HOFSTRA UNIVERSITY
1C FIELD GUIDEBOOK
NEW YORK CITY TO NEW JERSEY
Physiographic block diagram of northern Manhattan, the Bronx, the Hudson River and New Jersey showing the generalized structural geology of the region. (Drawing by A. K. Lobeck, Columbia University.)
Field Trip Notes by Professor Charles Merguerian
© 2002
HOFSTRA UNIVERSITY
Department of Geology
Hempstead, NY 11549
Core Course 01C
Physical Geological Science
Geology Field Trip Guidebook
By: Charles Merguerian
Welcome to Hofstra's "Geology Express", an all-day geo-excursion designed to introduce you to the "real world" of how geologists work in the great out-of-doors, or in their terms, "in the field." You are going to see for yourself many of the features that you have been reading about or hearing about in Geology 1C to date plus some features that you will be learning about later in the semester. This trip will take you back in time across terranes that formed as recently as 10,000 years ago, and past those that formed over a billion years ago! All of your favorite Professors from the Geology Department are aboard the geo-excursion buses ready to discuss the geological explanations of features visible out your windows and in the field. They will also serve as your guides and will be available to field questions (yes, dear friends, pun intended!).
So sit back and relax. To get the most from your day, be sure to read this guidebook while we are riding in the bus – rest rooms and reading sickness bags have been provided! The guidebook will explain where you are and where you're going. Participate in the discussions. Ask questions. And, of course, take pictures. In order to maintain a sense of scale, geologists commonly place a ruler or a well-known object in the corner of a photogenic area. (For example, a pen, hammer, coin, B-1 bomber, etc.) Remember, you must produce a photo essay to illustrate twenty different rocks, structures, or products of geologic processes as seen at the field-trip stops. Advanced geology students may be on the trip and will help you, along with your lab and lecture professors, to identify geologically interesting photo opportunities. The guidebook you hold in your hands will help in formulating your captions and detailed instructions on the format and preparation of your photo essay are available from your laboratory professor or instructor.
The foremost point to keep in mind today is James Hutton's concept of the "Great Geological Cycle" -- how the operations of the natural processes going on around us create a geologic record and how geologists can look at the geologic record and work backward to figure out what happened. In this scheme of working backward from the geologic record into geologic history, geologists operate in a totally different way from experimental scientists, such as chemists, for example. In an experimental science, the investigator tries to control the conditions as carefully as possible in order to relate condition to the result. A geologist usually is faced with the result (the geologic record) and is asked, in effect, to figure out what experiment took place.
The geologic record consists of three components that exist in three dimensions: (1) the bedrock, (2) the regolith, and (3) the physiography of the Earth's surface. Geologists commonly use two-dimensional maps and diagrams to illustrate the three dimensions of study and will invoke a fourth dimension into the equation - namely time. That is, we are concerned with the developmental history of a geologic terrain over the broad expanse of time. In the following introductory paragraphs, we review each of these components and show how they are connected.
Bedrock
To the casual observer, bedrock is known as the place that the Flintstones and Rubbles live. Bedrock, to geologists, means the continuous solid crustal rock of a continent that exists everywhere, either exposed at the Earth's surface or occurs buried by loose "dirt" or "alluvium" ("soil" to the engineers). The unconsolidated material that covers the bedrock is collectively designated as the regolith. Bedrock has to be dealt with using hammers and hand lenses. By contrast, one uses a shovel to do business with the regolith.
Bedrock is a collective designation that includes igneous, sedimentary, or metamorphic rocks. On Long Island, bedrock exposures are extremely scarce. A few examples are known near the Queens County Courthouse, and in Long Island City, at the extreme western edge of the island. In Manhattan, the Bronx, and Staten Island bedrock is exposed at the surface in scattered rocky knolls known as outcrops. Elsewhere, the bedrock has been buried by sandy regolith to depths of hundreds of meters. The ancient metamorphic and igneous rocks of the continents are largely covered by a veneer sedimentary rock. In areas where no natural outcrops exist, regolith covers the bedrock sequence. Because we want to show you some different kinds of bedrock and regolith, we must travel off Long Island to the Bronx and to northern New Jersey. (See figure on cover.) Let us begin by discussing some basic geological aspects of sedimentary strata.
The regolith collectively designates the loose, diggable material that overlies the bedrock. The upper part of the regolith may have an organic component and be capable of supporting plant growth. If so, then this upper part is known as a soil.
Regolith forms from bedrock or from lava being extruded from a volcano. Some regolith forms by the weathering of bedrock (a slow process). Other regolith is the product of several fast-acting processes by which bedrock can be broken to bits quickly, so to speak. The bedrock forming steep rocky cliffs may cascade downward rapidly in a rock avalanche. A product of such an avalanche is a local body of regolith. The impact of an extraterrestrial object such as a meteorite may create regolith, known as ejecta. Beneath a glacier bedrock is ground into regolith. For those who cannot wait for geologic time to do its thing, during certain kinds of volcanic activity vast quantities of "instant" new regolith are created by explosions (which may involve new igneous rocks in their formative stages or old rocks through which the magma passes on its way to the surface). Because of its enormous surface area and glass content, volcanic regolith can be converted to soil very rapidly. Much volcanic regolith is propelled high into the atmosphere and is responsible for beautiful sunsets.
An important point to be determined about regolith is whether it is forming by weathering of the underlying bedrock (= residual regolith) or whether it is unlike the underlying bedrock and thus has been moved in from some other place (transported regolith). Two common processes by which transported regolith is brought into an area are: (1) glaciers, and (2) the wind.
The Earth's landscape has been fashioned by the operation of many distinctive processes that we shall be studying in the next few weeks. Examples are valleys carved by streams, beaches built by waves, intertidal salt marshes growing at the edge of the sea, dunes blown by the wind, ridges heaped up by glaciers, various features eroded by glaciers, deltas built at the mouths of rivers, and the general shapes of the hills and valleys as a result of the general process of erosion. Your instructor will explain the origin of any distinctive surface features that we encounter. Regions within which the bedrock or the regolith display distinctive landscapes are known as physiographic provinces.
In summary, the three components of the geologic record are: (1) the bedrock, (2) the regolith, and (3) the physiography or “shape” of the Earth's surface. All are connected by the operation of the rock cycle by which rocks that form beneath the Earth's surface are rearranged at the Earth's surface and then may be still further changed by being buried again to great depths. Our objectives are to be able to identify the features found in the bedrock, to determine if any regolith is residual or transported, and to understand distinctive surficial features.
This field trip guidebook is arranged to discuss the geologic scenery of driving segments (called legs) and descriptions of the three sites (called stops) that you will examine in detail. Through the aid of sketches, drawings, index maps, geologic maps and cross-sections (collectively called figures in the text), you can follow your field trip along each step of the way and gain some insight into identifying what is “important” in this world of Geology. Most of the detailed stop descriptions and tables provided herein are excerpted from field-trip guides written by Merguerian and Sanders in the interval 1988 to 1998 and from more contemporary research.
