Sources of Ground Water 

in Southeastern New York

GEOLOGICAL SURVEY 

CIRCULAR 417
Sources of Ground Water in 

Southeastern New York

By Nathaniel M. Perlmutter

Prepared in cooperation with the New York State Water Power and Control Commission

GEOLOGICAL SURVEY CIRCULAR 417

Washington, D. C. 

7960
United States Department of the Interior

FRED A. SEATON, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on qpp/iccrfion to the U.S. Geofogko/ Survey, Wosnington 25, 0. C.
CONTENTS

Page Page

Introduction _ _____________ 1 Ground-water hydrology of southeastern

Ground-water hydrology of southeastern New York Continued

New York _ 1 Rockland County 7

Dutchess County ..._... __ 3 Ulster County ._ __ __ __ _ 8

Orange County. . _ 5 Westchester County _._ _____ 8

Putnam County ___ __ .____ 6 References cited .__ _ 9

ILLUSTRATION

Page

Figure 1. Map of southeastern New York showing the metropolitan counties, location of 

wells for which water analyses are given, and the part of the area included 

in the Delaware River basin _ __. __ -_ _ -__ - - 2

TABLE

Page 

Table 1. Chemical analyses of water from selected wells in Dutchess, Orange, Putnam,

Rockland, Ulster, and Westchester Counties ___ _ _- --- 4

III
Sources of Ground Water in Southeastern New York

By Nathaniel M. Perlmutter

INTRODUCTION

This report summarizes the ground-water 

situation in six metropolitan counties of 

southeastern New York exclusive of those of 

New York City and Long Island. Conditions 

in the latter areas are covered in many published reports and are not discussed further. 

The six metropolitan counties of Dutchess, 

Orange, Putnam, Rockland, Ulster, and Westchester together include an area of about 

3,600 square miles extending northward from 

New York City (fig. 1). Small parts of Ulster 

and Orange Counties lie in the Delaware River 

drainage basin, but the rest of the area is 

almost entirely in the adjoining Hudson River 

drainage basin. Some of the counties are 

densely populated and use relatively large 

quantities of ground water and surface water. 

Owing to the probable increase in population 

and the corresponding increase in water use, 

these counties are considered to be possible 

users of water from the Delaware River 

basin. Accordingly, a brief summary of the 

sources of ground water within these counties 

is pertinent to the overall investigation of the 

water resources of the Delaware River basin.

The data contained in this report were collected as part of the investigation of the 

ground-water resources of New York conducted by the U. S. Geological Survey in cooperation with the New York State Water Power 

and Control Commission. Reports on the investigations in Putnam County (Grossman, 

1957) and in Westchester County (Asselstine 

and Grossman, 1955) have been published. At 

the time of publication of this circular a report on Rockland County (Perlmutter, 1959) 

was in press and one on Dutchess County 

(Simmons and others, 1959) was nearly ready 

for publication, but no investigations had been 

started in Orange and Ulster Counties. However, all the unpublished open-file data for

these counties may be examined in the off ices 

of the Geological Survey at Albany and 

Mineola, N. Y.

The information given in this report is 

based on interpretation of open-file data of 

the Geological Survey; published bulletins of 

the New York State Water Power and Control 

Commission, New York State Museum and 

New York State Department of Health, and the 

geologic map of New York State; and on field 

observations by the author, mostly in Rockland and Orange Counties. The preparation 

of the report was supervised by G. C. Taylo'r, 

Jr., district geologist, Ground Water Branch, 

Mineola District. Acknowledgment is due 

R. C. Heath, geologist-in-charge, subdistrict 

office, Albany, N. Y., for making available 

unpublished data.

GROUND-WATER HYDROLOGY SOUTHEASTERN 

NEW YORK

The source of most of the ground water in 

southeastern New York is the precipitation 

that falls directly on the area. The average 

annual precipitation ranges from about 50 

inches in the southern part to about 42 inches 

in the northern part. Some ground water is 

derived locally by induced infiltration from 

lakes and streams into water-bearing deposits.

