9Th NYSGA Annual Meeting 1933 (6.510Mb)

,. EXCURSION OF THE NEW YORK STATE GEOLOGICAL ASSOCIATION IN THE NEW YORK C ITY REG I ON May 12th - 13th, 1933 G. MARSHALL KAY COLUMBIA UNIVERSITY (Note: Much of the region is described in Guidebqok 9, New York City and Vicinity, by c. P. Berkey, G. I. Finlay, R. J. Colony and others, to be issued by the International Geological Congress, Xvith sessiont Washington, 1933. The following guide has made use of the material in that guidebook.J INTRODUCTION Stop 1. Snake Hill is just southwest of Newburgh along State Highway 32. The locali The region to be visited in this excur ty has an outlier of resistant Pochuck gneiss sion is one including parts of several geolo that is an isolated remnant of a once-conti gical and physiographic provinces {see Figure nuous crystalline mass that was overthrust 1). The first stops are to be made adjacent northward from the Highlands upDn the Hudson to the Highlands of the Hudson, where the re River beds (see Figure 3). This overthrust sistant crystalline rocks of the Reading prong was cut later by a normal fault that tran of the New ·England Upland have been dissected sects the eastern side of the Hill and brings by the superimposed Hudson River. Northwest the Hudson River beds in contact with the of the Highlands, these crystalline rocks are older Wappinger limestone on the east. in faulted contact with the folded Paleozoic sediments of the Hudson Valley extension of the Folded Appalachians. The Catskill Moun tains farther northwest are the northern end of the Appalachian Plateau. East of the Hudson River from Peekskill to New York City, the meta-sediments lie in the Manhattan Prong of the New England Upland that terminates on Staten Island, New York City. To the west of the Hudson is the Trias sic Lowland margined on the east by the Pali sades, the river occupying a subsequent valley ---,oo_.__o_ _..___~lOO---:-:Mc-:'~:::.:··· ( a.ft er CO l O '1.y) in the underlying, gently westward-dipping, relatively non-resistant basal Triassic sedi Figure 3. Section through Snake Hill. ments. Thus there are two faults in the s action. The geologic column and history of each The older overthrust might be from the evi of theRe parts of the region will be treated dence in this section of Taconic (post-Ordo separately as the route traverses each area. vician, pre-Silurian) age; deformation of this age is lmown east of the Catskill Moun tains at Kingston. But from the fact that ITINERARY: NEWBURGH TO PEEKSKILL similar faults in the imnediate region affect Devonian sediments, it is inferred that the The first of the route maps (Figure 2) thrust on Snake Hill is post-Devonian. This shows the course of the excursion from New faulting may have occurred in the Acadic burgh to New York City. The first stops are disturbance {immediately post-Devonian) or on the northwest border of the Highlands at the Appalachian revolution (post-Permian). Snake Hill and Cornwall. The section in this Inasnmch as there are no sediments younger district includes an older series of crystal than Devonian in eastern New York north of line gneisses and Schists, the Pochuck, the Highlands, it is not possible to learn that have been intruded by younger granites with assurance which per iod of deformation is such as· the Storm King and Canada Hill represented; perhaps both affected the region. granites; these rocks are Pre-Cambrian. The faulting is thought to have occurred in In some localities they are un.coni"ormably the Appalachian revolution. overlain by the Cambrian Pough~ag quartzite The fault that cuts the gneiss on the (600 feet thick), the Cambro-Or ovician east side of the hill is believed to have Waapinger limestone (about 1000 feet) and the occurred in the Palisade disturbance (post Hu son River phyllite · (a few thousand feet Triassic) because similar normal faults of thick). Along the route of the excursion, the that age occur south of the Highlands. sediments are in fault contact with the Pre Cambrian crys~llines Stop 2, southwest of Cornwall-on-Hudson (1) 40~ is a locality where a thrust fault simi lit-·par-lit injections and dike-like and lar to that at Snake Hill brings the sill-like intrusions that commonly 60 per crystalline rocks over the Hudson River cent of it is igneous. The thickness is phyllites. unknown. The overlying Inwood marble is a massive, strongly-bedded, coarsely-crystal Between stops 2 and 3, the latter line magnesian limestone, the upper part of at the east end of Bear Mountain Bridge, which is generally impure and micaceous. north of Peekskill, the crystall~ne rocks of It is about 700 feet thick in New York Cit~ the Highlands are passed in many exposures. The