An important point to be gained from this trip is familiarity with some parts of the geologic time scale. To help you with this, we have included Table 1, a geological time scale that should be consulted while reading the following discussion. We will introduce a given geologic feature by referring to its age in years, but then will shift to the corresponding term from the geologic time scale. Table 2 is a generalized description of the major geologic "layers" (described below) found in southeastern New York State and vicinity. Table 3 provides a stratigraphic classification of the Pleistocene deposits of New York City and vicinity. Now, for you, a brief geological primer to give you some instant insights into the world of geology.
In dealing with the geologic structures in sedimentary rocks, the first surface one tries to identify positively is bedding or stratification. The boundaries of strata mark original sub-horizontal surfaces imparted to sediment in the earliest stage of the formation of sedimentary rock. As such, strata represent sequences of time. Imagine how such strata, buried by the weight of overlying strata are compressed and lithified to produce sedimentary rocks. If they are subject to compressive forces generated by the advance of convergent lithospheric plates, differential force necessary for rock deformation (folds and faults) can occur. Contrary to older ideas, we now realize that vertical burial cannot cause regional folds (although small-scale slumping, stratal disharmony, and clastic dikes are possible). Rather, resolved differential stress must be applied to provide the driving force to bring about deformation (folds and faults).
Surfaces of unconformity mark temporal gaps in the geologic record and commonly result from periods of uplift and erosion. Such uplift and erosion is commonly caused during the terminal phase of regional mountain-building episodes. As correctly interpreted by James Hutton at the now-famous surface of unconformity exposed in the cliff face of the River Jed (Figure 1), such surfaces represent mysterious intervals of geologic time where the local evidence contains no clues as to what went on! Usually, they mark periods of tectonism, uplift, and erosion produced during mountain building in adjacent areas. Thus, by looking elsewhere, the effects of a surface of unconformity of regional extent can be recognized and piecemeal discovery of evidence for filling in the missing interval may be found.
Unconformities occur in three basic varieties - angular unconformities, nonconformities, and disconformities. Angular unconformities (such as the River Jed) truncate dipping strata below the surface of unconformity and thus exhibit angular discordance at the erosion surface. Nonconformities separate sedimentary strata above the erosion surface from eroded igneous- or metamorphic rocks below. Disconformities are the most-subtle variety, separating subparallel sedimentary strata. They are commonly identified by paleontologic means, by the presence of channels cut into the underlying strata, or by clasts of the underlying strata in their basal part. The strata above a surface of unconformity may or may not include clasts of the underlying strata in the form of a coarse-grained, often bouldery basal facies.
During today’s trip we will see many different types of unconformities in the field. For example, as we drive across the Hudson River later in the day we travel across a nonconformity, with tilted sedimentary rocks of the Newark Basin resting unconformably upon metamorphic rocks of the Manhattan Prong. (See cover figure.) In this case the unconformity is tilted westward along with the overlying Newark strata.
Following the proposal made in 1963 by L. L. Sloss, surfaces of unconformity of regional extent within a craton are used as boundaries to define Stratigraphic Sequences. It's now time to turn to some geometric aspects of the features formed as a result of deformation of rocks in the Earth. We start with folds.
Figure 1 - Unconformity with basal conglomerate along the River Jed, south of Edinburgh, Scotland. From James Hutton's "Theory of the Earth", (1795).
The layers present in bedrock may be horizontal, vertical, or disposed at some intermediate angle. An important goal of many geologic field investigations is to work out the arrangement of the layers and to determine the geologic structure of a region. A widespread kind of geologic structural feature is a fold, defined as a bend in the layers. When rocks bend, they are behaving in a condition defined as ductile (as contrasted with brittle). We have not yet discussed folds in class, but we need to know some things about them. At Orchard Beach (Stop 1), for example, it is impossible to find any bedrock that has not been folded.
When subjected to differential forces, under high confining pressures and elevated temperatures, rocks (like humans) begin to behave foolishly, squirming in many directions and upsetting the original orientation of primary- or secondary planar- and linear features within them. New planar and linear fabrics develop in the rock mass. Geologists try to sort out the effects of deformation by working out the relative order in which these structural surfaces or linear features formed. In dealing with the structural geology of sedimentary rocks, the first surface to positively identify is bedding or stratification. At crustal levels below 10 km folds and faults are accompanied by recrystallization and reorientation of newly formed metamorphic minerals. More on metamorphic textures below - for now let's discuss some geometric aspects of structural geology.
If layers are folded into convex upward forms we call them anticlines. Convex-downward fold forms are called synclines. In Figure 2, note the geometric relationship of anticlines and synclines. In eroded anticlines, strata forming the limbs of the fold dip away from the central hinge area or axis of the structure. In synclines, the layers forming the limbs dip toward the hinge area. As such, older stratigraphic layers are expected to peek through in the axes of eroded anticlines but younger strata are preserved in the eroded axes of synclines. In metamorphic terranes, field geologists are not always sure of the stratigraphic topping direction of the metamorphosed strata. Thus, we tend to use the terms "antiform" and "synform" which describe the shapes of folds but do not imply anything about the relatative ages of the strata.
Axial surfaces of folds physically divide the fold in half. Note that in Figure 2, some folds are deformed about a vertical axial surface and are cylindrical about a linear fold axis that lies within the axial surface. The locus of points connected through the domain of maximum curvature of the bedding (or any other folded surface of the fold) is known as the hinge line (which is typically parallel to the fold axis). This is geometry folks and we have to keep it simple so that geologists can understand it.
Realize that in the upright folds shown in Figure 2, axial surfaces are vertical and fold axes, horizontal. Keep in mind that folding under metamorphic conditions commonly produces a penetrative mineral fabric with neocrystallized minerals (typically micas and amphiboles in matrix of recrystallized quartz and feldspar) aligned parallel to the axial surfaces of folds. Such metamorphic fabrics are called foliation, if primary, and schistosity, if secondary. Minerals can also align in a linear fashion producing a metamorphic lineation. Such features can be useful in interpreting a unique direction of tectonic transport or flow direction in ductile rock masses. Because folds in metamorphic rocks are commonly isoclinal (high amplitude to wavelength aspect ratio) with limbs generally parallel to axial surfaces, a penetrative foliation produced during regional dynamothermal metamorphism will generally parallel the reoriented remnants of stratification (except of course in the hinge area of folds). Thus, a composite foliation + remnant compositional layering is commonly observed in the field. Departures from this common norm are important to identify as they mark regional fold hinge areas.