Water occurs under water-table and artesian conditions both in the unconsolidated deposits of Pleistocene and Recent age and in 

the consolidated bedrock which is Precambrian to Devonian and Triassic in age. The 

unconsolidated deposits of Pleistocene age 

that are composed chiefly of poorly sorted 

andunstratified deposits of gravel, sand, silt, 

clay, and boulders laid down by glaciers are 

called till; those composed of well-sorted and
GROUND WATER IN SOUTHEASTERN NEW YORK

75° 74°

' ' ' /

f 0 T S EGO ^-- 'K, SCHOHARIEJ ALBANY 1

RENSSELAER

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NEW 

JERSEY

Chemical analysis of water is given in table 1 

Boundary of Delaware River basinINDEX MAP OF NEW YORK 

SHOWING AREA OF REPORT

42°

41°

75° 

Base from U. S. Geological Survey 1:500000 state map

Figure 1. Map of southeastern New York showing the metropolitan counties, location of wells for which water analyses are given, and the

part of the area included in the Delaware River basin.
GROUND-WATER HYDROLOGY OF SOUTHERN NEW YORK

.stratified sediments laid down by glacial melt 

waters are called outwash. Till covers most 

of the bedrock surface in the interstream 

areas and in minor stream valleys, and 

occurs also beneath outwash in the valleys of 

the major streams. Large bodies of outwash 

are distributed extensively in the valleys of 

the streams, shown on the map (fig. 1); numerous small, scattered bodies that occur in 

the valleys of minor streams and in other 

lowlands are riot shown on the map. Thin deposits of alluvium of Recent age occur in 

most valleys but they are generally unimportant as sources of water.

Water in the unconsolidated deposits moves 

through minute openings (pores) between the 

constituent grains. The size, shape, and interconnection of the pores determine the 

permeability the capacity of the deposits to 

transmit water. Owing to large differences 

in grain size and in sorting, the permeability 

ranges widely from place to place. For example, it is very high in outwash composed 

of well-sorted coarse sand and gravel and 

very small in outwash composed of silt and 

clay, and in till. Thus, in many places the 

outwash is capable of supplying more than 

1,000 gpm (gallons per minute) for public sup - 

ply and industrial use, whereas the yield from 

wells in till is generally small and sufficient 

for domestic use only.

The bedrock includes a large number of 

units of igneous, sedimentary and metamorphic rocks. In structure and composition the 

units range from the nearly horizontally bedded sandstone and shale of the Cat skill Mountains on the west to the complexly folded and 

faulted granite, and gneiss, and schist of the 

Hudson Highlands on the east. Many of the 

units extend across several counties. Their 

distribution is on the geologic map of New 

York (Merrill, 1901).

Water in dense crystalline rocks, such as 

granite, gneiss, schist, and quartzite,occurs 

chiefly in openings along joints, irregular 

fractures, faults, and cleavage planes; water 

in sedimentary rocks, such as sandstone, 

shale, limestone, and dolomite, occurs in 

openings chiefly along joints, bedding planes, 

solution channels (restricted to limestone and 

dolomite), and faults. The width of the openings ranges from a hairline to several inches. 

The number and size of the openings and 

their connection with a good source of recharge largely determine the yield of wells

drilled in rock. Most of the bedrock aquifers 

yield less than 10 gpm to individual wells; but 

several, such "as the sandstone and shale of 

the Newark group of Triassic age, the Inwood 

limestone of Precambrian age, and the Stockbridge limestone of Cambrian and Ordovician 

age, commonly yield several hundred gallons 

per minute to individual wells.

After reachingthe water table, water moves 

down the hydraulic gradient from areas of 

relatively high points on the water table to 

areas of relatively low points. Thus, the general pattern of movement of the ground water 

in bedrock is from areas of recharge in the 

uplands to areas of discharge on hillsides 

and in the bottoms of valleys. Similarly, 

ground water in the outwash, which is largely 

confined to lowlands, moves from relatively 

high points on the water table toward points 

of discharge at lower altitudes in streams 

and lakes. The small volume of water of 

many streams during the low-flow stage is 

furnished almost entirely by ground water 

issuing from springs and by direct percolation of ground water from the bedrock and 

outwash into the bottom of the streams.

