreted to be metamorphosed sedimentary to volcaniclastic rock that was deposited in a high energy environment. The hornblende-rich rock reflects volcanic input. The apparent fining upward sequence might reflect a prograding fan or delta or shifting channel sequence. The folds were generated during westward directed fold nappe emplacement. These recumbent folds are observed at all scales. They appear to accompany peak metamorphic conditions. This tectonic event is interpreted to have been a Himalayan type continental collision.


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Stop 2 Outcrop sized nappe stage folds
Photo courtesy of Gates et al, 2003
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Stop 2 Nappe-stage folds Outcrop, Mostly shows nice scenery

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Stop 2 Small-scale nappe stage folds

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Stop 2 Small-scale nappe stage folds

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 Stop 2 Possible turbidite sequence. Thick quartzo-feldspathic beds would be sandstone.
Thin metapelites would be muds.


STOP 3: Diorite Intrusion

( 7 Lakes Drive at Lake Tiorati , across from small rock island past Cedar Pond Camp and before PIPC camping office)

Pluton of coarse to very coarse-grained black and white speckled diorite. On the south side of the outcrop, the diorite is equigranular in texture with random grain orientation. It contains a roof pendant of well-foliated biotite quartzofeldspathic gneiss that exhibits crenulation cleavage. The xenolith contains drag folds along it's contact with the diorite. It also contains a rim of granitic pegmatite that connects to pegmatite and quartz veins within the diorite. The diorite contains plagioclase and hornblende and clinopyroxene but with brown cores of orthopyroxene. Other phases include magnetite and ilmenite. In the northern part of the exposure, the diorite is crossed my anastomosing mylonite bands. The mylonite strikes northeast and is near vertical. Lineations plunge shallowly to the northeast. Kinematic indicators include rotated porphyroclasts and S-C fabric. Where it can be determined, shear sense is consistently dextral.

Subsequent to the first tectonic event which included the nappe emplacement and granulite facies metamorphism, there was a period of intermediate plutonism. The xenolith was deformed and metamorphosed prior to intrusion. The xenolith became more ductile as a result of the heat of the pluton. Thus drag folds formed along its edges as it fell into the magma. The magma was hot enough to cause partial melting of the rim of the xenolith, producing a granitic melt. The diorite crystallized at higher temperature than the granitic melt. Fractures opened in the newly crystallized rock and the remaining granitic melt squeezed into them forming the veins. Later deformation produced the mylonitic fabric in the diorite. This outcrop is at the eastern edge of a large dextral strike-slip shear zone with similar orientation.  

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Stop 3 Diorite intrusion. The diorite, darker colored rock on left, has a roof pendant or xenolith of felsic gneiss on the right with a thin layer of melt rock along the contact. Some of the melt rock can be seen in small veins in the diorite.
Photo courtesy Gates et al, 2003
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Stop 3 Diorite intrusion. Close up. The hammer  is on the diorite. The contact with a ca 4 inch pink pegmatoidal melt vein is about a foot to the right of the hammer head.  

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Stop 3, Orthopyroxene, the brownish mineral, surrounded by clinopyroxene. The orthopyroxene is thought to be the earlier ferromagnesian phase to crystallize from the diorite melt, followed by clinopyroxene then hornblende. 

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Stop 3 Alec Gates is pointing to mylonite band that cuts the diorite. The minerals in the diorite have a strong fabric whereas in the previous part of the outcrop the diorite does not have a strong deformational fabric.

STOP 4a: Migmatitic Metavolcanics

 Black and white, strongly interlayered mafic and intermediate gneiss with migmatitic veins. The mafic layers in the melanosome are composed of clinopyroxene, hornblende, plagioclase, magnetite, sphene and apatite. The intermediate layers are dominantly plagioclase with minor quartz, K-spar locally, apatite, hornblende and biotite. The leucosome is composed of coarse interlocking plagioclase, quartz, and K-spar and form net veins and clots. Minerals are aligned in the gneiss and granular in the leucosome.

 The interlayered mafic-intermediate gneiss are interpreted as metavolcanics of island arc affinity. During the nappe emplacement event, metamorphism achieved granulite facies. Locally, the gneiss underwent anatexis and formed migmatite. Note that this rock still preserves the evidence of the first tectonic event with no overprinting. Contrast this rock with Stop 4b.