Folds could care less about the orientation of their axes or axial surfaces and you can certainly imagine tilting of the axial surface, to form inclined or overturned folds, or a sub-horizontal axial surface, to form recumbent folds, all accomplished by keeping the fold axis sub-horizontal (Figure 2). In addition, we can keep the axial surface vertical and alter the plunge of the axis from horizontal to some angle other than 0° to produce a plunging fold. Such folds can be plunging anticlines (antiforms) or plunging synclines (synforms). Vertical folds (plunging 90°) also occur, in which case the terms anticline and syncline are not meaningful. Most folds in complexly deformed mountain ranges show the effects of more than one episode of deformation and as such their ultimate configuration can be quite complex (i.e., plunging folds with inclined axial surfaces and overturned limbs).
Figure 2 - Composite diagram from many introductory texts showing various fold styles and nomenclature as discussed in the text.
Depending upon the direction in which the rocks were being transported when the folds formed, one of three categories of folds may come into being. These three categories of folds are named from the letters of the English alphabet that they resemble: S, M, and Z. As soon as you spot a fold, study it to find out if it belongs to the S category, the M category, or to the Z category. Typically, fold categories run in packs and the place where the folds of the S pack change over to those of the Z pack (or vice versa) is along the median line (axial surface) of a larger fold where M-folds develop.
Structural geologists use a relative nomenclature to discuss superimposed episodes of deformation (Dn), folding (Fn), foliation (Sn), and metamorphism (Mn), where n is a whole number starting with 1. Bedding is commonly designated as S0 (or surface number zero) as it is commonly overprinted by S1 (the first foliation). To use this relative nomenclature to describe the structural geology of an area, for example... "during the second deformation (D2), F2 folds formed with the development of an axial planar S2 schistosity under progressive M1 metamorphic conditions”.
One final note on folding -- it is generally agreed, in geologically simple areas, that axial surfaces form perpendicular to the main stress (force) that produced the fold. Therefore, the orientation of regional fold axial surfaces gives some hint as to the direction of application of the active forces (often a regional indicator of relative lithospheric plate convergence). In complex regions, the final regional orientation of the structures is a composite result of many protracted pulses of deformation, each with its unique geometric attributes.
A fault is defined as a fracture along which the opposite sides have been displaced. The surface of displacement is known as the fault plane (or fault surface). The enormous forces released during earthquakes produce elongate gouges within the fault surface called slickensides. They may possess asymmetric linear ridges that enable one to determine the relative motion between the moving sides (Figure 3, inset). The block situated below the fault plane is called the footwall block and the block situated above the fault plane, the hanging-wall block. Extensional force causes the hanging-wall block to slide down the fault plane producing a normal fault. [See Figure 3 (a).] Compressive forces drive the hanging-wall block up the fault plane to make a reverse fault. A reverse fault with a low angle (<30°) is called a thrust fault. [See Figure 3 (b).] In all of these cases, the slickensides on the fault will be oriented more or less down the dip of the fault plane and the relationship between the tiny "risers" that are perpendicular to the striae make it possible to determine the relative sense of motion along the fault. Experimental- and field evidence indicate that the asymmetry of slickensides is not always an ironcled indicator of relative fault motion. As such, displaced geological marker beds or veins are necessary to verify relative offset. Fault motion up- or down the dip (as in normal faults, reverse faults, or thrusts faults) is named dip-slip motion.
Rather than simply extending or compressing a rock, imagine that the block of rock is sheared along its sides (i. e., that is, one attempts to rotate the block about a vertical axis but does not allow the block to rotate). This situation is referred to as a shearing couple and could generate a strike-slip fault. [See Figure 3 (c).] On a strike-slip-fault plane, slickensides are oriented subhorizontally and again may provide information as to which direction the blocks athwart the fault surface moved.
Figure 3 - The three main types of faults shown in schematic blocks. Along a normal fault (a) the hanging-wall block has moved relatively downward. On a thrust fault (or reverse fault) (b) the hanging-wall block has moved relatively upward. Along a strike-slip fault (c), the vertical reference layer (black) has been offset by horizontal movement (left-lateral offset shown here). Inset (d) shows segments of two blocks along a slickensided surface show how the jagged "risers" of the stairsteps (formed as pull-apart tensional fractures) can be used to infer sense of relative motion. [(a), (b), (c), Composite diagram from introductory texts. (Inset from J. E. Sanders, 1981, fig. 16.11 (b), p. 397.)
Two basic kinds of shearing couples and/or strike-slip motion are possible: left lateral and right lateral. These are defined as follows. Imagine standing on one of the fault blocks and looking across the fault plane to the other block. If the block across the fault from you appears to have moved to the left, the fault is left lateral [illustrated in Figure 3 (c)]. If the block across the fault appears to have moved to the right, the motion is right lateral. Convince yourself that no matter from which block you can choose to observe the fault, you will get the same result! Naturally, complex faults show movements that can show components of dip-slip- and strike-slip motion, rotation about axes perpendicular to the fault plane, or reactivation in a number of contrasting directions or variety. This, however, is no fault of ours.
Tensional- or compressional faulting resulting from brittle deformation, at crustal levels above 10 to 15 km, is accompanied by seismicicity and the development of highly crushed and granulated rocks called cataclasites (including fault gouge, fault breccia, and others). Begining at roughly 10 to 15 km and continuing downward, rocks under stress behave aseismically and relieve strain by recrystallization and internal flow. These unique metamorphic conditions prompt the development of highly strained (ribboned) quartz, feldspar porphyroclasts (augen), and frayed micas and results in highly laminated ductile-fault rocks called mylonites.
The identification of such ductile fault rocks in complexly deformed terranes can be accomplished only by detailed mapping of metamorphic lithologies and establishing their geometric relationship to suspected mylonite zones. Unfortunately, continued deformation under load often causes early formed mylonites to recrystallize and thus to produce annealed mylonitic textures (Merguerian, 1988), which can easily be "missed" in the field without careful microscopic analysis. Cameron's Line, is an original ductile fault zone (mylonite) having a complex geologic history that includes recrystallization and post-tectonic brittle reactivation.
Although we have defined bedrock as being "continuous" and "solid," in reality it may consist of various layers and display several kinds of cracks (syn.: fractures where it has been broken) or partings, which are surfaces between layers along which blocks of the rock can separate. Along the cracks, the facing sides may or may not have been displaced. A fracture along which the adjacent blocks have not been displaced is known as a joint. The existence of joints is a signal that the rock behaved in response to deformation or, perhaps, simple unloading, as a brittle solid. A fracture along which the adjacent blocks have been displaced is known as a fault. Joints rarely exist in isolation. Typically they form a group of parallel fractures known as joint sets. Several joint sets may intersect in such a manner as to break the solid bedrock into many large blocks, each ending at a joint.
