Watchung Outliers
Encyclopedia
The Watchung Outliers include six areas of isolated low hills and rock outcrops of volcanic origin in the U.S. states of New York
, New Jersey
, and Pennsylvania
. These geologic outliers are primarily diminutive and detached remnants of the Triassic
/Jurassic
age Watchung Mountain basalt flows
. All six of the outliers are found along the western edge of the Newark Basin
, occupying small synclines
adjacent to the Ramapo fault system
. The outliers include, from north to south: the Ladentown basalt, Union Hill, the New Germantown/Oldwick basalt (including Round Top), Prospect Hill, the Sand Brook basalt, and the Jacksonwald basalt.
and Ramapo, New York
, approximately seven miles north of the maximum extent of the Watchung Mountains at Campgaw Mountain
. The trap rock
of the outlier covers a considerable area, running at or beneath the surface for a length of about two miles with a width of about a mile. Originally, because of its close proximity to the Palisades, the Ladentown basalt was thought to be an extension of the intrusive sill. It is now known from magnetic and gravity measurements that the Ladentown basalt is linked to the Palisades by a significant subsurface volcanic dike, but it is definitely an extrusive feature, displaying Orange Mountain basalt characteristic of First Watchung Mountain. This makes the Ladentown basalt unique among the Watchung basalts in that it appears to be rooted to an intrusive feature. In this respect, the Ladentown Basalt can be thought of as the remains of a fissure volcano
, owing to the extrusion’s slab-like character and ramping profile. New research indicates that the vicinity of Ladentown may have served as the primary vent from which the magma of the Palisades Sill
erupted to the surface to form the Watchung flood basalts.
in Ramapo, New York
, about three and a half miles north of the Watchung Mountains, or about halfway between the Ladentown basalt and Campgaw Mountain. The hill originally rose 150 feet above the surrounding terrain, but extensive quarrying for trap rock has since obliterated most of the hill except for the slopes of its perimeter. Like the Ladentown basalt, Union Hill was once considered to be an extension of the Palisades. It has since been determined that the hill consists of a flow of Orange Mountain basalt with a minimum thickness of 125 ft (38 m).
and Readington Township, New Jersey
, approximately seven miles southwest of the southern extent of the Watchungs and about three miles north of Cushetunk Mountain
.
The structure of the New Germantown basalt appears to mirror the construction of the Watchung Mountains on a tiny scale. The eastern side of the feature consists of a horseshoe shaped ridge with prongs facing to the west. This ridge is composed of Orange Mountain (First Watchung Mountain) basalt which is actively quarried. West of this First Watchung-like ridge is a valley occupied by sedimentary layers of the Feltville Formation
, characteristic of the valley between the First and Second ridges of the Watchungs. Finally, at the center of the horseshoe is a dome of volcanic rock forming a hill known as Round Top. The hill is composed of Preakness basalt, making it the equivalent of Second Watchung Mountain in the New Germantown complex. The thickness of basalt in this outlier is estimated at close to 425 ft (~130 m).
and Raritan Township, New Jersey
, Prospect Hill constitutes the smallest of the Watchung Outliers, being approximately one fourth of a mile wide and extending north to south for less than half a mile. The hill is composed of Orange Mountain basalt, with no overlying Jurassic sediments apparent. Dikes branching south from the vicinity of the outlier appear to be connected to the intrusive igneous Sourland Mountain
, an extension of the Palisades Sill
. In this respect, Prospect Hill likely marks another location where the Palisades magma found a way to the surface, albeit not as significant as the Ladentown basalt.
. The outlier shares some similarity with the Sand Brook and New Germantown basalts. As in the other two outliers, the Jacksonwald basalt features a low, horseshoe shaped ridge of Orange Mountain basalt that embraces an area composed of sedimentary strata from the Feltville Formation. However, unlike in the Sand Brook and New Germantown outliers, the Jacksonwald basalt lacks a central dome of Preakness basalt. The thickness of the basalt in this outlier is estimated to be in excess of 325 ft (~100 m).
An interesting trait of the Jacksonwald outlier is that it is surrounded by a ridge of intrusive diabase
on its eastern side. The diabase, which lies stratagraphically below the basalt, is thought to have been intruded into the syncline containing the Jacksonwald outlier during or after some degree of folding had affected local strata. The diabase is therefore considered to constitute a phacolith
.
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
, New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
, and Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
. These geologic outliers are primarily diminutive and detached remnants of the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
/Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
age Watchung Mountain basalt flows
Flood basalt
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
. All six of the outliers are found along the western edge of the Newark Basin
Newark Basin
The Newark Basin is a sediment-filled rift basin located mainly in northern New Jersey but also stretching into south-eastern Pennsylvania and southern New York. It is part of the system of Eastern North America Rift Basins.-Geology:...
