MILFORD ANTIFORM

BLOODY

BLUFF

FAULT

BLOODY BLUFF FAULT

ASSABET

RIVER

FAULT

SPENCER BROOK FAULT

D

U

U

D

U

D

BLACKSTONE VALLEY FAULT

FISHERVILLE

POND

FAULT

FISHERVILLE POND FAULT

D

U

D

U

U

D

U

D

PURGATORY

CHASM FAULT

60

28

60

25

54

59

2

35

11

34

47

74

38

25

58

32

25

78

17

22

52

26

28

38

30

35

28 51

40

72

35

9

40

54

31

36

63 51 81

20

55 28

70

18 61

11 60

14

75

65

10

35

7

48

67

50

2 20

42

35

20

11 31

18

5

64

48

60

49

47

35

75

46

39

59

65 74

11

52

40 38

30

21

21 70

40

19

18

25

16 12 32

28

60 30

2

71

35

56

43

68

43

35

34 35

46

35

25

74

62

62

47

44

38

5

30

47 52 15

60

52

12

40

25

38

52 10

36

35

42

42

50

26

68

79

17

35 32

31 20 6

1

14 40

6

16

16

45

58

14

78

72 57

52

64

76

27

38

13

10

37 62

34

82

79

55

25

68

63

63

82 63

50

61

33 65

67

65

72

77

16 7

54 56 47

51 51 78

55

5040

46

65 68 55 58 54

30 68

62

25

33

68

75

80 33 80

65

64

15

61 25

41

58

17 46 52

43

62

49

46

72

59

51

45

42

18

44

29

51

9

22 17 38

41

68 40

78 60

40

16

39

21

22

1111

25

23

11

20

10

14 16

10

11

22

11

14

14

14

17

15

16

10

28

15

30

24

18

31 36

31 4015

26 40

34

40

32 28

19

50

21

74 69

68 60 24

64

45

65

60

11

71

66

78

69

67

65 80

64

36

43

54

53

50

39 80

23

26

26

45

57

44

47 22

1

39

43

38

55

61

64

30

14

1

20

18

47

35

8

5

28 45

41

54 45

64

21 57

36

12

6

15 14 24 11

15

28

16

34

21

17

25

24

16

9

24

18

24 13

24

36

7

15

10

10

18

18

87

17

20

13

18

17

67

21

10

22

25

24

23

9

89

19 22

20

31

31

34

34 18 18

26

23 22 24

17

16

17 16

18

18

28

24

25

21

16

14

18

18

21 20

78

18

22

2

18

15 25

24

16

10

10 16

24

19

16

25

12

14

19

13

12

71

15

8

18 46

21

12

11

6

11

14

7

12

20

24 58

14

12

12

2917

57

64

31 12

20

11

14

12

10

30

18 17 14

67 11

9

18

14

9 11

13 21

15 6

16

16

85

16

8 29

14

13

16

14

14

13

14 13

9

23

16

21

21

22

22

34

20

18

24

28 22

25

19

15

13

7

15 7

13

3121

25

19

18

13

20

24

17

16

20

13 18

29 17

26

18

18

17

25 16

27

21

13

11

78

58

16

16

9

9

20 18

76

34

22

22

74

38 34

12

41 16

31

21

25

22

70

28

22

13

10

19 31

20 6

10

16

20

27

12

10

21

7

21

27

27

22

41

27

31

26

32

42

24

16 24

15

16

12 18 13

22

8

20

25

18

16

11

18

12

24

11 24

15

19

20

17

15

10 76

16

12

15

15

12

12

26

21