Many joint faces are simply planar surfaces that display the fresh bedrock. Other joint faces have been coated with one or more mineral linings. Pyrite is a common deposit on joint faces. The proof that pyrite lines some joint faces comes from deep borings in quarries and holes drilled for other purposes. The rocks from these borings are fresh; the samples come from depths where conditions have excluded rainwater. When it is brought close to the Earth's surface and into contact with oxygen-bearing rainwater, pyrite (a sulfide mineral) decomposes readily. The iron from the pyrite is oxidized and the yellow-tan mineral limonite and/or its reddish cohort, hematite forms during chemical weathering. The sulfur from the pyrite is oxidized and combines with water to form sulfuric acid. Other minerals commonly found on joint faces include calcite, quartz, and chlorite (a green mineral of the mica group).
The minerals that grow along the joint faces may fill the formerly empty space. If they do this, they form a tabular mineral deposit known as a vein. In their shapes, many veins resemble dikes, which you may recall are discordant tabular plutons - bodies of igneous rock. The difference between a vein and a dike reflects the mode of growth of the crystals. In a vein, the minerals grew outward from a solid-rock face into an opening toward the center. In a dike, the magma wedged itself into the crack and the minerals forming the igneous rock crystallized from countless nucleii scattered throughout the magma.
Joints are the perfect locii for physical and chemical weathering to occur. During a rainstorm, rainwater seeps into the openings along joints. The seeped-in water evaporates very slowly; thus, it persists between rain showers. The water stored in the openings found along joints can be used as a solvent for chemically active fluids and by tree roots. The initial roots that grow downward along a joint may be tiny, hairlike features that can insert themselves into the most minute cracks. With time, however, the sizes of the roots enlarge. Eventually, the roots may pry loose large blocks that are bounded by joints. See if you can spot any examples of tree roots growing along joints.
We start by examining the shape of the Earth's surface in the region we plan to visit. Figure 4 is an oblique bird's eye view of the territory included on our field trip. The diagram, drawn to emphasize the contrasts in physiographic characteristics, has numbers of our intended Stops 1 through 3 shown. Because the group is so large, half of the buses will go first to Stop 1 and the other half to Stop 2. Those in the first group will proceed in the order 1, 2, and then 3. Those in the second group will proceed in the order 2, 3, and 1. We hope that you can adjust to the relative order of your field trip route by flipping to the appropriate pages in this guide. If this is confusing to you, we then suggest you take two weeks off from Hofstra – then quit.
The first driving leg of your journey will take you across a number of distinctive geologic belts (called physiographic provinces) that are roughly oriented northeasterly, parallel to the main trend of the Appalachian mountain belt. We will travel from the buried coastal plain of Long Island across the Manhattan Prong of New York City to the Newark basin of New Jersey. In doing so we will cross major unconformities and former plate boundaries. The major geologic layers on this part of the Appalachian mountain belt are listed, from oldest [Layer I] to youngest [Layer VII] in Table 2 and shown in Figure 4. For ease of discussion, we describe the major geological layers found in the area of your field trip from the top down (meaning from youngest to oldest).
Figure 4 - Physiographic diagram of northern New Jersey and adjacent regions of New York with cut-away vertical slice to show geologic structure. Note positions of our field-trip Stops (1, 2, and 3). Drawing by A. K. Lobeck, Columbia University.
Long Island is capped by a thin veneer of Pleistocene sediment [Layer VII in Table 2] ranging in age from 10,000 years old to possibly much older (perhaps 200,000 years ago). The glacial sediment is collectively called drift but consists of till and outwash. It is not unusual to find large exotic boulders (called erratics) in Long Island's glacial drift from New England and Canada. In fact, because the glaciers ground southward from the north in possibly five separate advances from two contrasting directions (NNE and NW), the astute observer can find different striae and erratics from Canada and New England as well as from New Jersey and Pennsylvania on our trip. We will see a number of glacial striae and erratics at all of our stops today!
The prominent lobate, curvilinear fork-like spines of Long Island are ridges composed of material bulldozed at the snout of a glacier to form the Harbor Hill and Ronkonkoma "moraines" (Figure 5). Recent work by Sanders and Merguerian (1991a, b; 1992a; 1994a, b, c) and Sanders, Merguerian, and Mills (1993), indicate that they are largely outwash and not till as advertised and promoted by previous workers (with top-notch PR departments on Madison Avenue). According to these prevailing "experts", the Long Island "moraines" mark the southward terminus of a 10,000 year old event (“Woodfordian” glacier) when meltback of the glacial front and a sudden surge heaped up linear ridges of ground up rocks, clay, and debris (including mature sediments from the underlying Cretaceous) to produce moraine lobes. Not so fast! We have a different idea based on our moronic studies.
After over twenty-five years of combined research, Professors Sanders and Merguerian have compiled ample evidence in the form of crosscutting glacial striae, roche moutonnée structures, superposed tills of contrasting sedimentology and boulder content and by the petrologic and field identification of unique indicator stones, to help develop a more complicated, protracted, and more ancient glacial history for the region (Tables 2, 3). The glaciation of Westchester, New York City, and Long Island included a number of advancing glaciers (five, we think) which typically terminated their southward advance in the present location of Long Island Sound (Figure 6) and deposited outwash fans farther south on Long Island by melt waters above a tilted sequence of older, eroded sediment strata of the Cretaceous Coastal Plain (Layer VI in Table 2). A few advances covered Long Island to create the famous terminal moraine ridges but these are from older glaciers that flowed from the NW and were deposited on older outwash terraces as shown correctly by Fuller (1914) in Figure 5.
According to our combined analysis, the earliest glacial advance (V) flowed from the NW to SE, deposited reddish brown till and outwash in Staten Island and Garvies Point, Long Island and includes the Jameco Gravel. Glacier IV advanced from the NNE to SSW and deposited a gray till at Teller’s Point in Westchester and the lower till at Target Rock, Long Island. Glacier III was from the NW and deposited deltaic sediments (Manhasset Formation of Fuller [1914]) into Glacial Lake Long Island and eventually the Ronkonkoma moraine. The Harbor Hill moraine was deposited by Glacier II, also from the NW. The final (“Woodfordian”) glacier flowed from the NNE and barely touched Long Island in our view. (See Figure 6.) Instead, great volumes of water once again shed stratified drift southward to cover Long Island. We have no age dating yet but base our model on superposition and careful regional studies. Most modern workers have adopted a multi-glacier hypothesis in recent years based on this new work but the number and timing of glacial advances remains controversial. Suffice to say that the “One glacier did it all” school of geology has increased their acceptance level 100% - they now admit to two. We say, “it’s a good start and a step in the right direction”.
Figure 5 - Map of Long Island showing the two prominent terminal-moraine ridges and profile-sections illustrating Fuller's interpretation of the subsurface relationships. Further explanation in text. (Map from A. K. Lobeck, 1939, fig. on p. 309 with location lines of Fuller's sections added. Profile-sections from M. L. Fuller, 1914, fig. 107, p. 120, rearranged to place easternmost section at top, westernmost at bottom.)