In chemical quality, the ground water 

ranges from the soft, almost mineral-free 

water in Devonian sandstone in the Catskill 

Mountains to the hard, salty water in parts 

of the bedrock and unconsolidated deposits 

that border and underlie the Hudson River 

about as far north as Wappinger Creek in 

southwestern Dutchess County. The temperature of the water generally ranges between 

51° and 56°F. Chemical analyses of water 

from selected wells tapping the principal 

aquifers are given in table 1.

In the following sections the general geology 

and water-bearing characteristics of the deposits are summarized by counties.

DUTCHESS COUNTY

Dutchess County has an area of approximately 800 square miles. The permanent 

population is about 145,000 (1953 census). 

Most of the county is a gently rolling plain 

ranging in altitude from about sea level along 

the Hudson River to about 500 feet in the central part. However, summit levels generally 

range from about 1,000 to 1,300 feet in the 

Taconic Mountains section along the eastern 

border and are as much as 1,600 feet in the
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GROUND-WATER HYDROLOGY OF SOUTHERN NEW YORK

Hudson Highlands along the southern border. 

Several prominent lowlands, developed on 

relatively soft rocks, occur at altitudes of 

400 to 600 feet above sea level.

GENERAL GEOLOGY

The bedrock of Dutchess County consists 

of igneous, metamorphic, and sedimentary 

rocks ranging in age from Precambrian to 

Ordovician. Most of the rocks are closely 

folded and have a northeasterly strike.

The principal bedrock units are (a) undifferentiated granite and gneiss and diorite of 

Precambrian age, which crop out mainly along 

the southern border of the county; (b) the 

Hudson River formation (of former usage) 

that, as defined in this county, includes gray 

and red shale, slate, phyllite, and schist of 

the Nassau and Schodack formations of Cambrian age and the Deepkill and Normanskill 

shales of Ordovician age; and (c) the Cheshire quartzite of Cambrian age and the Stockbridge limestone of Cambrian and Ordovician 

age, which crop out in scattered elongate 

belts.

The Hudson River formation (of former usage) underlies more than two-thirds of the 

county. Unc on soli dated deposits of till and 

outwash of Pleistocene age cover most of the 

bedrock surface.

WATER-BEARING CHARACTERISTICS OF THE 

DEPOSITS

Ground water is obtained from all three 

principal rock types: till, outwash, and bedrock. Deposits of till range in thickness 

from less than a foot to more than 100 feet 

and generally are thinnest on the upland areas. 

Yields of wells in till average only a few gallons per minute. Deposits of outwash composed of sand and gravel yield the largest 

quantities of water in the county. Most of 

these deposits are restricted to the valleys 

of the major streams, Wappinger Creek, 

Fishkill Creek, and the Hudson River. Yields 

of 34 screened wells tapping outwash range 

from 10 to 800 gpm and average 300 gpm.

About 90 percent of the existing wells tap 

the Hudson River formation (of former usage) 

and the Stockbridge limestone. Yields of 439 

wells tapping the former range from 0 to

135 gpm and average 16 gpm; yields of 153 

wells tapping the Stockbridge limestone range 

from 0 to 220 gpm and average 22 gpm. Yields 

from the other bedrock units average about 

10 gpm. Chemical analyses of water from 5 

wells are given in table 1.

Twenty-four public-supply systems serve 

about half the population. Most of the water 

used is taken from surface sources, but 16 

public-supply systems are supplied wholly 

or in part with ground water. Withdrawals 

from these systems averaged about 1 mgd 

(million gallons per day) in T954. Total 

ground-water pumpage is about 7 mgd.

ORANGE COUNTY

Orange County has an area of about 840 

square miles. The permanent population is 

about 159,000 (1953 census). The land surface 

in Orange County is of four main physiographic types: (a) a broad, gently rolling 

plain in the central and northeastern part of 

the county, ranging in altitude from a few feet 

above sea level along the Hudson River to 

about 1,000 feet in the interior; (b) a dissected 

highland to the southeast, where the land surface reaches an altitude of more than 1,500 

feet; (c) a northeasterly trending belt of narrow ridges and valleys bordering the central 

plain on the northwest; v and (d) a small plateau in the extreme western corner of the 

county.

GENERAL GEOLOGY

The bedrock of Orange County consists of 

igneous, metamorphic, and sedimentary rocks 

ranging in age from Precambrian to Devonian. 

