Addenda to the Geology 01 Field Guide for Isham and Inwood Parks
By: Charles Merguerian © 2005
The
geology of the bedrock units of New York City can best be explained in a layer
cake fashion (Figure 1). To simplify matters, we will concentrate only on the major units
found in the vicinity of Isham and Inwood Parks. The oldest bedrock unit is
known as the Fordham Gneiss (Yf) which is overlain by the Inwood Marble (C-Oi).
The Inwood is overlain by various schistose rocks (Ow, C-Om, and C-Oh)
which are here collectively designated as the Manhattan Schist. The detailed
mapping and analysis
by Merguerian
(1981, 1983, 1985, 1987, 1994, 1996, 2005); Merguerian and Merguerian (2004); and Merguerian and Sanders (1991)
suggests that these units can be subdivided into various subunits but for our
purposes they are best left lumped together and discussed collectively as the
Manhattan Schist. The interested reader is directed to the publications listed
above for the sordid subunit details.
Fordham Gneiss. The Fordham Gneiss (basal unit Yf in Figure 1) constitutes the oldest underpinning of rock formations in the NYC area and consists of a complex assemblage of massive Proterozoic (~ 1.1 billion year old) ortho- and paragneiss, granitoid rocks, metavolcanic- and metasedimentary rocks. In NYC, only a few attempts have been made to decipher the internal stratigraphic relationships, hence, the three-dimensional structural relationships remain obscure. They have experienced a very high grade of metamorphic recrystallization with internal melting developed on a regional scale and the production of migmatite (mixed igneous and metamorphic rock – the transition to the end game of the rock cycle).
Figure 1 – Bedrock stratigraphy of New York City as described in text. Note that the polydeformed bedrock units are nonconformably overlain by west-dipping Triassic and younger strata (TrJns) and the Palisades intrusive sheet (Jp).
Inwood
Marble. The
Inwood (C-Oi in Figure 1) consists of Paleozoic (~ 500 million year old)
calcitic and dolomitic marble. The Inwood Marble underlies the Inwood section
of northern Manhattan, the Harlem lowland NE of Central Park, occurs as thin
belts in the East River channel and in the subsurface of southeastern
Manhattan, and also crops out in The Bronx and Westchester County. These
exposures are correlative with a laterally continuous outcrop belt of Cambrian
to Ordovician rocks formed along the entire Appalachian chain along the east
coast of North America.
Manhattan Schist. The Manhattan Schist (C-Om in Figure 1) consists
of Paleozoic (~ 450 million year old) massive rusty- to sometimes
maroon-weathering, medium- to coarse-textured,
biotite-muscovite-plagioclase-quartz-garnet-kyanite-sillimanite gneiss and, to
a lesser degree, schist. The unit is characterized by the lack of internal
layering, the presence of kyanite+sillimanite+quartz+magnetite layers and
lenses up to 10 cm thick, cm- to m-scale layers of blackish amphibolite
(metabasalt), and minor quartzose granofels. The unit is a major ridge former
in northern Manhattan, a testament to its durability owing to the lack of
layering and presence of weathering-resistant minerals quartz, garnet, kyanite,
and sillimanite. All three bedrock units (Fordham, Inwood, and Manhattan) are exposed in the vicinity of Inwood and Isham parks.
Inwood Marble and Inwood-Manhattan contact, Isham and Inwood Hill Parks, Inwood section of Manhattan.
[UTM Coordinates: 590.97E / 4524.72N, Central Park quadrangle and 590.66E / 4525.40N, Yonkers quadrangle, respectively.]
Inwood Hill Park is located in the extreme northwest corner of Manhattan Island (Figure 2). The park is bordered by Dyckman Street on the south, the Hudson River on the west, Spuyten Duyvil (Harlem Ship Canal) on the north, and Payson and Seaman Avenues on the east. Isham Park occupies the flat area northeast of Inwood Hill Park extending eastward to Broadway between Isham and West 214th Streets.
The area of Manhattan north of Dyckman Street is known as the Inwood section. Except for Inwood Hill Park, the region is underlain by the Inwood Marble marking the type-locality for that particular unit of NYC bedrock. This unit was originally called the Inwood Limestone by Merrill (1890). Isham Park contains near continuous low relief exposure of the Inwood Marble cut by high-angle conjugate joints which have facilitated the weathering process by allowing aqueous solutions to permeate the rocks (Figure 3). The marble ranges from white- to blue-white to gray-white. Several lithologies occur such as coarse-grained dolomitic marble, fine-grained calcite marble, foliated calc-schist, and marble containing disarticulated siliceous layers (former chert?) and calc-silicate aggregates that stand in relief as cm-scale knots on the weathered surface (Figure 4). Abundant examples of boudinage of the siliceous and calc-silicate layers into lenses occur due to the marked ductility contrast between them and the surrounding marble (Figure 5). Depending on the amount of impurities it weathers gray or tan and produces a sugary-textured surface on outcrops that ultimately develops into residual calcareous sand. Overall, the outcrops illustrate differential weathering with dolomite-silicate units standing in high relief and calcite marble forming depressions.
Figure 2 – Index map showing the location of our ad-hoc “in the rain” field trip stops in Isham and Inwood parks in northern Manhattan.
Figure 3 – Northward view of highly jointed east-dipping Inwood Marble exposed in Isham Park in Manhattan. Although well-foliated, the obvious compositional layering preserves ancient bedding in the rock mass.