23

24 22

21

18

14

14

17

16

17

12

61

35

24

25

21

20

52

30

30 16

16

15

10

25

25

21

20

20 16

11

10

28

26

38

44

26

30

82

27

24

24

34

28 21 50

15

81

22

28

24

36

19 18

21

16

27

18 12

17

15

15

22

21 24

23

17

16

21

21

15

19

17

17

13

31

22

21

19

25

16

18

15

24

24

77

24

75

19

14

18

16

21

21

21

17

21

16

15

18

22

18 18

17

22 21

27 22

223

20 24

20

27

27

86 24

24

21

76

25

80

30 28

16

17

29

26

18

28

25

86

20

22

22 21

20

15

28

26

22

75

22

21 28 15

20

29

11

16

22

12

31

37

18 15

25

48 21

21 26

25

22

24

27

31

23 27

82

22

21

18

16

38

18 18

10

19

18

34

30

21

20

18

28

19 22

18

20

26 21

29

17 19

23

82

24

19 18 24 82

28

21

25

21

26

20

76

31 5

15

17

32 16

19

25

21

19 19

17 17

19

22

83

32

35

19

28

62

18

20

20

47

3

58

68

38

85

89

42

8

80

19

45

16

56

25

20

68

64

Zw

m

Sn Sn

Zw Zw

Zw

m

Sd

m

Sn

Sn

Sn

Za

Zd

m

m

m

m

m

m

Za

Za

mg

Zhv

Zhv

Zpg

Zwa

Zwa

Zwq

Zwq

Sns

Sns

Sna

gg

gg

Zpg

Zpg

Zpg

Zpg

Zpg

Zwq

Zhv

Zhv

Zpg

Zwa

Zpg

Zpg

Zhv

Zhv

Zwa

Zhv

Zpg

Zpg

Zhv

Zpg

Zhv

Zpg

Zwq

Zhvb

Zhvb

Zhvb

Zhvb

Zhvb

Zhvb

Zhvb

Zhvb Zgdn

Zhvb

Zncg

Zhvb

Zpgh 

Zncg

Zncg

Zncg

Znclg

Sna

Sng

Zw

Sns

GR–352

607±5 Ma

GR–352

607±5 Ma

GR–23

612±5 Ma

GR–512

606±5 Ma

GR–32

356±4 Ma

339±17 Ma

GR–203

515±4 Ma

GR–202

2D

2A

2D

GR–202

2E

GR–45

13B

GR–52

13A

8D

8E

GR–247

GR–141

4

GR–039

8A

GR–067

8C

GR–385

2B

GR–485

2C

GR–109

8B

GR–275

12C

GR–276

12A

12B

GR–030

18

GR–040

20

GR–261

21

GR–286

15B

11

GR–244

BLOODY BLUFF FAULT

FISHERVILLE POND FAULT

BURLINGTON MYLONITE ZONE

Sn

Zd Sn Sn Zw

Sn

Sn Zw Zw

Zhv

Zhv

Zwq

Zhv

Zhv

Sna

Sns Zpg

Zpg

Zwa

Zhvb

Zhvb

Zhvb Zgdn

T A

A T

SPENCER BROOK FAULT

ASSABET RIVER FAULT

m gg m

DIABASE DIKE

MILFORD ANTIFORM

SURFACE

SEA LEVEL

4000

2000

FEET

3000

4000

1000

3000

1000

2000

A

NO VERTICAL EXAGGERATION 1000 feet = 305 meters

Surficial deposits not shown

SEA

LEVEL

4000

2000

3000

4000

1000

3000

1000

2000

B

NO VERTICAL EXAGGERATION 1000 feet = 305 meters

Surficial deposits not shown

C

NO VERTICAL EXAGGERATION 1000 feet = 305 meters

Surficial deposits not shown

FEET

SEA

LEVEL

4000

2000

3000

4000

1000

3000

1000

2000

FEET

A'B'

4000

FEET

3000

1000

2000

4000

3000

2000

1000

SEA LEVEL

C'

Apparent dip of dominant foliation Apparent dip of dominant foliation Apparent dip of dominant foliation

