INTRODUCTION
We have offered to lead this trip in order to lay before those interested in
Long Island geology some implications (we would go so far as to assert,
"far-reaching" implications) that follow from evidence we have
discovered during the past several years on Staten Island and in Westchester
County, chiefly, with respect to the number of and flow directions of the
Pleistocene glaciers. We are convinced that as many as 5 continental
glaciers spread into the New York metropolitan area and
that each of them flowed from one of two contrasting directions, NNW to SSE and
NNE to SSW. What we have to say is based on the following 3 propositions:
Proposition No. 1 in our interpretation may be paraphrased from a recent U. S.
Supreme Court ruling on voting rights: "one person, one
vote." In terms of tills, we would express this as "one
glacier, one flow direction." (Farther along, we discuss all the
caveats that go with this vision of "one glacier, one flow
direction". Suffice it to say here, that if this point is accepted,
then a series of drastic revisions in the Pleistocene geology of Long Island necessarily
follows. Thus, the best strategy to be used by defenders of the status
quo in Long Island's glacial geology is to try to "head us off at the
pass" by denying our Proposition No. 1). "To the
barricades!"
Proposition No. 2 holds that the youngest of the Wisconsinan glaciers, the
Woodfordian, flowed from the NNE to the SSW. Flint (1961) demonstrated
this in Connecticut; he named the youngest
till there the Hamden Till. Our evidence, best seen at Croton Point Park, Westchester County, supports Flint's result: that
is, the youngest glacier flowed down the Hudson Valley.
Proposition No. 3 holds that several pre-Woodfordian glaciers flowed from NNW
to SSE. In Connecticut, Flint's (1961) Lake
Chamberlain Till, which underlies the Hamden Till, is a product of a glacier
that flowed from NNW to SSE. Glacier flow from the NNW to the SSE
dominates Manhattan Island, and is especially well
displayed in Central
Park.
What is even-more significant is the evidence that ice flowing from NNW to SSE
deposited the Harbor Hill moraine. We think Woodworth's results in
western Queens demonstrate this, but
that further support comes from the distribution of erratics along Long Island's north-shore
beaches. We will devote much of today's trip to a study of the erratics
as they relate to this significant proposition.
BIG IMPLICATION: If ice flowing from the NNW to SSE deposited the
Harbor Hill moraine, then the age of that moraine must be
pre-Woodfordian. We do not know its age exactly, but concur with C. A.
Kaye's (1964b) interpretation that the age of two prominent moraines on
Martha's Vineyard (correlatives of those on Long Island?) are Early Wisconsinan
and Illinoian. This represents a major downward shifting of the age
assignments of Long Island's famous moraines. Such a proposed age
shift is the only way in which we differ significantly from the stratigraphic
results of M. L. Fuller (1914), who assigned both of these moraines to the
latest Wisconsinan (in modern terms, Woodfordian).
Our next point is based on our new discovery at Garvies Point of a till
containing decayed granite erratics resting on the Cretaceous. Without
doubt, this contact is on a piece of material that has been displaced upward by
ice-thrust deformation and subsequent to that, has slumped down to beach
level. Nevertheless, whatever its post depositional vicissitudes have
been, we think this ancient till matches Fuller's Mannetto Gravel and that it
is the product of a glacier of Early Pleistocene age. We have found a
comparably old till containing decayed granitic stones on Staten Island and at Croton Point
(Westchester Co.). Therefore, we think the small exposures that we have
dug out at Garvies Point are very significant.
Our approach to the study of Long Island's glacial deposits has
differed from that of most other workers. We are just now starting to
become familiar with the details of the exposures and the subsurface
relationships on Long Island. We compare what we have done with what
the glaciers themselves did--cover the bedrock and transport blocks of it onto Long Island. As our main
qualifications for daring to intrude into the hallowed precints of the study of
the Long Island glacial deposits, we list only: (1) familiarity with the
regional geologic relationships of many of the pre-glacial formations in the
territory north and west of Long Island and related study of the tills and the
directional features made by the flowing glaciers; and (2) possession of (and
willingness to use) GI trenching tools (i. e., small shovels) and to expend
whatever effort is necessary to "undress" an exposure of Quaternary
sediments by stripping away the surficial layer of washed-down debris from
higher up the slope and thus to disclose the Quaternary sediments "au
naturel," so to speak.
To date, our studies have led us to diametrically opposite conclusions from all
previous workers who have accepted the latest-glacial age for the Long Island terminal moraines.
The interpretations involved are so contrasting that they can be described only
as follows: both could be wrong, but only one of them can be right.
Our results are directly contrary to those that have been published by others
during the last 20 years or so and that have been given the "good
mapmaking seal of approval" by the "establishment" in the form
of a new state map (Cadwell and others, 1986).