Figure 6 - Restored profile-section from Connecticut to Long Island showing terminus of continental glacier standing in what is now Long Island Sound and spreading compositionally mature outwash sand and gravel southward to bury the Upper Cretaceous strata of Long Island. Extension of Cretaceous beneath glacier is schematic, but is based on the lack of feldspar in much of the Long Island outwash. (Drawn by J. E. Sanders in 1985 using regional relationships shown in W. deLaguna, 1963, fig. 2, p. A10.)
The Cretaceous Coastal Plain strata of eastern North America are found to underlie broad areas of the continental margin (Figure 7). Except for limited exposures at Garvey's Point, Sand's Point, and elsewhere on the north shore of Long Island, Cretaceous Coastal Plain strata (Layer VI in Table 2) are generally not directly observed cropping out on Long Island.
Cretaceous strata exist in the subsurface of Long Island, Queens, and in Brooklyn as a sequence of southward tilted layers of gravel, sand, and clay. (See Figure 6.) They overlie crystalline bedrock and Newark strata based on seismic reflection data. North-south-trending stream channels are etched onto the Cretaceous strata. These stream channels were controlled by and etched into the southward tilted Cretaceous layers during and after Pliocene uplift, southward tilting of the Cretaceous strata, and subsequent erosion. (See Table 1.) In addition, the channels were undoubtedly modified by the erosive action of the earliest glacier (V in Table 2) and glacial meltwaters. Thus, after Pliocene oceanward tilting, differential uplift, and erosion, the Cretaceous sequence was beveled and unconformably overlain by several glacial sequences of Pleistocene age (Figure 7).
Figure 7 - Diagrammatic map of part of Atlantic Coastal Plain (open stipple) from Washington, D. C. (W at lower L) to Cape Cod, Massachusetts showing how the inner lowland (close stipple) at the preserved edge of the coastal-plain strata has been submerged northeast of New York City (N.Y.). Inset schematic profile-section (large vertical exaggeration) extends SE from Philadelphia, PA (P) to Atlantic Ocean off Atlantic City, NJ (not shown on map). (A. K. Lobeck, 1939, p. 456.)
The overlying glacial deposits form a permeable layer that blankets the Cretaceous strata, two of which (the Magothy and Lloyd formations), consist of highly porous, nonlithified sands. As such, they form an important subterranean reservoir of fresh water (called an aquifer) that is recharged with rain water and partially depleted everytime you turn on a faucet, wash your car, water your lawn, water your hamster, or fill up your swimming pool. Strata of Cretaceous age do crop out locally, as mentioned, and in regions to the south of Long Island from Staten Island to Florida and forms the Atlantic Coastal Plain physiographic province. Professor Bennington has worked on the stratigraphy and fossil distribution patterns of the Cretaceous strata in New Jersey. Autographed copies of his papers and a complete “signature” line of sneakers are available at Hip-Hop clubs across the country and at Geology Club meetings, Wednesdays, during the common hour in Gittleson 135!
The tilted and eroded remnants of the Newark strata are exposed along the west side of the Hudson River, from Stony Point south to Staten Island (Figure 8). As shown there and on the diagonal cut-away slice in Figure 4, the Newark strata generally dip about 15° to the northwest. The west side of the Hudson River channel is marked by an impressive cliff face of highly jointed mafic rock. These spectacular cliffs mark the raw, eroded edge of the Palisades intrusive sheet, a concordant tabular sill-like igneous body, formerly intruded at depths of 3 to 4 km into formerly buried Newark strata.
Figure 8 - Interpretive geologic section across the Hudson River in the vicinity of the George Washington Bridge showing westward tilted strata of the Newark Basin and the Palisades intrusive sheet and their nonconformable relationship to folded metamorphic rocks of New York City. (After Berkey, 1948; digitally enhanced by Geology 18 students.)
The formal stratigraphic name for the Newark strata is Newark Supergroup. Included are various sedimentary formations, of which the basal unit is the Stockton Arkose. Above it is the Lockatong Formation, the unit into which the Palisades sheet has been intruded. Higher up are other sedimentary units and three interbedded sheets of mafic extrusive igneous rock (ancient lava flows), whose tilted edges are resistant and underlie ridges known as the Watchungs. The age of the sedimentary units beneath the oldest extrusive sheet is Late Triassic. The remainder of the formations are of Early Jurassic age. At Stops 2 and 3 you will examine some of the red-colored sedimentary rocks of the Newark Basin and also see basaltic volcanic rocks of the Orange Mountain Formation. (See Figure 4.)
The Newark sedimentary strata were deposited in a fault-bounded basin (Figure 9) to which the sea never gained access. In this basin, the filling strata were deposited in various nonmarine environments, including subaerial fans, streams, and shallow- and deep lakes. Lake levels varied according to changes in climate. After the Newark strata had been deeply buried, they were elevated and tilted, probably during a period of mid-Jurassic tectonic activities (Merguerian and Sanders, 1994b).
Figure 9 - Sketch map and geologic profile-section, southeastern New York and adjacent New Jersey. Note lack of correspondence in scale between profile-section and map, with resulting expansion of the length of AB and shortening of line segment BC. (From Wolff, Sichko, and Liebling, 1987.)
An important point to be established about the Palisades intrusive sheet is its date of intrusion. Its time of intrusion is thought to coincide with the time of the extrusion of one or more of the Watchung extrusive sheets; the problem is to prove that the Palisades was intruded at the same time as one of the Watchung extrusives. According to Sichko (1970 ms.), Puffer (1988), and Husch (1990), a likely correlation is between the high-Ti magma that solidified to form the Palisades sheet and the various lavas that cooled to form the multiple flows of the Orange Mountain Formation (First Watchung). The general absence of chilled zones within the Palisades sheet implies that all pulses of magmatic activity took place before the igneous rock had cooled. Based on their interpretation of the time value of sediments deposited under the influence of climate cycles in the associated sedimentary strata, Olsen and Fedosh (1988) calculate that approximately 2.5 Ma elapsed between the time of extrusion of the Orange Mountain Formation and that of the Preakness Formation. This means that if igneous activity within the Palisades sheet took place at the same time as that of the extrusion of these two ancient lava flows, then more than 2.5 Ma were required for the sheet-like Palisades intrusive to cool.
If one can prove the synchroneity of intrusion of the Palisades intrusive sheet with one or more of the Watchung flows, then a further point is settled: depth of intrusion. The depth of intrusion then becomes the stratigraphic thickness of Newark strata between the Palisades and the First or Second Watchung basalt (roughly 3 to 4 km). Thus, the Palisades intrusive sheet may have been dropped to lower (warmer) crustal levels while it was attempting to cool. Using the average geothermal gradient of 30°C/km the increase in temperature would exceed the boiling point for water! This brings the discussion of cooling to a full boil and provides an explanation for long duration cooling history for the Palisades intrusive sheet. Studies by Merguerian and Sanders (1992a, 1994f, 1995a, b) suggest that the Palisades magma was intruded under shallow conditions (~3 to 4 km) and that the magma may have originated from the vicinity of Staten Island and flowed northeastward, rather than toward the southeast from fractures related to the Ramapo fault as most previous workers have argued. Vertical flow features, the great thickness of the Palisades in NYC, and the central location of Staten Island support this hypothesis.