The rocks are folded and most of them strike 

northeasterly. Dips range from gentle to 

steep, and major faults are numerous. The 

central two -thirds of the county is underlain 

by gray slaty shale and sandstone composing 

the Normanskill shale and the Snake Hill for - 

mation of Ordovician age. To the southeast 

the Normanskill is in contact with elongate 

belts of infolded and faulted beds of limestone of Precambrian, Cambrian, and Ordovician age, and sandstone, conglomerate, 

and shale of Devonian age. These rocks, in 

turn,lie on and against the complex crystalline 

rocks of the Hudson Highlands area, which 

consist mainly of Precambrian granite and
GROUND WATER IN SOUTHEASTERN NEW YORK

gneiss. The Normanskill shale and Snake Hill 

formation are overlain unconformably to the 

northwest by the northwesterly dipping beds 

of the Shawangunk conglomerate and the High 

Falls shale of Silurian age, which form the 

Shawangunk Mountains. A prominent valley 

that parallels the northwest flank of the 

Shawangunk Mountains is underlain by northwesterly dipping beds of gray limestone, 

shale, and sandstone divided into several 

formations ranging in age from Late Silurian 

to Middle Devonian. The plateau to the northwest of the valley is underlain by sandstone 

and shale of Middle to Late Devonian age.

The bedrock in most of the county is covered by a mantle of unconsolidated deposits 

of till and outwash of Pleistocene age.

WATER-BEARING CHARACTERISTICS OF THE 

DEPOSITS

Records for about 200 wells and springs 

are available for Orange County. Because 

most of the data are for the extreme northwestern part, in the Neversink River valley, 

a detailed appraisal of the ground-water conditions requires considerably more fie Id work 

in other parts of the county than has been 

done to date.

The principal water-bearing units are till, 

outwash, and bedrock. The till covers the upland areas throughout most of the county. It 

yields to dug wells small quantities of water 

sufficient for domestic use. Deposits of outwash composed of sand and gravel and ranging 

from a few feet to several hundred feet in 

thickness occur in many of the stream valleys and other lowland areas; the largest deposit is in the valley of the Neversink River, 

and has a maximum thickness of more than 

200 feet. In 1957,total withdrawals from this 

deposit were only a small fraction of the recharge. Other deposits of outwash, chiefly 

those in the valley of the Walkill River and its 

tributaries, are tapped by a relatively small 

number of wells and represent potential 

sources of substantial quantities of water. 

The yields of 13 screened wells in outwash 

range from 3 to 380 gpm and average 85gpm.

Most of the wells in the county are completed in bedrock. The chief bedrock aquifer 

is the Normanskill shale of Ordovician age. 

Yields of 16 wells in the Normanskill range

from 2 to 80 gpm and average 30 gpm. Yields 

of 21 wells in sandstone and shale of Devonian 

age range from 1 to 80 gpm and average 20 

gpm. Yields of wells tapping limestone of 

Cambrian and Ordovician age are among the 

largest for bedrock aquifers; for 11 wells, the 

yields range from 18 to 150 gpm and average 

80 gpm. Wells in granite and gneiss have the 

lowest yields; for 10 wells, the yields range 

from 1 to 25 gpm and average 10 gpm. In 

quality the water ranges from soft to hard; 

locally, the iron content is excessive. Table 1 

gives analyses of water from 10 wells.

About 90 percent of the water used in the 

county is pumped from streams and other 

bodies of surface water. Public-supply systems for 28 communities ranging from small 

real-estate developments to municipalities 

obtain all or part of their water from wells 

and springs. These systems pumped about 

2 mgd in 1954, chiefly from deposits of outwash. Ground water is used also for domestic, agricultural, and some industrial purposes. Total ground-water pumpage is estimated to be about 7 mgd.

PUTNAM COUNTY

A detailed report (Grossman, 1957) on the 

ground-water resources of Putnam County 

has been published by the New York State 

Water Power and Control Commission. The 

following data are summarized from that 

report.

Putnam County has an area of 235 square 

miles. Its permanent population (1953 census) 

is about 22,000. The county is in the Hudson 

Highlands, a dissected belt of low mountains 

that rise from about sea level along the Hud -< 

son River to as much as 1,400 feet in the upland areas.