While we were pondering over how we could possibly put together a case strong
enough to instill some doubts in the minds of the "true believers"
and were in the midst of preparing the guidebook for our
"On-The-Rocks" trip to Montauk Point in November 1990, we
found our road to "salvation." We inadvertently discovered what
we consider to be a true "hidden treasure." From its innocuous
title, one would never suspect that the paper written by H. C. Ricketts (1986)
about two borings made near Kings Point (on Great Neck, west side of Manhasset
Bay) amounts to a breakthrough and is (in our opinion) the most-significant
paper written since Fuller's monograph of 1914. What Ricketts did that no
one would ever suspect merely from reading the title of his paper is to obtain
amino-acid racemization results from shell material from interglacial formations.
Ricketts collected shell material from the two borings that penetrated the
Gardiners Clay and from the shell beds exposed in the Port Washington sand
pits, on Manhasset Neck, west side of Hempstead Harbor, He submitted
these shells to Professor J. H. Wehmiller, of the University of Delaware, who
is a specialist in determining the ages of shells by the changes with time
(racemization) of the original amino acids. On the specimens collected by
Ricketts, Wehmiller found D/L leucine values between 0.26 and 0.34, which
implies that the age of the shells is about 225,000 years. (By
comparison, a D/L value in shells from Nantucket that are thought to be 125,000
years old is about 0.20; Oldale and others, 1982.) After we knew about
the Ricketts paper, we volunteered to organize and lead today's trip.
We have summarized our interpretations on a new correlation chart (Table
1). Despite our lack of chronologic data, we present it as a logical
statement of our current thinking and as a guide for others.
OBJECTIVES
Our objectives for today's field trip for the Long Island Geologists are to
take the participants to 3 localities on the north shore of Long Island (Figure 1, cover) to
view evidence that we think requires a total revision in previously
published interpretations of Long Island's glacial
geology. We shall also see exposures of the Cretaceous strata, look at
some evidence on the rate of retreat of the coastal cliffs, and study the
relationships between the fans at the mouths of gullies eroded into the
sediments of the coastal cliffs and the modern beach.
Specifically, our objectives are:
1. To examine the evidence for the proposition that the
Harbor Hill Moraine was deposited by a pre-
Woodfordian glacier which flowed from NNW to SSE.
This will involve us in a study of the erratics on
the north-shore beaches and discussions of their
provenance.
2. To study an exposure of a till containing decayed
granitic stones (equivalent of Fuller's Mannetto
Gravel), and its contact relationships with the
underlying Cretaceous.
3. To examine some exposed Cretaceous strata and to become
familiar with two distinctive varieties of Cretaceous
rocks, not exposed, that are common in the beach
gravels: (1) a hematite-cemented conglomerate
(erroneously identified by others as coming from the
Silurian Green Pond Conglomerate); and (2) a fine-
textured red sandstone (erroneously identified by
others as coming from the Newark basin-filling
strata).
4. To study the fans at the toes of the eroding bluffs and
how fan sediments and beach sediments are
interrelated. We shall use these fans as small-scale
models for interpreting the widespread outwash
underlying Long Island.
So much for the introductory comments for today's trip. Now on to a
general summary of the bedrock geology to the northwest and north of Long Island and the specifics of
glacial-flow directions.
BEDROCK IN SOUTHEASTERN NEW YORK AND CONNECTICUT
Viewed as a first approximation, any reasonable geologic intuition holds that
the erratic boulders found along the north shore of Long Island come from the bedrock
of Connecticut. The central
point where our interpretation of the Pleistocene glacial deposits differs from
that of most other workers has to do with the total number of glaciations and
the flow directions of the glaciers. One of our important conclusions is
that at certain times, glacial ice flowed rectilinearly (to use C. A. Kaye's
term) from two contrasting directions [from the NW to the SE and from the NE to
SW (Sanders and Merguerian, 1991)]. This can be demonstrated without any
question in Westchester County, in Manhattan, and in western Queens, where glaciers flowing
southeastward transported distinctive erratics from the rocks of the Newark basin and from the
anthracite district of northeastern Pennsylvania. What is more,
the color of the till deposited by such glaciers always bears the reddish-brown
color of the Newark sedimentary strata.
Rocks virtually identical with those of the Newark basin are present in
central Connecticut (Figure 2).
Because of the geographic relationship between the northeastern termination of
the belt of outcrop of the Newark rocks in Rockland County, New York, and the
southern termination of the belt of outcrop of the rocks in the Hartford basin
in New Haven harbor, Connecticut, a crystalline "corridor" exists
through which a glacier flowing from NW to SE could pass, extend to Long
Island, and not pass over Newark-age rocks, and thus the color of its till
would not be the diagnostic reddish brown. We would expect the erratics
to be different, as well. Note that in Figure 2 our Stops 1, 2 and 3 are,
for our inferred NW to SE glacial flow, geographically located within the
crystalline "corridor" of southeastern New York and western Connecticut.
In the following paragraphs, we review the distinctive features of the bedrock
in southeastern New York and the state of Connecticut. Our objective is
to help "soft-rockers" pinpoint distinctive kinds of bedrock so that
the erratics found on Long Island's north shore beaches
can serve as indicator stones.