, occupying small synclines
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
adjacent to the Ramapo fault system
Ramapo Fault
The Ramapo Fault zone, spanning more than 185 miles in New York, New Jersey, and Pennsylvania, is a system of faults between the northern Appalachian Mountains and Piedmont areas to the east. This fault is perhaps the best known fault zone in the Mid-Atlantic region, and some small earthquakes...
. The outliers include, from north to south: the Ladentown basalt, Union Hill, the New Germantown/Oldwick basalt (including Round Top), Prospect Hill, the Sand Brook basalt, and the Jacksonwald basalt.
Ladentown Basalt
The Ladentown basalt is the most northern of the Watchung Outliers, located along the border of the Town of HaverstrawHaverstraw (town), New York
Haverstraw is a town in Rockland County, New York, United States located north of the Town of Clarkstown and the Town of Ramapo; east of Orange County, New York; south of the Town of Stony Point and west of the Hudson River. The town runs from the west to the east border of the county in its...
and Ramapo, New York
Ramapo, New York
Ramapo , formerly known as New Hempstead and then Hampstead, is a town in Rockland County, New York, United States located north of New Jersey; southeast of Orange County, New York; south of the Town of Haverstraw and west of the Town of Clarkstown and the Town of Orangetown...
, approximately seven miles north of the maximum extent of the Watchung Mountains at Campgaw Mountain
Campgaw Mountain
Campgaw Mountain is the northernmost ridge of the volcanically formed Watchung Mountains, straddling the border of Franklin Lakes, Oakland, and Mahwah in Bergen County, New Jersey. Located almost entirely within the bounds of Campgaw Mountain Reservation, the mountain offers numerous outdoor...
. The trap rock
Trap rock
Trap rock is a form of igneous rock that tends to form polygonal vertical fractures, most typically hexagonal, but also four to eight sided. The fracture pattern forms when magma of suitable chemical composition intrudes as a sill or extrudes as a thick lava flow, and slowly cools.Because of the...
of the outlier covers a considerable area, running at or beneath the surface for a length of about two miles with a width of about a mile. Originally, because of its close proximity to the Palisades, the Ladentown basalt was thought to be an extension of the intrusive sill. It is now known from magnetic and gravity measurements that the Ladentown basalt is linked to the Palisades by a significant subsurface volcanic dike, but it is definitely an extrusive feature, displaying Orange Mountain basalt characteristic of First Watchung Mountain. This makes the Ladentown basalt unique among the Watchung basalts in that it appears to be rooted to an intrusive feature. In this respect, the Ladentown Basalt can be thought of as the remains of a fissure volcano
Fissure vent
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is usually a few meters wide and may be many kilometers long. Fissure vents can cause large flood basalts and lava channels...
, owing to the extrusion’s slab-like character and ramping profile. New research indicates that the vicinity of Ladentown may have served as the primary vent from which the magma of the Palisades Sill
Palisades Sill
The Palisades Sill is a Triassic, 200 Ma diabase intrusion. It extends through portions of New York and New Jersey. It is most noteworthy for The Palisades, the cliffs that rise steeply above the western bank of the Hudson River...
erupted to the surface to form the Watchung flood basalts.
Union Hill
Union Hill is located near the village of SuffernSuffern, New York
Suffern is a village in the Town of Ramapo, Rockland County, New York, United States located north of the State of New Jersey; east of Hillburn; south of Montebello and west of Airmont...
in Ramapo, New York
Ramapo, New York
Ramapo , formerly known as New Hempstead and then Hampstead, is a town in Rockland County, New York, United States located north of New Jersey; southeast of Orange County, New York; south of the Town of Haverstraw and west of the Town of Clarkstown and the Town of Orangetown...
, about three and a half miles north of the Watchung Mountains, or about halfway between the Ladentown basalt and Campgaw Mountain. The hill originally rose 150 feet above the surrounding terrain, but extensive quarrying for trap rock has since obliterated most of the hill except for the slopes of its perimeter. Like the Ladentown basalt, Union Hill was once considered to be an extension of the Palisades. It has since been determined that the hill consists of a flow of Orange Mountain basalt with a minimum thickness of 125 ft (38 m).
New Germantown Basalt
The New Germantown basalt (also known as the Oldwick basalt) is the most northern of the outliers south of the Watchung Mountains. The hills of the New Germantown basalt are located along the border of Tewksbury TownshipTewksbury Township, New Jersey
Tewksbury Township is a Township located in Hunterdon County, New Jersey and is located within the New York Metropolitan Area. As of the United States 2010 Census, the township population was 5,993...
and Readington Township, New Jersey
Readington Township, New Jersey
Readington Township is a Township located in the easternmost portion of Hunterdon County, New Jersey. As of the 2010 United States Census, the township population was 16,126...