Figures 4, 7, 8, and 9 show that the deepest layer in this area consists of Paleozoic and Proterozoic metamorphic and metamorphosed igneous rocks crystalline basement rocks. These older strata (Layers I and II in Table 2) are continuous with rocks exposed along the deeply-eroded spine of the Appalachian mountain belt that extends from Georgia northeastward through Maine. Some of these crystalline rocks crop out in the Bronx (Stop 1) and in Manhattan and form the stable bedrock core atop which the skyscrapers of New York City. They are subdivided into two basic sub-layers, an older sequence consisting of ~1.0 Ga (billion year old) gneisses (Layer I) and a younger sequence of complexly deformed and internally sheared schist, gneiss, amphibolite, and marble (Layer II). Included in Layer II are Late Proterozoic former rift-facies rocks found in the Bronx and Westchester known as the Ned Mountain Formation. Rocks of Layers I and II are overlain by Devonian strata of the Catskills (Layer III in Table 2). Dr. Wolff has performed important research on the Catskills. Copies of his publications can be sought at all Geology Club meetings on Wednesdays during the Common Hour in Gittleson 135.
The rocks of the Grenville cycle (Layer I of Table 2) are the oldest recognized strata in southeastern New York. (Note the black stippled areas in Figure 4.) They include the Fordham Gneiss in New York City area and the Hudson Highlands gneisses (Figure 10). The Highlands gneisses are composed of complexly deformed layered feldspathic gneiss, schist, amphibolite, calc-silicate rocks, and massive granitoid gneiss. They constitute a complex where metamorphosed intrusive rocks form an integral part of the sequence but whose internal stratigraphic relationships are poorly understood. Grenville-aged (Proterozoic Y) basement rocks include the Fordham Gneiss of Westchester County, the Bronx, and the subsurface of western Long Island (Queens and Brooklyn Sections, NYC Water Tunnel #3), the Hudson Highland-Reading Prong terrane, the Franklin Marble Belt and associated rocks, and the New Milford, Housatonic, Berkshire, and Green Mountain Massifs of New England. (See Tables 1 and 2.) Taken as a whole, the ancient Grenville-cycle sequence unconformably underlies the younger Appalachian-cycle rocks (Layer II) described in the next section.
Southeast of the Hudson Highlands, the Grenville rocks are known as the Fordham Gneiss of the Manhattan Prong. Here they have been intricately folded with the Paleozoic-aged rocks of the Appalachian cycle. In the Pound Ridge area (PR in Figure 10), the Fordham Gneiss has yielded 1.1 Ga 207Pb/206Pb zircon ages (Grauert and Hall, 1973) that falls well within the range of the Grenville orogeny. Rb/Sr data of Mose (1982) suggest that metasedimentary- and metavolcanic protoliths (parent material) of the Fordham date back to 1.35 Ga.
In Westchester County, subunits in the Fordham are cut by the Pound Ridge Gneiss and correlative Yonkers Gneiss. Using Rb-Sr techniques, Mose and Hayes (1975) have dated the Pound Ridge Gneiss as Proterozoic Z in age (579+21 Ma). This gneiss body shows an intrusive or possibly a nonconformable relationship with the Grenvillian basement sequence (Dr. Patrick Brock, personal communication). The Yonkers Granitic Gneiss (Y in Figure 10) has yielded ages of 563+30 Ma (Long, 1969b) and 530+43 Ma (Mose, 1981). The Pound Ridge along with the Yonkers Gneiss, are thought to be the products of latest Proterozoic alkali-calcic plutonism (Yonkers) and/or -volcanism (Pound Ridge) in response to rifting of the ancient Gondwanan supercontinent.
The Grenville-cycle units are unconformably overlain either by the Lower Cambrian Lowerre quartzite (Hall, 1968a, b, 1976; Brock, 1989) or by a vast rift sequence (now metamorphosed) of potash feldspar-rich felsic gneiss, calc-silicate rock, volcaniclastic rock, amphibolite gneiss, and minor quartzite (Ned Mountain Formation of Brock 1989, 1993). Thus, the Grenville-cycle sequence represents the ancient continental crust of proto-North America that became a trailing edge, passive continental margin early in the Paleozoic Era.
Figure 10 - Simplified geologic map of the Manhattan Prong showing the distribution of metamorphic rocks from Grenville cycle (Layer I; rocks of Proterozoic Y age) and early phases of Appalachian cycle (Layer II; rocks of late Proterozoic to Early Paleozoic age). Most faults and intrusive rocks have been omitted. (From Mose and Merguerian, 1985, fig. 1, p. 21.)
The crystalline bedrock of New York City originated as sedimentary and volcanic rocks that originally formed adjacent to the early Paleozoic shelf edge of eastern North America (Figure 11). The sedimentary apron was deposited across a passive continental margin and produced two sub-parallel belts – a shallow water sequence adjacent to the shoreline consisting of sandstone, limestone, and shale and a deeper water sequence away from the shelf edge consisting of greywacke, shale, and volcanic strata (Figure 12). During the Taconic orogeny of medial Ordovician age (See Tables 1 and 2), these disparate sequences were juxtaposed, highly folded and metamorphosed in a subduction zone that formerly operated adjacent to the east coast of North America. An arc-continent collision (Figure 13) was the first in a series of Paleozoic plate tectonic events that ultimately produced the Appalachian Mountain chain.
Figure 11 - Paleogeographic map of North America in Early Paleozoic time showing how the east coast on North America was awash in volcanic island arcs (Kay, 1951).
Figure 12 - Block diagram showing the Lower Paleozoic continental shelf edge of embryonic North America immediately before the deposition of Layer IIB. Current state outlines are dotted. The depositional areas for Layers IIA(W) and IIA(E), and the position of the Taconic arc and foreland basin are shown.
Thus, in western Connecticut and southeastern New York, Layer I rocks of the Grenville cycle are overlain by Cambro-Ordovician formations that are products of the early (Taconian) part of the Appalachian cycle (Layer II of Table 2). These sedimentary- and igneous rock units have been highly metamorphosed, folded, and faulted. They began their geologic lives roughly 550-450 million years ago as thick accumulations of both shallow- and deep-water sediments adjacent to the Early Paleozoic shores of proto-North America. (See Figures 10-12.) For ease of discussion, Layer II can be divided into two sub-layers, IIA and IIB.