Southeastern New York
The bedrock geology of southeastern New York state is dominated by a
complexly deformed, metamorphosed sequence of autochthonous- and allocthonous
Proterozoic Y through Cambro-Ordovician rocks which form the Manhattan Prong
(Figure 3). The Manhattan Prong is bounded on the northwest by
Grenvillian Proterozoic Y rocks of the Hudson-Reading Prong and to the east is
in ductile-fault contact with essentially coeval, dominantly eugeosynclinal
Cambro-Ordovician rocks of western Connecticut. The contact is marked by
a zone of syntectonically intercalated, mylonitic rocks and is mapped as
Cameron's Line.
Figure
2.
Geotectonic map of western Connecticut and southeastern New York. The location of
our three field-trip stops and their geographic relationships to the
crystalline "corridor" are shown. (Merguerian, 1983.)
Figure
3.
Simplified geologic map of Manhattan Prong showing the distribution of
metamorphic rocks ranging in ages from Proterozoic to Early Paleozoic.
Most intrusive rocks have been omitted. (Mose and Merguerian, 1985).
Figure
4.
Maps of Connecticut and southern New England showing major tectonic features
and rock bodies.
A.
Tectonic map of Connecticut showing Cameron's Line
and three major terranes. (John Rodgers, 1985.)
B.
Tectonic sketch map of southern New England showing major domes,
folds, and faults. (P. Robinson and L. Hall, 1980, fig. 3, p. 78.)
Cameron's Line, which skirts the New York - Connecticut state boundaries,
separates rocks of the Manhattan Prong to the west from coeval rocks to the
east that were deposited in a dramatically different paleogeographic
setting. Rocks to the west of Cameron's Line include the metamorphic
rocks of the Manhattan Prong consisting of the Proterozoic Y Fordham Gneiss,
the Proterozoic Z Yonkers and Poundridge gneisses and coeval rift-facies strata
mapped as the Ned Mountain Formation, the Cambrian Lowerre (Cheshire)
Quartzite, the Cambrian-to-Ordovician Inwood (=Woodville, and Stockbridge)
marbles, and overlying Middle Ordovician Manhattan Schist (Unit A) and
correlative Annsville Phyllite. In addition, these basement-cover rocks are
structurally overlain by allochthonous rocks of the Taconic sequence and their
metamorphosed, dominantly massive southerly equivalents (the Manhattan Schist
and related amphibolite (Units B and C), the Waramaug Formation, and locally,
the Hartland Formation). Farther north, beyond the Hudson-Reading Prong
in New York State, less-metamorphosed
lithostratigraphic equivalents of the Early Paleozoic rocks are found including
the Wappinger carbonates, the
Walloomsac slate and shale, the Normanskill graywackes and shale, and rocks of
the Taconic allochthons.
Extending toward the southwest of the Hudson River valley from
southeastern New York into New Jersey, are Mesozoic igneous-
and sedimentary rocks of the Newark basin. (See
Figure 2.) Distinctive in their reddish-brown color, the west-dipping
sedimentary strata range in age from Late Triassic to Early Jurassic and are
time- and lithostratigraphic equivalents of rocks of the Hartford basin of central Connecticut (as discussed
below). The west-dipping Palisades Sill, which forms a prominent ridge on
the west side of the Hudson River valley from the New York Bight to the
vicinity of Stony Point, New York, is a medium- to coarse-grained diabase that
was originally intrusive into the the Newark strata based on baked contacts
above and below the sill. Basaltic lava flows of the Watchung mountains
and intercalated red-brown Jurassic sedimentary rocks crop out farther westward
and up-section in New Jersey in an outcrop belt that
extends beyond the New York State border. Many of
these Mesozoic rocks are found as erratics on Manhattan Island. Near the Ramapo
fault, the border fault on the western edge of the Newark Basin, distinctive
clast-supported fanglomerates occur which bear Paleozoic boulders eroded from
above the Hudson-Reading Prong and from areas to the north- and northwest of
the highlands, cemented in a red-brown matrix.
Given the varied lithologies found in southeastern New York and adjacent regions of
New
Jersey, erratics from these areas could only be
deposited on Long
Island
by a glacier that flowed from the northwest to the southeast.
Connecticut
Connecticut, like all of Caesar's Gaul, can be subdivided into
three parts (physiographic provinces, geotectonic terranes, or whatever).
These are western, central, and eastern terranes, respectively (Figure 4).
The
Western Terrane
(1) The western terrane includes mostly metamorphic rocks that form part of the
central crystalline core of the Appalachians. The age range of
these rocks is from Proterozoic Y through Lower Paleozoic. Although
volumetrically most of the metamorphic rocks are metasedimentary units, some
distinctive igneous rocks (both intrusive and extrusive) are present as are some
distinctive mylonites associated with a large-scale regional overthrust known
as Cameron's Line (Figures 2, 4).