, approximately seven miles southwest of the southern extent of the Watchungs and about three miles north of Cushetunk Mountain
Cushetunk Mountain
Cushetunk Mountain is a ring shaped mountain in the U.S. state of New Jersey, located in Readington and Clinton Township. The Lenape called the mountain "Cushetunk" meaning "place of hogs" after the wild hogs found there...
.
The structure of the New Germantown basalt appears to mirror the construction of the Watchung Mountains on a tiny scale. The eastern side of the feature consists of a horseshoe shaped ridge with prongs facing to the west. This ridge is composed of Orange Mountain (First Watchung Mountain) basalt which is actively quarried. West of this First Watchung-like ridge is a valley occupied by sedimentary layers of the Feltville Formation
Feltville Formation
The Feltville Formation is a mapped bedrock unit primarily in New Jersey, with one known outlier in Pennsylvania. It is named for the Deserted Village of Feltville in Watchung Reservation, New Jersey, which is near where its type section was described by paleontologist Paul E...
, characteristic of the valley between the First and Second ridges of the Watchungs. Finally, at the center of the horseshoe is a dome of volcanic rock forming a hill known as Round Top. The hill is composed of Preakness basalt, making it the equivalent of Second Watchung Mountain in the New Germantown complex. The thickness of basalt in this outlier is estimated at close to 425 ft (~130 m).
Prospect Hill
Located along the border of western FlemingtonFlemington, New Jersey
Flemington is a borough in Hunterdon County, New Jersey, United States. As of the United States 2010 Census, the borough population was 4,581. It is the county seat of Hunterdon County....
and Raritan Township, New Jersey
Raritan Township, New Jersey
Raritan Township is a Township in Hunterdon County, New Jersey, United States. As of the United States 2010 Census, the township population was 22,185...
, Prospect Hill constitutes the smallest of the Watchung Outliers, being approximately one fourth of a mile wide and extending north to south for less than half a mile. The hill is composed of Orange Mountain basalt, with no overlying Jurassic sediments apparent. Dikes branching south from the vicinity of the outlier appear to be connected to the intrusive igneous Sourland Mountain
Sourland Mountain
Sourland Mountain is a long ridge in central New Jersey, extending from the Delaware River at Lambertville to the western end of Hillsborough Township near the community of Neshanic , through Montgomery Township and into Hopewell Township in Mercer County. It comprises the largest contiguous...
, an extension of the Palisades Sill
Palisades Sill
The Palisades Sill is a Triassic, 200 Ma diabase intrusion. It extends through portions of New York and New Jersey. It is most noteworthy for The Palisades, the cliffs that rise steeply above the western bank of the Hudson River...
. In this respect, Prospect Hill likely marks another location where the Palisades magma found a way to the surface, albeit not as significant as the Ladentown basalt.
Sand Brook Basalt
The Sand Brook basalt, located in the eastern part of Delaware Township, New Jersey, appears nearly identical in construction to the New Germantown basalt. A low ridge composed of Orange Mountain basalt forms a horseshoe around the eastern edge of a belt of sedimentary strata from the Feltville Formation. In the middle of the outlier is a dome-like structure of what is likely Preakness Mountain basalt, not unlike Round Top in the New Germantown outlier. The thickness of the basalt in this outlier is estimated at nearly 400 ft (~120 m), similar to the New Germantown basalt.Jacksonwald Basalt
The Jacksonwald basalt is the only Watchung outlier in Pennsylvania. It is located over 100 km southwest of the actual Watchung Mountains in Exeter TownshipExeter Township, Berks County, Pennsylvania
Exeter Township is a township in Berks County, Pennsylvania, United States. The population was 25,550 as of the 2010 Census, making it the third most populous municipality in Berks County after the city of Reading and Spring Township. Daniel Boone Homestead is within its borders. Betsy King, the...
. The outlier shares some similarity with the Sand Brook and New Germantown basalts. As in the other two outliers, the Jacksonwald basalt features a low, horseshoe shaped ridge of Orange Mountain basalt that embraces an area composed of sedimentary strata from the Feltville Formation. However, unlike in the Sand Brook and New Germantown outliers, the Jacksonwald basalt lacks a central dome of Preakness basalt. The thickness of the basalt in this outlier is estimated to be in excess of 325 ft (~100 m).
An interesting trait of the Jacksonwald outlier is that it is surrounded by a ridge of intrusive diabase
Diabase
Diabase or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material...
on its eastern side. The diabase, which lies stratagraphically below the basalt, is thought to have been intruded into the syncline containing the Jacksonwald outlier during or after some degree of folding had affected local strata. The diabase is therefore considered to constitute a phacolith
Phacolith
A phacolith is a pluton parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline. In rare cases the body may extend as a sill from the crest of an anticline...
.
External links
- Geocaching.com – Two Hundred Million Year-Old Lava Flow! Earthcache at the base of Prospect Hill described by the New Jersey Geological Survey.