The older of these, IIA, represents strata deposited along the ancient passive-margin of North America. The passive-margin deposits of Layer IIA can be subdivided into two varieties (facies) [IIA(W) and IIA(E)] that differ in their original geographic positions with respect to the shoreline and shelf edge. A nearshore facies [Layer IIA(W)], deposited in shallow water, is collectively designated as the Sauk Sequence. This sequence includes former conglomerate, feldspathic sands, and volcanic rocks of the late Proterozoic Ned Mountain Formation, basal Cambrian sandy sediment and overlying thick Cambro-Ordovician carbonate sediments, which were predominantly dolomitic in nearshore areas. The Sauk clastics and -carbonates in New York City are the Lowerre Quartzite and Inwood Marble. In western Connecticut and Massachusetts, the basal-Sauk sandy unit is the Cheshire Quartzite and the carbonate rocks, here containing more limestone than in localities closer to the ancient shoreline, are named the Woodville- and Stockbridge Marble. Thus, the Sauk strata began life as sandy- and limey sediments in an environment not significantly different from the present-day Bahama Banks. In fact, during the Appalachian cycle, New York City was situated in the tropical parts of the Southern Hemisphere (~20°S latitude); what is now east was then south and what is now west, north (Figure 14).
Figure 13 - Sequential tectonic cross sections for the Taconic orogeny in New England. From the top down the collision of a volcanic arc (on right) with the passive continental margin of North America (on left) produced the ancestral Appalachians. (From Rowley and Kidd, 1981.)
Figure 14 - Paleogeographic map showing North America in its Early Paleozoic position astride the Earth's Equator. (C. Merguerian and J. E. Sanders, 1996, fig. 2, p.118; after C. K. Seyfert and L. A. Sirkin, 1979.)
Farther offshore, fine-textured terrigenous time-stratigraphic equivalents of the shallow-water Sauk strata (shelf sequence) were deposited in deep water on oceanic crust [Layer IIA(E)]. This deep-water sequence is also of Cambrian- to Ordovician age. In upstate New York, it is known as the Taconic Sequence.
Layer IIB consists of younger strata designated collectively as the Tippecanoe Sequence. The Tippecanoe strata overlie the Sauk Sequence [Layer IIA(W)] above a surface of unconformity of regional extent. The change from passive margin to convergent margin took place while the Tippecanoe Sequence was accumulating. The basal unit of the Tippecanoe Sequence is a limestone (the "Balmville") deposited at the end of the passive-margin phase. Overlying this limestone is a thick body of dark-colored terrigenous strata, the filling of a foreland basin that formed during the earliest part of the convergent-margin regime that supplanted the passive-margin regime in mid-Ordovician time. (See Figure 13.)
Merrill (1890) and Merrill et al. (1902) established the name Manhattan Schist for the well-exposed schistose rocks of Manhattan Island. A new picture of the geology of New York City has evolved from the combined work of many geologists over the past century. Huge capital construction projects, both on the surface and in the subsurface, have allowed geologists to examine and map large parts of the city where no natural bedrock is exposed. Since the early 1980s CM has been able to examine most of the NYC Water Tunnel #3 during various construction phases. Based on this work and surface mapping in NYC over the same time period, a much more complex stratigraphy and structure has been found in comparison to earlier maps and reports. Indeed, the bedrock of New York City consists of three ductile fault bounded sheets of rock that are intricately folded together as shown on Figures 15, 16, and 17.
Figure 15 – Geologic map of New York City showing the generalized structural geology of the region. Blue dot shows the epicenter of the 17 January 2001 magnitude 2.4 earthquake that struck NYC. It is located along the famous 125th Street “Manhattanville” fault. (Adapted from Merguerian and Baskerville, 1987.)
The
bedrock geology of New
York City
can best be described using the concept of sequence stratigraphy. All of the
major sequences (Sauk, Taconic, Tippecanoe) are represented there (Figure 16). As such,
the various metamorphic rocks found in NYC were formerly deposited across the Cambro-Ordovician
shelf edge of embryonic North
America.
The former shelf (Sauk Sequence) is preserved as the Cambro-Ordovician Inwood
Marble (C-Oi) that is locally interlayered with autochthonous
calcite-marble bearing Middle Ordovician Manhattan Schist (Om) of the Tippecanoe
Sequence.
The Saint Nicholas thrust (Taconic frontal
thrust) separates lower-plate Tippecanoe (Om) and Sauk (C-Oi) rocks from
upper-plate gneiss, schist, and amphibolite of the former Cambro-Ordovician
slope- and rise (Manhattan Formation; C-Om). The structurally higher
ductile fault mapped as Cameron's Line, juxtaposes muscovite-rich schist and
gneiss, amphibolite, serpentinite, and coticule of a former deep-water realm
(Hartland Terrane; C-Oh) with C-Om rocks. All combined together
as the Manhattan Schist Formation by past workers, the subunits C-Om and
C-Oh are here considered to be metamorphosed and sheared facies of the Taconic
Sequence. During Ordovician Taconian arc-continent suturing, the Saint
Nicholas thrust and Cameron's Line juxtaposed former shelf-, rise-, and
deep-water facies in a continentward-facing subduction complex (Merguerian
1986, 1996c; Merguerian and Sanders 1991b, 1991g, 1993d).
Figure 16 - Geologic map of Manhattan Island showing a new interpretation of the stratigraphy and structure of Manhattan Island. Drawn and mapped by C. Merguerian (unpublished data).
Two cross-sections (Figure 17) show a simplified view of the geologic structure of Manhattan Island. The larger section cuts across northern Manhattan from the Hudson River to the Bronx. The W-E section shows the general structure of New York City and how the St. Nicholas thrust and Cameron's Line place the middle unit of the Manhattan Schist, and the Hartland Formation respectively, above the Fordham-Inwood-lower schist unit basement-cover sequence. The major F3 folds produce digitations of the structural- and lithostratigraphic contacts that dip gently south, downward out of the page toward the viewer. The N-S section, along Fifth Avenue in Manhattan, illustrates the southward topping of lithostratigraphic units and the effects of the late NW-trending upright folds.
Figure 17 - Geologic cross-sections, keyed to Figures 15 and 16, showing an interpretive west-east and north-south structure sections across northern Manhattan and the Bronx. Drawn by C. Merguerian.
Metamorphic index minerals such as garnet, sillimanite, and kyanite are found throughout the metamorphic rocks of New York City and the Bronx indicating deep conditions (~30 km or more) during their metamorphism. Dr. Ratcliffe has performed important research on the mineral kyanite. Copies of his publications are available in all public areas on campus or can be sought at all Geology Club meetings on Wednesdays during the Common Hour in Gittleson 135.
Enough Geological Background for one day. Lots of additional information is available by visiting the Geology Department webpage, visiting our links, and by downloading our publications. On to our Road Log and description of individual field trip localities (stops).