We begin with the contrasting metasedimentary rocks found adjacent to Cameron's
Line and then summarize some of the distinctive mylonitic rocks and igneous
rocks. In westernmost Connecticut north of the
"panhandle", rocks of the Manhattan Prong crop out. As they
were described above we will not discuss them here.
East of Cameron's Line, the bedrock formations differ significantly from those
of the Manhattan Prong to the west of this line. To the east, the
Cambrian-to-Ordovician Hartland Formation is the dominant formation underlying
the crystalline highlands of western Connecticut. The Hartland
Formation consists of a thick sequence of dominantly well-layered
muscovite-rich schist, gneiss, amphibolite, and intercalated mafic- to felsic
metavolcanic- and metavolcaniclastic rocks (Merguerian, 1983). Throughout
the Hartland terrane of western Connecticut, local bodies of unique
ferruginous- and manganiferous garnet-quartz granofels (coticules) are found as
highly laminated rocks within the sequence (Merguerian, 1980, 1981). The
Cambrian-to-Ordovician Hartland Formation is unconformably overlain by
Siluro-Devonian metamorphic rocks of the Straits Schist.
To the east of the panhandle area of southwesternmost Connecticut, Silurian-to-Devonian
metamorphic rocks occur as the Straits Schist to the south and north of the
Cambrian (?) gneisses of the Waterbury Dome. (See Figure 4.)
Farther east and cropping out in the vicinty of New Haven, in the extreme
southeastern corner of the western terrane, are relatively low-grade (chlorite
to garnet) schistose-, phyllitic-, and metavolcanic rocks of the Allingtown and
Maltby Lake volcanics (Fritts 1962,
1963). These rocks are a part of the Middle Ordovician Bronson
Hill-Ammonoosuc volcanic terrane which trends northeasterly through Connecticut, Massachusetts, and New Hampshire. Thus, as
initially pointed out by Crowley (1968) and elaborated on by Merguerian (1983,
1985), in a transect extending from northwest to southeast across the western
terrane, the interpreted protoliths of Paleozoic metamorphic rocks of the
western terrane of southeastern New York and western Connecticut become less
"continental" and more "oceanic"; an abrupt lithologic
change occurs at Cameron's Line. (See Figures 2, 4.)
CM interprets Cameron's Line as a thrust fault within a deep-seated subduction
complex that formed during the medial Ordovician Taconic orogeny adjacent to
the Early Paleozoic shelf edge of eastern North America. This might
explain the northwest- to-southeast imbrication of early Paleozoic
shallow-water "continental" (Fordham-Lowerre-Inwood-Manhattan A plus
correlative) lithologies with transitional slope- and rise- lithologies
(Manhattan B and C, Waramaug, and parts of the Hartland Formation), from purely
deep-water (including volcanic) rocks found west of the New Haven area. Thus,
according to many workers, the juxtaposition of these largely coeval belts
occurred during the arc-continent collision of the Taconic orogeny and resulted
in telescoping of the continental-margin sequence and overthrusting of the
volcanic arc and its fringing oceanic- basin deposits (Merguerian, 1983).
The rocks along the Taconic suture (Cameron's Line) form an impressive zone of
mylonitic rocks that experienced abnormally high shear strain under deep burial
during the Taconic arc-continent collision. Mylonites, or ductile-fault
rocks, bear unique metamorphic textures that can be easily identified in the
field by their highly-laminated appearance and distinctive microtextures under
the microscope.
With respect to the Taconic orogeny, local plutons are both synorogenic and
post-tectonic. The older group of synorogenic plutons cut across Cameron's Line
in western Connecticut and southeastern New York. These include a
series of mafic- to ultramafic plutons (now largely metamorphosed) that are
similar in mineral composition and texture to the Cortlandt Complex of
Peekskill, New York (On-The-Rocks Trips #10 and #14). In the panhandle
area of southeastern New York and southwestern Connecticut, high-grade Ordovician
granitoid- and dioritic orthogneisses (including various phases of the Harrison
Gneiss, Brookfield Diorite Gneiss, and Bedford Augen Gneiss) are in great
abundance. Similar metaplutonic rocks including norite, hornblendite, and
pyroxenite occur farther north near Litchfield and Torrington, Connecticut and are known as the Mount Prospect and Hodges Complexes
and the Tyler Lake Granite (Cameron, 1951; Merguerian, 1977, 1985).
Together, these orthogneisses represent late syn-orogenic plutons that were
intruded into the developing suture zone during the waning stages of the
Taconic orogeny (Merguerian, Mose, and Nagel, 1984). As such, these
mineralogically and texturally distinct metaplutonic rocks should serve as
valuable indicator stones.
The younger group of post-orogenic intrusives include Devonian lamprophyre and
potash feldspar-phyric (meaning a porphyritic rock containing feldspar
phenocrysts to translate for those of who have not been exposed to the
"new" petrologic language) Nonewaug Granite. Other plutonic
rocks of still-younger ages include isolated bodies of Permian syenite,
-adamellite, and -dacite porphyry. Of additional help, we are
investigating the distribution of economic ore deposits in the crystalline
terranes to the north- and northwest of Long Island in an effort to locate scarce,
but highly useful indicator stones. During an On-The-Rocks field trip in
November, 1990, one such erratic, containing sulfide ore minerals, was found
eroding out of the Montauk Point "till".