FISHERVILLE POND FAULT

Za

Zwq

Zw Zpg Zhv Zpg

Zhv

Zpg

Zhvb

Zncg

Zhvb

Zpgh

Zhvb

SURFACE

gg

m ASSABET RIVER FAULT

SPENCER BROOK FAULT

MILFORD ANTIFORM

TL UAF YELL AV E NOTSKCALB

Zpg Zpg Zpg

Zhv

Zhv

Zpg

Zhvb

Zpgh

Zpgh

Zncg

Zncg

Zncg

Zncg

Zncg

Znclg

SURFACE

Carpenter

MILFORD ANTIFORM Reservoir

BLOODY BLUFF FAULT

Sn

Sn

Sn

S Sns ns

B' C

C'

B

A

A'

Summary of U-Pb zircon ages from the Grafton quadrangle

Sample no. Map unit and rock type Age1 (Ma)

GR–203 Grafton Gneiss—Biotite granite gneiss (|gg) 515±4

GR–32 Marlboro Formation—Amphibolite gneiss (|m) 

Metamorphic zircon rims 339±17

Metamorphic zircon cores 356±4

GR–512 Hope Valley Alaskite Gneiss (Zhvb) 606±5

GR–352 Northbridge Granite Gneiss—Biotite granite gneiss (Zncg) 607±5

GR–23 Ponaganset Gneiss—Megacrystic biotite granite gneiss (Zpg) 612±5

1

Ages are from igneous cores, except for sample no. GR–32. 

Ages from the Grafton Gneiss and Marlboro Formation of the Nashoba zone obtained by sensitive high-resolution ion microprobe 

(SHRIMP) at Stanford University, Palo Alto, Calif. Ages from rocks in the Avalon zone obtained by SHRIMP II, Research School of 

Earth Sciences, Australian National University, Canberra, Australia.

Sns and Sna

Amphibolite

Megacryst

Garnet

Epidote

Sd intrudes |gg

Sd

|gg

GR–057

GR–061

GR–023

GR–550

GR–120

GR–040

GR–190

GR–201

GR–366 

GR–354 

GR–014 

GR–548 

GR–299 

GR–462

GR–064

GR–046

GR–274

GR–202

GR–202

GR–121

GR–070

GR–098

GR–272

GR–063

GR–492

GR–091

GR–466

Zhv

Zhvb

GR–305

1 MILE

CONTOUR INTERVAL 10 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 .5 0 1 KILOMETER

SCALE 1:24 000

1 1/2 0

APPROXIMATE MEAN

DECLINATION, 2011

MAGNETIC

 NORTH

TRUE NORTH

14 / ° 1 2

MAP LOCATION

MASSACHUSETTS

WORCESTER

NORTH SHREWSBURY

OXFORD

GRAFTON

UXBRIDGE BLACKSTONE

MILFORD

MARLBOROUGH

WORCESTER

SOUTH

7.5-MINUTE QUADRANGLE LOCATIONS

Merrimack zone

Nashoba zone

Avalon zone

DESCRIPTION OF MAP UNITS

[Stations labeled on the photographs in the Description of Map Units correspond to 

locations shown in the GIS database. Major minerals listed in order of increasing abundance]

MESOZOIC INTRUSIVE ROCK

Diabase dike (Jurassic)—Black, fine-grained to aphanitic, diabase dike 

with plagioclase phenocrysts as much as 1 cm long and red-weathering 

biotite phenocrysts as much as 5 cm across. Occurs as a 2-m-thick dike 

in the Hope Valley Alaskite (Zhv) unit. Contains flattened amygdules 

elongated parallel to the contact of the dike. Observed at a single 

roadcut on the southern side of the Massachusetts Turnpike in 

Westborough. Location and orientation shown on the map with a 

green strike and dip symbol. The age of the dike is probably Jurassic on 

the basis of a correlation with the dated Medford diabase shown on the 

State map (Zen, 1983), which yielded a K-Ar biotite age of 194±6 Ma 

(Zartman and Marvin, 1991; Wones and Goldsmith, 1991)