Figure 4 – View of differentially weathered Inwood Marble exposed in Isham Park in northern Manhattan. Note the small knots of tremolite (a member of the amphibole family) weathering in high relief.
Figure 5 – View of disarticulated layer of quartzite (former chert?) in differentially weathered Inwood Marble exposed in Isham Park in northern Manhattan. Such features result from the mechanical differences between the competent quartzite and the less competent marble which undoubtedly flowed around the resilient quartzite layers and lenses. Note the black pocket knife for scale.
Although locally variable, the exposure of Inwood Marble in Isham Park trends about N45°E and dips 73°SE. It forms the eastern overturned limb of a large syncline which is cored to the west (in Inwood Park) by kyanite-garnet gneiss of the Manhattan Schist formation. Tight south-plunging folds are locally developed in the Inwood. Older internal structures are not obvious but do occur as isoclinal and asymmetrical folds with shallow plunges.
We enter Inwood Park by following the path past the playground. The first prominent ridge to your left is composed of Manhattan Schist which also dips steeply toward the SE, essentially parallel to the orientation of the Inwood Marble exposed in Isham Park. Both units are part of a huge south-plunging syncline with the Manhattan Schist preserved in the central core of the structure (Figure 6). Inverted topography results from the marked contrast in weathering susceptibility afforded by the marble and schist. In the overall wet temperate climates such as we experience in this region, carbonate rocks (such as the Inwood Marble) weather and dissolve much more readily than do silica-rich rocks of the Manhattan Schist. As a result, structural downfolds (synclines) tend to hold up the topographically high ridges and structural upfolds (anticlines) typically underlie the low valleys. Such topographic inversions are well known in the folded central and northern Appalachians.
Figure 6 – Block diagram illustrating the structural geology of Inwood and Isham parks. Note that the topographically higher portions of Inwood Park are underlain by the Manhattan Schist (green) and that the topographically lower portions are underlain by the Inwood Marble (yellow). This is the result of the difference in weathering susceptibility of the Inwood and Manhattan. In overall humid, wet climates such as we experience in this region, carbonate rocks (such as the Inwood) weather much more readily than do silica-rich rocks of the Manhattan Schist. Note how the topographically higher ridges are structural synclines (downfolds) yet the valleys are underlain by structural upfolds (anticlines). Such relationships are common in the folded Appalachians and are termed “inverted topography”.
The path going north (up-slope) along the westernmost synclinal ridge exposes a zone of massive brown-weathering, blackish amphibolite (metabasalt) of the Manhattan Schist. The structure of the westernmost ridge is another south-plunging syncline overturned toward the northwest. (See Figure 6.) The foliation in the schist is related to folds with axial surfaces oriented N41°E, 75°SE and south-plunging hingelines.
The contact between the middle and lower schist units (the St. Nicholas thrust of Figure 1) is exposed in a 20 m zone from beneath the Henry Hudson Bridge abutment to river level. Structurally beneath the Manhattan Schist unit, a 0.5 m layer of sheared (mylonitic) amphibolite is deformed by folds. Unlike the amphibolite in the schist unit above, which contains subidioblastic hornblende, this exposure of Manhattan amphibolite has been retrograded by intense shearing. Green hornblende porphyroclasts are set in a wavy, anastomosing foliation consisting of colorless amphibole, biotite, and quartz ribbons (Merguerian and Sanders 1991). The thrust zone is structurally complex consisting of intercalated lithologies of the lower and middle schist units together with mylonitic amphibolite.
Directly beneath the bridge, where a dirt trail leads down to the river, a coarse-grained gray-white calcite marble with differentially eroded calc-silicate nodules is exposed at low tide. It is unknown whether the marble exposed at the low-tide mark is an interlayer in the lower schist unit (Ow in Figure 1) or the Inwood Marble. Unquestionably, the Inwood Marble lurks nearby as it wraps around the westernmost ridge of Manhattan Schist and underlies the Spuyten Duyvil, Marble Hill in the Bronx, and the Hudson River. As a geometric result of the southward plunge of the major folds, the oldest unit of the NYC bedrock (Fordham Gneiss) projects up to the surface in the Bronx in a huge vertical exposure immediately across the Harlem Ship Canal. Here, in the Bronx, the Fordham is painted blue with the Columbia University “C”.
REFERENCES
Merguerian, Charles, 1983, Tectonic significance of Cameron's Line in the vicinity of the Hodges Complex - an imbricate thrust model for western Connecticut: American Journal of Science, v. 283, p. 341-368.
Merguerian, Charles, 1985, Geology in the vicinity of the Hodges Complex and the Tyler Lake granite, West Torrington, Connecticut: in R. J. Tracy, editor, Guidebook for fieldtrips, New England Intercollegiate Geological Conference, 77th Annual Meeting, New Haven, Connecticut, p.C2-1-C2-32.
Merguerian, Charles, 1987, The geology of Cameron's Line, West Torrington, Connecticut: in Roy, D.C., ed., Northeastern Section of the Geological Society of America, Centennial Fieldguide, p. 159-164.
Merguerian, Charles; and Sanders, J. E., 1991, Geology of Manhattan and the Bronx: Guidebook for On-The-Rocks 1990-91 Fieldtrip Series, Trip 16, 21 April 1991, Section of Geological Sciences, New York Academy of Sciences, 141 p.
Filename: Isham_Inwood_Park.htm