The
Central Terrane
The rocks of the central terrane consist of Mesozoic sedimentary- and igneous
rocks filling the Hartford basin. The Hartford basin-fill rocks
underlie a north-south-trending lowland in the central part of Connecticut. This lowland
continues northward into Massachusetts (Longwell, 1922, 1928,
1933, 1937). To the west, the basin-filling rocks lie in fault-modified
unconformable contact with Paleozoic metamorphic rocks of the western
crystalline terrane. On the east, the basin filling rocks end abruptly at
the basin-marginal fault (On-The-Rocks Trip #18). East of this fault are
the rocks of the eastern crystalline terrane of Connecticut and Rhode Island. [In this
discussion, we include the Mesozoic rocks found in an isolated half-graben
known as the Pomeraug Basin within the western
terrane. (See Figure 2.)]
Lithologically distinct, the strata filling the Mesozoic basins consist
predominantly of east-dipping, red-colored sedimentary rocks and intercalated
basalts with local intrusive mafic rocks (for example, the Buttress and West
Rock dolerites). Correlative with the Upper Triassic to Lower Jurassic
Newark Supergroup of New Jersey (On-The-Rocks Trip #5), the rocks of the
Hartford and Pomeraug Basins include the New Haven, Shuttle Meadow, East
Berlin, and Portland formations consisting of red- to maroon-colored micaceous
arkose and quartzose sandstone and siltstone, shale, and local conglomerate and
fanglomerate, together with subordinate black shale and local dolostone, and
intercalated dark-colored mafic volcanic rocks of the Talcott, Holyoke, and
Hampden basalts.
The basins are internally cut by a myriad of faults and, as discussed below,
trend southward and project into Mesozoic grabens in the subsurface of Long
Island and the New York Bight that have been identified by samples from drill
holes and data from geophysical surveys (Klitgord and Hutchinson, 1985;
Hutchinson, Klitgord, and Detrick, 1986; Hutchinson and Klitgord, 1988).
The distinctive color and lithology of these rocks make them ideally suited for
use in analysis as indicator stones and as sources for the generation of
red-colored tills but similarity with rocks of the Newark Basin complicates direct
correlation. All is not lost however, as the presence of low-grade
(chlorite- and epidote-bearing) schist and metavolcanic rocks as erratics in
concert with these distinctive Mesozoic lithologies would identify a unique Connecticut source as such
low-grade rocks of volcaniclastic parentage do not occur in the vicinity of the
Newark Basin of New Jersey.
The
Eastern Terrane
The crystalline rocks to the east of the basin-marginal fault along the east
side of the Hartford Basin underlie eastern Connecticut and Rhode Island. The bedrock formations
here include exceedingly complex suites of metamorphic- and metaigneous rocks
that range in age from Proterozoic through Permian. They have been cut by
a regionally important ductile shear zone having the unlikely but nonetheless
real name of Lake Chargoggagoggmanchauggagoggchaubunagungamaugg - Honey Hill
Fault Zone [also known as the Lake Char - Honey Hill Fault Zone], which
separates metavolcanic-, metaplutonic-, and metasedimentary rocks of the
Bronson Hill-Ammonoosuc terrane to the north and west from Proterozoic Z
gneisses and Permian intrusive rocks of the Avalonian terrane to the south and
east. The Proterozoic Z rocks include the Plainfield Quartzite (a
distinctive vitreous feldpathic +/- biotite quartzite), the Waterford Group, and
the Sterling Plutonic Group. Many unusual porphyritic gneisses are
present within this sequence that should serve as excellent indicator stones.
The Ordovician rocks of the Bronson Hill - Ammonoosuc volcanic terrane include
the Monson Gneiss and overlying Middletown and Brimfield
formations as well as the Glastonbury Gneiss. These rocks are overlain by
Silurian and Devonian metamorphic rocks of the Bolton Group and cut by the
Devonian Maromas Granite Gneiss. To the east, correlatives include the
Ordovician Quinebaug, Tatnic Hill, and Brimfield formations and overlying
Siluro-Devonian units known as the Hebron Gneiss and equivalents.
Intrusive into these crystalline rocks are many plutons ranging in age from
Ordovician to Permian. The distinctive rocks among this group on the
Connecticut side of the Lake Char - Honey Hill Fault Zone include the
Ordovician Preston Gabbro (+/- diorite), the Devonian Lebanon Gabbro (+/-
diorite), and unnamed Devonian norite, diorite, and granitoid gneiss. In
places where the mafic rocks are in close proximity to the Lake Char - Honey Hill Fault
Zone, the rocks have been transformed into distinctive mafic mylonites.