GENERAL GEOLOGY

The bedrock of Putnam County consists of 

folded and faulted igneous and metamorphic 

rocks ranging in age from Precambrian to 

Ordovician. More than 90 percent of the bedrock is composed of undifferentiated granite 

and gneiss of Precambrian age. The remainder consists of some scattered bodies 

of thePochuck gabbro gneiss of Precambrian 

age and a few northeast-trending narrow
GROUND-WATER HYDROLOGY OF SOUTHERN NEW YORK

infolded and faulted beds of the Cheshire 

quartzite of Cambrian age, the Stockbridge 

limestone of Cambrian and Ordovician age, 

and argillaceous and schistose beds of the 

Hudson River formation (of former usage). 

The consolidated rocks are overlain by an 

irregular mantle of unconsolidated till and 

outwash of Pleistocene age. In the upland 

areas, the till generally is less than 30 feet 

thick, but in some valleys it is as much as 

200 feet thick. Outwash consisting chiefly of 

sand and gravel is limited mostly to the valleys of the Hudson River, Peekskill Hollow 

Creek, Canopus Creek, and the Croton River.

WATER-BEARING CHARACTERISTICS OF THE 

DEPOSITS

Ground water is contained in till, outwash, 

and bedrock. Records of a few wells in till 

indicate an average yield of about 2 gpm. 

Many wells in till go dry during periods of 

low rainfall, owing to decline of the water 

table. Yields of 50 wells tapping outwash sand 

and gravel range from 1 to 450 gpm and average 33 gpm.

The yields of about 370 wells that tap bedrock, chiefly granite and gneiss, range from 

0 to 120 gpm and average 12 gpm. There is 

little difference in the average yields of wells 

tapping the different bedrock units. The 

average hardness of all the ground-water 

samples collected is 70 ppm, but it is as high 

as 480 ppm in water from the Stockbridge 

limestone. Locally the water has high iron 

content. Chemical analyses of water from 

six wells are given in table 1.

About one -third of the total estimated pump - 

age of 1.5 mgd in 1950 came from groundwater sources. Eleven public -supply systems 

use ground water wholly or in part. Ground 

water is used also for domestic, agricultural, 

and some industrial purposes.

ROCKLAND COUNTY

Rockland County has an area of approximately 200 square miles. The permanent 

population is about 114,000 (1957). The land 

surface ranges in altitude from about sea 

level along the Hudson River in the eastern 

part to about 1,300 feet in the mountainous 

northwestern part. The eastern two-thirds

of the county is a gently rolling plain which 

is bordered on its eastern margin by a narrow, curving ridge that rises as much as 800 

feet above the plain. The plain terminates 

against a rugged highland,the RamapoMountains, in the northwestern part of the county.

GENERAL GEOLOGY

The bedrock of Rockland County includes igneous, sedimentary, and metamorphic rocks; 

age is Precambrian, Cambrian and Ordovician, and Triassic. The eastern two-thirds 

of the county is underlain by beds of conglomerate, sandstone, and shale, a part of the 

Newark group, and igneous rocks, all of Triassic age. The largest differentiated unit of 

the igneous rocks is the Palisade diabase, a 

sill intruded into the sedimentary rocks in 

the eastern part of the county. The rocks of 

Triassic age dip gently northwest and terminate along a major fault marking the southeastern border of a belt of crystalline rocks, 

mostly granite and gneiss, of Precambrian 

age. A small area in the northeastern part 

of the county is underlain by an infolded and 

faulted belt of rocks of Cambrian and Ordovician age consisting of beds of quartzite, 

limestone, and phyllite. Deposits of till and 

outwash of Pleistocene age cover the bedrock 

in most of the county; in most places their 

total thickness is less than 30 feet but in some 

it is more than 300 feet.

WATER-BEARING CHARACTERISTICS OF THE 

DEPOSITS

Ground water is contained in till, outwash, 

and bedrock. Deposits of till are tapped by a 

small number of domestic wells, but yields 

generally are less than 5 gpm. Deposits of 

outwash composed chiefly of sand and gravel 

are restricted to valleys of such major 

streams as the Ramapo River, the Mahwah 

River, the Hackensack River, and Sparkill 

Creek. The outwash is generally less than 

100feet thickand yields about 50to 1,500gpm 

to individual wells; at some places it is composed largely of beds of clay and silt and 

very fine sand which do not yield large 

supplies.