formation is locally cut by pegmatite Most are ~of the granites of the Storm King dikes, but as a whole is quite free of ig and Canada Hill formations. At stop 3, the neous matter. The contact with the Manhat Canada Hill granite has been intruded by tan schist is a transitional one with in ~ounger pegmatite dikes. terbeddiruz: of the two sorts of rock. The course of the Hudson River through the Highlands is believed to be that of a stream superimposed on the crystalline rocks from a co ver of sedimentary rocks that has been removed. Recent studies sug gest that Cretaceous sediments once transgressed tne area, and that the ancient Hudson that flowed conse quently on the plain that was ex posed with the retreat of the Cre taceous sea was "let down" on the "barrier" of crystallines. The less resistant sedlmenta have been removed, but the more resistant granites and gneisses remain as the Highlands of the Hudson. Below the Highlands, the river has become adjusted to the resis tance of the belts of rock that it traverses. This subsequent drain age will be discussed more fully in connection with the stop on the top of the Palisades at Edgewater (stop 7). Figure 1. Block Diagram of the region from Newburgh to New York City (by Raisz after Berkey). From Peekskill to New York City the ~oute travdrses a region underlain by the crystalline Ford ham. gneiss, Inwood marble and Man hattan schist; the formations will be ~xamined 1n Manhattan and The Bronx, New York City. They are be lieved to be of Pre-Cambrian age, though some have thought the younga two to be of early Paleozoic age. 0 C f AN The Fordham gneiss is typical ly a black and white banded, qua.rtzose and granitic gneiss, in places schistoae, and so thoroughly impregnated with pegmatitic and 5 0 25 MILES granitic matter in the form of ._._....._.~s--=-o--'---..L.--L---2...IS_K_IL_O--JMETERS 11.o..Pu.l.._l... _..___._..a._~-..JI ( 2) ,4,a;s FIGURE 6 FIGURE 2 ROUTE MAP RO UTE MAP EDGEWATER NEvVBURGH TO TO STATEN ISLAl'JD NEW YORK Scale Mcl es Sea.le Mi le.s The Route~ Th e. Route~ Border of t,e Highlands · · · · · ·· ... Othu Roo.d, -- Ma.rgin of Seep£ntine _.,_ ___s _ _ Ma.rgin of Termin<ll. Mo>-a.in~ •··········• Ma.nho.He1n Schist in New Ju~e!I ~ ( 3) 492~ The Manhattan schist is a dark, deepened to make it navigable. streaked, strongly micaceous, coarsely crys talline, markedly foliated rock, locally Stop 5, at 204th Street and Nagle Ave carrying large quantities of quartz, feld nue, Manhattan, is an excavation in these apar and garnet, and exceeding 3000 feet in westward dipping Inwood marble beds; a large thickness. It is filled with irregular pegmatite dike cuts across the structure. masses of pegmatitic material and pegmatite From an examination of quartzitic beds above dikes and contains a few layers of horn the west face of the excavation, one gains blende schist that are believed to represent an impression of the more con~etent charac metamorphosed sills of basic igneous rock ter of such strata. The surface of the hill intruded prior to the foliation of the rock. is strewn with boulders of glacial drift, and tbe marble has locally disintegrated to This series of crystalline rocks is be sand. The varying resistances of the beds lieved to have had the following history. have produced miniature subsequent monoclin Fine elastic sediments, shales, were deposi al ridges and valleys on the weathered sur ted in a sea; the source of the materials faces. and character of the under lying rocks are unknown. These shales were succeeded by Stop 6, along the Harlem Speedway, is limestones and later by more shales, the beneath the same Washington bridge that was three constituting the darker part of . the crossed earlier in the afternoon. The Fordham, the Inwood and the Manhattan. schist is the Manhattan, and in addition to Basic igneous rocks were intruded into these the pegmatitic material permeating the sediments during or after their deposition formation, there are remarkably parallel (and are now represented by the hornblende pegmatite dikes that dip gently to the west. schists). These undisturbed pegmatites reach a thick These sediments were buried under a ness of ten feet, and frequently show in the thiclc cover of since-removed sediments, and finer crystalline te~ture of their periphery the region was affected by the intrusion of the effect of having been chilled by cooler large quantities of granitic material that wall rocks. The east end of Figure 5 gives thoroughly permeated the shaly beds; these a section such as that &cross the Harlem include the light-colored injections in the River from stop 6 to stop 4.

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