To the east of the Lake Char - Honey Hill Fault Zone
are Permian intrusives known as the Narragansett Pier Granite (including a
mafic phase) and the Westerly Granite, both distinctive lithologies.
Thus, given the complexities of the geology of the state of Connecticut, there appears to be
room for identification of distinctive rocks and lithologic assemblages in the
erratics that we will examine on our trip this weekend. Pleistocene
boulder chasing is an exercise that best demonstrates the necessity of having a
well-rounded knowledge of all fields in geology in order to arrive at a
satisfactory conclusion.
DIRECTIONS OF GLACIER
FLOW
The directions of flow of a former glacier can be determined easily and
unambiguously in the field by recording the azimuths of features eroded on
bedrock by the flowing ice (such as striae and grooves, and the long axes and
slope asymmetry of roches moutonées and rock drumlins), by plotting the
distribution of erratics (especially indicator stones), and by recording
certain asymmetrical properties of glacial deposits (such as directions of
orientations of elongate clasts and the long axes of drumlins).
In the nineteenth century, the raw data on ice-flow marks on the bedrock were
collected systematically. Indeed, many papers dealing solely with data on
striae on bedrock surfaces were published. Before long, the novelty of
such papers wore off. And a worn-off geologic novelty is like yesterday's
newspaper. Novel or not, the straightforwardness of the directional data
disappear when one begins to interpret them. If all the flow-directional
data in a given region are about the same, interpretations are not
controversial. But, if more than one set of flow-directional indicators
have been found, then that is another story. In the following paragraphs,
we review the evidence that in the New York City region, more than one
set of ice-flow indicators can be demonstrated. After we have reviewed
these, we take up their interpretation.
Glacial
striae on bedrock surfaces
Perhaps the easiest of the ice-flow indicators to record are the directions of
striae and grooves on the bedrock. In New York City, such a survey was
first carried out in 1828-29 by L. D. Gale (Mather, 1843). As was common
in his day, Gale supposed that the grooves and scratches had been made by water
currents, perhaps assisted by icebergs. The presumed significance of
water is implied in the use of the term diluvial. Gale's report written
in 1839 and entitled: "Geological Report of New-York; New-York
island" was published in Mather (1843). Gale's results on flow
directions appear on p. 209-210).
Details of Gale's observations are contained in our On-The-Rocks guidebook for
Trip 15 to Montauk
Point
(On-the-Rocks guidebooks from our past season's trips are available from Duke Geological
Laboratory - See order form at the back of this guidebook). To summarize,
Gale's measurements led him to conclude that he had observed the effects of two
contrasting flow directions, (a) nearly all the "diluvial scratches and
furrows" indicated flow from the NW to the SE and (b) the displacement of
indicator erratics (an anthophyllite-bearing rock and a white limestone)
implied transport from the NNE to the SSW. Despite the apparent contrast
in flow directions, Gale interpreted his data in terms of a single event, which
he expressed as "the diluvial current." Gale tried to show how
the changes in flow of a single such current could account for both the
regional trends of the scratches and furrows on the smoothed bedrock and the displaced
indicator erratics. In this regard, Gale began a pattern that would be
followed by most subsequent students of the "diluvial"
deposits: trying to account for all the disparate observations by
invoking only a single transport event. But Gale's single transport event
differed significantly from the one favored by later investigators. Gale
concluded that his single "diluvial current" had flowed from NW to SE
and he sought aberrations in this flow direction to account for the
displacement from NNE to SSW of indicator erratics. In contrast, the
single flow event for most later workers was taken to be from the NNE to the
SSW; they invoked aberrations to explain the scratches and furrows that trend
NW-SE.
That great genius of Connecticut geology, James Gates
Percival, was well acquainted with both the bedrock and what we would now refer
to as the Pleistocene deposits. He classified these as
"Diluvium," or the "unstratified materials," which he
contrasted with the Alluvium, "those arranged in strata." Percival
(1842, p. 453-456) cited many examples of distinctive rocks that had been
displaced from NW to SE. "The greater part of the Diluvium was
apparently deposited by a general current, traversing the surface from N. N. W.
to S. S. E. This is satisfactorily indicated both by the bowlders,
scattered over the surface, or imbedded in the diluvial earth, and by the
smaller fragments included in the latter, as well as by its general character
(sic)." (Percival, 1842, p. 453). Percival emphasized that knowledge
of the composition of the bedrock was absolutely essential to reconstructions
of the directions of the "diluvial currents:"
"In order to determine the direction of the diluvial currents, a
particular knowledge of the local character (sic) of the rocks, as indicated in
the account already given of the different local formations, is
indispensable. Several of these local formations are so peculiar in the
character (sic) of their rocks, that the latter cannot be mistaken, to whatever
distance they may have been transported. These, by the distribution of
their bowlders and fragments, furnish conclusive evidence that the more general
(sic) direction of the diluvial current was S. S. E. (Percival, 1842, p. 454)
Despite the numerous examples he cited that demonstrate transport from NW to
SE, Percival reported that some rocks had been moved from NNE to SSW. As
did Gale in Manhattan, Percival supposed that
this transport to the SSW had resulted from local deflections of the general
diluvial movement to the SSE:
"Although the general direction of the diluvial current was apparently S.