The source of water from most of the wells 

in the county is the bedrock. The main waterbearing rocks are the beds of sandstone,
8 GROUND WATER IN SOUTHEASTERN NEW YORK

conglomerate, and shale of the Newark group. 

Yields of 200 wells tappingthese rocks range 

from 4 to 1,500 gpm and average about 80 

gpm. Public-supply wells tapping the Newark 

group have an average yield of about 300 gpm 

and an average depth of about 400 feet. Yields 

of 13 wells tapping the Palisade diabase average about 11 gpm. Yields of 37 wells obtaining water from granite and gneiss average 

about 15 gpm but most wells yield less than 

10 gpm. Yields from the other, minor bedrock units are small, commonly less than 10 

gpm. Onthe average, the water is moderately 

hard and is slightly alkaline. Chemical analyses of water from six wells are given in 

table 1.

Most of the water used in the county is 

taken from ground-water sources. An average of about 10 mgd was pumped in 1956. 

About 80 percent of the water was pumped for 

public-supply and industrial use and the remainder was pumped for domestic, agricultural, and other uses. A detailed report on 

the ground-water resources of the county has 

been prepared (Perlmutter, 1959).

ULSTER COUNTY

Ulster County has an area of approximately 

1,140 square miles. The permanent population is about 96,000 (1953 census). The land 

surface ranges in altitude from about sea 

level along the Hudson River to almost 4,000 

feet in the northwestern part of the county. 

The eastern half of the county is a gently 

rolling, dissected lowland. The Shawangunk 

Mountains and some minor ridges and valleys 

which trend northeastward across the county 

separate the eastern lowland from the dissected plateau, which is a part of theCatskill 

Mountains, in the northwestern half of the 

county.

GENERAL GEOLOGY

The bedrock of Ulster County consists of 

folded sedimentary rocks of Ordovician to 

Devonian age. The rocks in the eastern half 

of the county have a northeasterly to northerly 

strike. They consist of folded beds of gray 

and black shale and sandstone which belong 

largely to the Normanskill sjiale of Ordovician age. Unconformably overlying the Normanskill shale in the central part of the 

county are the northwesterly dipping beds of

the Shawangunk conglomerate of Silurian age 

of the Shawangunk Mountains. Minor ridges 

and valleys immediately west and north of 

the Shawangunk Mountains are underlain by 

folded beds, mainly of marine shale, sandstone, and limestone, that constitute several 

formations of Late Silurian to Middle Devon - 

ian age. The Catskill Mountains area in the 

northwestern half of the county is underlain 

by nearly horizontal beds of red and gray 

sandstone, shale, and conglomerate of Middle 

and Late Devonian age. The bedrock is overlain in most of the area by unconsolidated 

deposits of till and outwash of Pleistocene 

age. The maximum thickness of these deposits is more than 200 feet.

WATER-BEARING CHARACTERISTICS OF THE 

DEPOSITS

Little is known of the water-bearing characteristics of the deposits in Ulster County 

because only a small number of records of 

wells and springs have been collected in the 

county to date. As in other nearby counties, 

the main water-bearing units are till, outwash, and bedrock. The till is of irregular 

thickness and covers most of the upland 

areas. Average yields of wells in till probably are a few gallons per minute. Scattered 

outwash deposits of sand and gravel occur in 

the major stream valleys^ those of Rondout 

Creek, Walkill River, and Esopus Creek, and 

probably can yield large quantities of water 

to individual wells. The bedrock is the source 

of water for most of the wells in the county. 

The yields of wells tapping bedrock units 

probably are of the same order of magnitude 

as the yields from wells in the same units in 

Orange County (p. 6). Chemical analyses 

of water from 3 wells are given in table 1.

Most of the ground water is used for public - 

supply, domestic, and agricultural purposes, 

but a small quantity is used for industrial and 

miscellaneous purposes. About 10 publicsupply systems depend on ground-water 

sources wholly or in part. The total volume 

of ground water pumped by these systems 

averaged somewhat less than half a million 

gallons per day in 1954.