Heading westward from Hofstra on the Long Island Expressway, the extraordinary lack of topographic relief is because of the thin layers of glacial deposits (called outwash) that were shed southward during post-glacial retreat. These sandy deposits were deposited southward from Connecticut, lapping onto the glacial moraines and thus together form the surface units of Long Island. Keep an eye out for our first turnoff from the LIE. Note how the relief has changed from veritable flatlands to a hilly terrain. We are now encountering areas of Queens underlain by the glacial ridges of Long Island and cut by tongues of advancing glacial ice and meltwater channels. Farther to the west on the LIE near Kissena Boulevard (not passed on this trip), Queens College is perched on the intersection point of the two terminal moraines described earlier. Heading northward on the Whitestone Expressway toward the Throgs Neck Bridge we pass across glacial outwash deposits which terminate along the northern border of Queens on the rubbly north shore of Long Island.
Passing over the Throgs Neck Bridge we pass through a time portal separating us from the Pleistocene strata and older, underlying Cretaceous deposits of the Coastal Plain onto Proterozoic and Paleozoic crystalline rocks exposed in the Bronx. A diagrammatic sketch illustrating the structure of these rocks is shown in Figure 18. Note how the Mesozoic sedimentary rocks dip toward the south above an erosional surface developed on the crystalline rocks. This depositional contact is known as a nonconformity, the first of three major ones we will cross today, as it separates rocks of vastly different age and lithology.
Figure 18 – Diagrammatic north-south sketch of the nonconformity beneath the Throgs Neck Bridge and the disconformity of Long Island found along our first driving leg. (Drawing by C. Merguerian.)
Driving northward from the Throgs Neck Bridge on Route 95 we do not see much exposed bedrock due to a deep weathering profile. The rocks we are driving on through (Layer IIA(E) in Table 2) are a sequence of highly deformed and metamorphosed rocks mapped as the Hutchinson River Group (Baskerville, 1982). Continuous and correlative with rocks of the Hartland Formation (See Figures 15 and 16), the Hutchinson River Group is interpreted as a former oceanic sequence deposited adjacent to the early Paleozoic shelf edge of eastern North America.
Together, the crystalline metamorphic rocks of Manhattan and the Bronx comprise another physiographic province known as the Manhattan Prong. As shown in the cover figure and in Figure 4, the Manhattan Prong consists of a northeast-trending, deeply-eroded sequence of metamorphosed Proterozoic to Lower Paleozoic rocks, including quartzite, marble, and schist, that plunge southward beneath unconformable Cretaceous sedimentary rocks and overlying Recent (glacial) sediments in New York City.
By contrast to coeval (age equivalent) metamorphic rocks cropping out in New York City (Layer IIA(W) in Table 2), the Hutchinson River Group contains abundant amphibolite (metabasalt) and feldspathic gneiss and does not contain appreciable quartzite (metamorphosed sandstone) or marble (metamorphosed limestone) that is so typical of the shallow water depositional environment. As such, compared to the dominantly miogeosynclinal (shallow water shelf deposits) character of the Manhattan Prong west of Cameron's Line, the Hutchinson River Group is decidedly eugeosynclinal (deep-water oceanic parentage). Thus, on either side of Cameron's Line, an important structural boundary in the New England Appalachians, we have disparate sequence of juxtaposed metamorphic rocks of roughly equivalent age.
In
summary, the rocks exposed along I-95 form a sequence of highly metamorphosed metasedimentary
and metavolcanic rocks of Early Paleozoic age [Layer IIA(E)] which trend
northeasterly through Orchard Beach (STOP 1) and City Island into western
Connecticut where they are mapped as the Hartland Formation (C-Oh in
Figures 10 and 15). Together with their northward extensions into Massachusetts, Vermont, and New Hampshire this largely metavolcanic
rock sequence marks a former oceanic terrane that collided with North America during the medial
Ordovician Taconic orogeny or mountain building event (Figure 13). Imagine the
Japanese volcanic islands colliding with China and you may picture a modern analog of the
Taconic orogeny. A diagram illustrating the pre-Taconic paleogeography of the
Early Paleozoic shelf edge of eastern North America is shown in Figure 11. The depositional
sites for Layers IIA(W) and IIA(E) are shown.
Rocks of the Hutchinson River Group occur in highly glaciated exposures (look for glacial striae, till, and erratics) on South and North Twin Islands to the north of Orchard Beach in the Bronx. Described by Leveson and Seyfert (1969), and Seyfert and Leveson (1968, 1969), these high- to medium- grade metamorphic rocks include gneiss, schist, and amphibolite all showing ample evidence for partial melting (fusion) into mixed igneous and metamorphic rocks known as migmatites. In addition, many pegmatites and veins of quartz occur. A geologic map of the region is reproduced in Figure 19. Try to identify some of the major folds on the ground.
The glacial features of South Twin Island are remarkable and take the form of glacial striae oriented N32°W, glacial polish, and roche moutonnée structure. In addition to these features, a thin red-brown till, consisting of rounded boulders set in a reddish-brown matrix of poorly sorted sand, silt, and clay, has been unearthed (dug out) at the northern part of South Twin Island. Beneath the till the NW-trending glacial grooves are quite obvious on the glaciated bedrock surface (Figure 20). Glacial rounding has produced what Sanders and Merguerian (1994b) describe as a roche moutonnée structure on the bedrock at the extreme north end of South Twin Island. Here, the bedrock shows evidence of being sculpted from both the NW- and NNE- directions. Two important indicator stones (large boulders of ultramafic rock) occur on the striated bedrock surface. They are derived from an exposure of identical plutonic rocks from the Cortlandt Complex found to the NNW in the vicinity of Peekskill, New York. As such, they are the products of a glacial advance from the NW (Glacier II or III in Table 3). The NW to SE trending striae are also the products of our Glaciers II and/or III (Table 3). An older glacier (Glacier IV in Table 3) is responsible for NNE initial sculpting of the roche moutonnée structure at the north end of South Twin Island. Here, we find no evidence for our youngest glacier (I).
Seyfert and Leveson (1968) have subdivided the metamorphosed bedrock into two major units for the purposes of mapping. The "Felsic Unit" includes 95% feldspathic gneiss and 5% sillimanite schist and underlies roughly 50% of North Twin Island and most of South Twin Island. Contacts between the felsic gneiss and schist are gradational over distances of several mm to 10s of cm. The gneisses consist of quartz, plagioclase (An33), and biotite with minor garnet, muscovite, microcline, sillimanite, magnetite, and apatite. The schist unit, although of volumetrically minor importance, consist of plagioclase, quartz, biotite, sillimanite, microcline, and garnet with subordinate magnetite and muscovite. The calculated chemical composition of the felsic unit suggests that their protoliths were interlayered graywackes and shales although CM would not discount the possibility that they are largely of volcaniclastic origin.