S. E., yet in some instances, from local obstructions, its course was deflected
to a S. S. W. direction. This is most distinctly obvious along the
Western border of the larger Secondary formation, where blocks and fragments of
the Trap and Sandstone of that formation are accumulated, sometimes quite
abundantly, in such a direction from their apparent source." (Percival,
1842, p. 457).
Striated bedrock lending further support for glacial flow from the NW or NNW to
SE or SSE was found by Woodworth (1901) in Long Island City.
Salisbury and assistants (1902)
found that the predominance of ice-flow indicators showed glacial flow from the
NNW to the SSE over the Palisades. By contrast,
such indicators demonstrated that glacial flow over the Watchung ridges had
been predominantly from the NNE to the SSW. In his interpretation of
these indicators of contrasting directions of ice flow, Salisbury argued that within the
margins of an ice sheet are localized zones within which the ice-flow paths are
faster than elsewhere. Accordingly, the ice-flow "streamlines"
are thought to be crowded close together, as in the sketch map of the region
surrounding Lake
Michigan
(Figure 5). On either side of such supposed zones of concentrated flow,
the ice tends to spread out toward each side. In applying this concept to
the New York metropolitan area, Salisbury inferred that during the latest
glaciation of the New York City region, the axis of fastest-flowing ice had not
been down the Hudson Valley, as one might expect on the basis of valley size,
but rather had followed the Hackensack Valley to the west (Salisbury and
assistants, 1902). From this inferred zone of concentrated flow down the Hackensack Valley, they thought that the
ice had flowed toward the south-southeast over the Palisades ridge and Manhattan, and toward the
south-southwest over the crests of the Watchung Ridges in New Jersey (Figure 6). Salisbury admitted that the
regional distribution of erratics of the distinctive Silurian Green Pond
Conglomerate from northwestern New Jersey constituted an anomaly
to this explanation of marginal-flow divergence within a single glacier as the
cause of the divergent orientations of the glacial grooves and
-scratches. Salisbury acknowledged that
another succession of events which could explain the distribution of erratics
of Green Pond Conglomerate involved two glaciations. He considered this
possibility briefly, but rejected it.
Hanley and Graff (1976) found numerous places in Central Park, Manhattan where the bedrock has
been prominently striated by glacial ice that flowed from the NW to the SE
(Figure 7).
Indicator
stones
Indicator stones implying flow from NW to SE have been found on Staten Island by Hollick (1908, 1915)
and at the Brooklyn Botanical Garden (Gager, 1932, based on
petrographic results by Robert Balk). Erratics of Green Pond Conglomerate
(Silurian, northwestern New Jersey) and of Pennsylvanian
anthracite from near Scranton, Pennsylvania have been found on Staten Island (Friedman and Sanders,
1978; Figure 8).
Figure
5.
Sketch map of area west of Lake Michigan (mostly in Wisconsin, but including parts of
Michigan and Illinois), showing concept of
divergent flow from a narrow zone (centered above Green Bay, Wisconsin) of rapid flow within
an ice sheet. (R. D. Salisbury, 1902, fig. 31.)
Figure
6.
Sketch map showing lobate margin of a terminal moraine (Harbor Hill?) in New York City and vicinity with
inferred divergent directions of flow of the most-recent continental
glacier. (R. D. Salisbury, in Merrill and others, 1902, fig. 12, p. 13;
also 1908, fig. 11; also, H. B. Kümmel, 1933, fig. 13, p. 66.)
Figure
7.
Sketch map of the geology of Umpire Rock, Central Park, New York showing
directions of glacial striae oriented NW to SE. (After Hanley and Graff,
1976, p. 43; modified by JES.)
Figure
8.
Distinctive erratics found in till in New York City, (1) anthracite from
northeastern Pennsylvania, and (2) Green Pond Conglomerate from northern New
Jersey, support interpretation of rectilinear flow of glacier from NW to
SE. Stippled area, outcrops of anthracite coal; S. I. = Staten Island; P. B. = Princess Bay. (Friedman and
Sanders, 1978, Figure 2-1, p. 27.)
After he had analyzed the Pleistocene stratigraphy of the Boston area, Kaye (1982) wrote
the following paragraph under the headings of directions of glacial flow:
"The direction of ice flow in the Boston Basin
and adjoining uplands was studied by means of
the orientation of striations (sic) and grooves
on the bedrock surface, the orientation (sic) of
the long axes of drumlins, the direction of
transport of erratics in till, and the direction
of thrusting and and overturning of bedding in
glacially deformed drift. These data range
through 360° in azimuth. Analysis of this
confusing message shows the existence not of
an ever-shifting ice current but of at least
four separate and distinct ice currents of
different ages. Three of these flowed fairly
rectilinearly, but one (the last) was
multicomponent and
marked by strong lobation"
(1982, p. 31).