WESTCHESTER COUNTY

Westchester County has an area of about 

450 square miles. The permanent population
REFERENCES CITED 9

is about 753,000 (1957). The altitude of the 

land surface ranges from about sea level 

along the Hudson River in the western part 

of the county to about 700 feet in the Hudson 

Highlands in the northern part. The land 

surface in most of the county is characterized 

by northeastward-trending low ridges and 

valleys.

GENERAL GEOLOGY

In Westchester County, bedrock consists of 

closely folded igneous and metamorphic rocks 

which range fromPrecambrian toOrdovician 

in age and form northeastward-trending belts. 

The principal bedrock units are (a) the Fordham gneiss, (b) the Inwood limestone, and 

(c) the Manhattan schist, all of Precambrian 

age. In addition, small areas are underlain 

bytheHarrisondiorite and the Yonkers granite, of Precambrian age, and infolded belts of 

the Cheshire quartzite of Cambrian age, the 

Stockbridge limestone, slate of the Hudson 

River formation, (of former usage) and miscellaneous igneous rocks such as granite, 

pegmatite, and the undifferentiated mafic 

rocks of the .Cortlandt series. Outcrops of 

bedrock are numerous, but the bedrock surface in most of the county is covered by unconsolidated deposits of till and outwash of 

Pleistocene age that range in total thickness 

from a few feet to as much as 200 feet.

At least 70 percent of the wells in Westchester County tap the bedrock. The average 

yield from these wells is less than 30 gpm. 

Most of the bedrock wells tap schist and 

gneiss, and range in yield from 1 to 400 gpm. 

Limestone is the most productive type of 

bedrock, particularly in those lowland areas 

where it is overlain by water-bearing deposits of outwash. Yields of individual wells 

penetrating limestone range from 2 to as 

much as 450 gpm. Yields from other minor 

bedrock units generally are small. Chemical 

analyses of water from 8 wells are given in 

the table.

Public-supply systems, which serve about 

90 percent of the population, pump most of 

their water from lakes and streams. However, about 21 systems use ground-water 

sources wholly or in part. The estimated 

pumpage from public -supply wells was about 

6mgd in 1956. Wells and springs also supply 

domestic, agricultural, and some industrial 

needs but the amount supplied is unknown.

Records and logs of about 1,100 wells in 

Westchester County are given in Bulletin GW35 (Asselstine and Grossman, 1955) of the 

New York State Water Power and Control 

Commission.

REFERENCES CITED

WATER-BEARING CHARACTERISTICS OF THE 

DEPOSITS

Ground water is obtained from till, outwash, and bedrock. Deposits of till having a 

wide range in thickness are extensively distributed on the uplands and in some valleys. 

The till has a relatively low permeability and 

except where it contains sandy lenses yields 

only a few gallons per minute to dug wells. 

Scattered sizable deposits of outwash occur 

in parts of the county; the largest and thickest 

are restricted to large valleys, such as those 

of the Hudson River, Sawmill River and Croton River. The outwash consists mostly of 

sand and gravel but in some places is largely 

composed of silt and clay. Outwash deposits 

of sand and gravel yield the largest supplies 

to wells in the county. Individual wells range 

in yield from about 3 to 600 gpm. The average 

yield is about 200 gpm.

Asselstine,E. S., and Grossman, I. G., 1955, 

The ground-water resources of Westchester County, N. Y.,part 1, Records of wells 

and test holes: New York Water Power and 

Control Comm. Bull. GW-35.

Grossman, I. G., 1957, The ground-water resources of Putnam County, N. Y.: New York 

Water Power and Control Comm. Bull. GW37.

Merrill, F. J. H., 1901, Geologic map of New 

York: New York State Mus.

New York State Department of Health, 1954, 

Public-water supply data: Bull. 19.

Perlmutter,N.M., 1959,Geology and groundwater resources of Rockland County,N. Y. 

with special emphasis on the Newark group 

(Triassic): New York Water Power and 

Control Comm. Bull. GW-42.

Simmons,E. T., Grossman, I. G., and Heath, 

R. C., 1959, The ground-water resources 

of Dutchess County,N. Y.: U. S. Geol. Survey open-file rept.