STOP 4b: Tectonic Blocks

            Lozenges of mafic gneiss contained within contorted layered biotite and hornblende quartzofeldspathic gneiss. The mafic gneiss is the same as in Stop 4a but it contains magnetite veins and contorted folds. The quartzofeldspathic gneiss is layered as defined by variations in biotite content and locally hornblende content. The layering is also contorted and wraps around the lozenges. The fold axes and long axes of the lozenges are parallel and plunge shallowly to the northeast.

            Stops 4a and 4b are grouped together because the compositions are similar and interpreted to be part of the same sequence. There is a large dextral strike-slip shear zone to the northwest. This deformation postdates the nappe emplacement phase. The rocks in stop 4a are considered to have been unaffected or only mildly affected by this later deformation. Deformation progressively increases towards the northwest as tracked by the progressive increase in linear fabric and steepening of the foliation.

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Stop 4 a. Outcrop of migmatitic meta volcanics
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Stop 4a Close up of migmatitic metavolcanics
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Stop 4. Lake is in fault zone. The two hills were originally one ridge that has been offset by recent faulting. Photo courtesy of Joe Malave
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Stop 4b Photo courtesy of Gates et al, 2003

STOP 6: Hogencamp Mine

The Hogencamp mine was active in the 18th and 19th centuries. Magnetite was mined from the mineralized veins. The vein that hosts the Hogencamp deposit is about 6 km long and ranges in thickness from about 2 to 15 m. The wall rock is mylonitic and in this area, it is composed of quartzofeldspathic calcsilicate, amphibole-pyroxene gneiss (metavolcanic), and diopside marble. The contact of the vein with the wall rock is sharp and generally parallel to mylonitic foliation. On the small scale, however, it crosses foliation and generally the vein appears to eat into the wall rock. There is a bleached zone in the wall rock at the contact with the vein. In quartzofeldspathic rock, the bleached zone is marked by retrogression of feldspar to mica and pyroxene to amphibole. It also contains scapolite, calcite locally and apatite. The vein is composed of distinct compositional band characterized by mineral assemblages. Nearest the wall rock, there is pargasite, scapolite, K-spar, and phlogopitic biotite. The next zone in contains mainly biotite pargasite and salite. The next band is salite and pargasite. Minerals in interior zones are salite, magnetite, and calcite in that order. The salite and locally magnetite crystallized in cavities because they are euhedral and locally form doubly terminated crystals. The bulk composition of the salite and pargasite rich zones is identical to an ultramafic rock. These are metamorphically produced ultramafic rocks. The mineralized veins are intruded by very coarse grained pegmatites which locally contain xenoliths of vein material. Ar/Ar dating of the hornblende in these deposits yields 924 Ma.

The veins are interpreted to have formed in dilational joints and fractures during the waning stages of dextral strike-slip shearing. Metamorphic fluids flushed through these fractures and reacted with the wall rock. The fluids mobilized elements from the reactions with the wall rock. These reactions buffered the composition of the fluids. When these fluids encountered the right conditions either physically or chemically, they deposited the ore and gaunge minerals. With the banding of different assemblages and compositions reflects the changing chemistries of the fluids. These changes may reflect changes in flux, fluid source, or physical conditions. The pegmatites may have intruded along the same pathways as the fluids. 

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Stop 6 Vertical iron mine shaft. Water at bottom.


STOP 8: Mylonite Zone

(Rt 17A near Sloatsburg, outcrop is on the north side of the westbound lane just before the first left turn lane to the west of Rt. 17)

             The rock is a biotite quartzofeldspathic mylonitic gneiss with interlayers of biotite gneiss locally. The mylonite is well foliated and lineated and composed of plagioclase, quartz, K-spar, and biotite. This mylonite exhibits well developed kinematic indicators including S-C fabric, reverse shear cleavage (RSC), rotated porphyroclasts, and shear bands. These kinematic indicators show a consistent dextral shear sense. The width of the zone and low S-C angle indicate significant offset.

Because this shear zone developed in a biotite-rich gneiss, it displays kinematic indicators better than most zones. It is another in the series of anastomosing dextral shear zones that were produced during the second event. The gneiss is interpreted to have a volcaniclastic origin.


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Stop 8 Alec Gates is showing the direction of shearing
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Stop 8 Close up of mylonite. Handle of hammer is parallel to axes of broad folds in mylonite. Direction of compressional stress which causes the dextral shearing in the mylonite is perpendicular to handle.
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Stop 8 Kinematic indicators. Courtesy of Vesna Kundic
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Stop 8 Late undeformed  hornblende-rich intrusion in mylonitic country rock. Hammer head is near contact. Hornblende crystals appear to be growing away from contact into the intrusive body.


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Photo of field trip participants