Kay numbered these tills from I (oldest) to IV (youngest). Tills I, II,
and III flowed from the NW to the SE, the mean direction being S23°E +/- 1° for
Till I; S64°E +/- 18° for Till II; and S31°E +/- 2° for Till III. Till IV
was from the NNE.
The regional geologic relationships in the New York City region are especially
helpful with regard to ice-flow reconstructions. The distribution of the
reddish-brown bedrock in the strata filling the Newark basin lying west of the
Hudson
River
provides an especially valuable indicator with respect to ice flow across the Hudson Valley in contrast to ice flow
down the valley. In many localities east of the Hudson River, it is possible to
distinguish distinctive tills and outwash on the basis of color. Any
reddish-brown sediments resulted from action of ice that flowed across the Hudson River. By contrast,
sediments that are yellowish brown or gray were deposited as a result of
glaciation by ice that flowed down the Hudson Valley.
If one grants that the ice-flow indicators presented imply flow from both the
NNW and the NNE, then several interpretations are possible. Three ideas
are analyzed here: (1a) that changing meteorological conditions caused
the sites of maximum snow accumulation to form "ice domes" on the top
of the glacier and that flow direction responded to the changing zones of
maximum ice thickness, thus enabling the flow directions within a single ice
sheet to shift with time; (1b) that the margin of the ice sheet was
characterized by lobes within which the ice flowed with contrasting directions;
and (2) each set of flow directions was made by a single glacier having only
one dominant flow direction.
(1a)
Changing meteorological conditions and shifting ice domes.
The theoretical background in support of the concept that changing meteorological
conditions could shift the locus of maximum snow accumulation to build ice
domes and that as a result, one and the same continental ice sheet could
display multiple flow directions was proposed at the time when the modern
version of the Laurentide Ice Sheet was advocated (Flint, 1943).
According to Flint, the Laurentide Ice
Sheet began as one or more snowfields in the highlands of northeastern Canada. With continued
additions of snow, an ice cap appeared and it began to spread southward and
westward. The azimuth from the northeastern Canadian highlands to New York City is 195° or along a line
from N15°E to S15°W. After this ice cap had become a full-fledged ice
sheet and had attained something close to its full thickness, it is presumed to
have itself become a factor in localizing where further snow would fall. Flint inferred that the ice
sheet could divert the flow of moisture-bearing winds from the Gulf of Mexico and thus would have
acted as an orographic source of precipitation. In other words, the ice
sheet forced the air to rise and to be cooled and thus to drop its
moisture. Enough snow is therefore thought to have been heaped up at
various localities near the outer margin of the ice sheet and thus to have formed
ice domes whose relief altered the direction of flow. Thus, the initial
direction of regional rectilinear flow toward the SSW as a result of snow
supply from northeastern Canada, could change locally
to centers of quasi-radial flow under of the influence of the ice domes each of
which could display divergent flow patterns, including zones of flow from NNW
to SSE. During retreat, the above-described situation would be
reversed. The factors responsible for radial ice-dome flow, including
sectors from NNW toward the SSE, would cease to operate and those causing
rectilinear flow toward the SSW to resume their former preeminence. The
predicted pattern of flow for each advance of such an ice sheet, therefore,
would involve three phases in the following order: (1) rectilinear flow
from the NNE toward the SSW; (2) quasi-radial flow from the glacier-marginal
ice domes, but locally from the NNW toward the SSE; and (3) rectilinear from
the NNE toward the SSW.
Flint opposed the
multiple-glacier hypothesis because he was convinced that if two glaciers had
flowed over an area and both had extended their influence deep enough to polish
and scratch the bedrock, then the younger glacier would obliterate all traces
of the older one. Accordingly, he argued that all the striae must have
been made by only one glacier, the youngest one.
(1b)
Ice lobes at glacier margin.
Another version of how a single glacier could create flow indicators having
several directions is based on the behavior of ice lobes. Such lobes
characterize the terminus of a valley glacier that has spread beyond the
confining bedrock valley walls. Although the main flow direction of ice
in a valley glacier is parallel to the trend of the valley, within the terminal
lobe, the spreading ice creates divergent flow paths.
(2)
One ice sheet, one flow direction.
Our field studies made mostly off Long Island, have led us to
conclude that in the New York metropolitan area, the
evidence about glacier-flow directions supports the view that each ice sheet
was characterized by a single dominant flow direction (Figures 9, 10). In
other words, the pattern was one ice sheet, one flow direction. Table 2
shows the relationships as we envisage them.
We make no claim that we have proved that our interpretation is correct and
that it should, therefore, supersede all others. Our scheme lacks a
chronologic basis; we are vulnerable to all the pitfalls associated with that
oft-used method of "counting down from the top." But, given the
current state of affairs in local Pleistocene geology, we believe that our
beliefs should be considered by the believers of contrasting sets of
beliefs. Our interpretation contains specific, checkable consequences
with regard to the provenance of the tills.