ROCKS OF THE NASHOBA ZONE

Paleozoic Intrusive Rocks

Quartz vein (Permian?)—Quartz veins locally contain variable amounts 

of graphite, muscovite, magnetite, epidote, hematite, biotite, chlorite, 

carbonate minerals, and K-feldspar. Milky to smoky quartz veins are 

generally tabular and locally irregularly shaped, show sharp boundaries 

with the host rock, and locally contain blocky to elongate quartz crystals 

with euhedral terminations in vugs as much as several centimeters 

across along the median line. Strike and dips symbols show the 

location and orientation of 55 veins. Measured veins are as much as 

0.6 m thick, dip steeply, and show variable strikes

Pegmatite dike (Permian) or sill (Ordovician to Mississippian)—Pink 

and white, white-weathering, coarse-grained, unfoliated, steeply 

dipping, tabular, muscovite granitic pegmatite dikes in the Avalon zone 

and foliated to weakly foliated syntectonic granitic pegmatite sills in the 

Nashoba zone. Red strike and dip symbols show the location and 

orientation of 10 pegmatite dikes in the Avalon zone and 2 pegmatite 

sills in the Nashoba zone. Pegmatites are as much as 3 m thick. 

Tabular dikes postdate the foliation and show a consistent strike to the 

northeast with a principal trend of 23°. Eight out of ten of the Avalon 

zone dikes were found in the southwestern part of the map. The two 

pegmatite sills in the Nashoba Formation occur in the plane of the 

foliation. Other smaller syntectonic pegmatite sills occur through the 

Nashoba Formation but were not mapped separately. A Permian age 

for the pegmatite dikes is inferred from regional correlations with 

similar dikes in Connecticut and Rhode Island (Zartman and Hermes, 

1987; Wintsch and Aleinikoff, 1987; Zartman and others, 1988; Walsh 

and others, 2007). An Ordovician to Mississippian age range exists for 

the granite pegmatites in the Nashoba Formation (Zartman and Naylor, 

1984; Hepburn and others, 1995, 1998; Acaster and Bickford, 1999)

Metadiorite (Silurian?)—Dark-gray, equigranular, fine- to coarse-grained, 

weakly to moderately foliated, biotite-hornblende diorite gneiss. 

Composition varies from quartz-biotite-hornblende-plagioclase to 

hornblende-quartz-biotite-plagioclase. The rock contains 7 to 13 

percent quartz, 3 to 38 percent hornblende, 15 to 22 percent biotite, 

and 38 to 49 percent plagioclase. Accessory minerals include 

K-feldspar, epidote-clinozoisite, chlorite, apatite, sphene, and 

magnetite. Outcrops show slabby weathering where the foliation is 

developed or blocky weathering where the foliation is poorly developed. 

Contains layers and xenoliths of Marlboro Formation. Occurs as a 

single map unit along the northern border of the map. An outcrop 

located along the power line in the adjacent Shrewsbury quadrangle 

(277885 E., 4681175 N.), approximately 300 m north of the 

quadrangle boundary, shows that the Marlboro Formation is intruded by 

the Grafton Gneiss, and the Grafton Gneiss is, in turn, intruded by the 

metadiorite. May be correlative along strike to the northeast with the 

Sharpners Pond Diorite (Zen, 1983; Wones and Goldsmith, 1991) 

dated as Silurian based on a conventional U-Pb zircon age of 430±5 

Ma (Zartman and Naylor, 1984; Wones and Goldsmith, 1991)

Paleozoic Metasedimentary and Metavolcanic Rocks

Nashoba Formation (Cambrian to Silurian)

Gray schist—Gray, medium- to coarse-grained, well-foliated, K-feldsparsillimanite-biotite-plagioclase-quartz-muscovite schist to gneiss to 

muscovite-K-feldspar-plagioclase-quartz-biotite schist. Contains 

accessory chlorite, monazite, epidote, clinozoisite, apatite, hematite, 

and opaques including magnetite. Locally contains garnet. Sericite and 

larger flakes of muscovite occur around K-feldspar. The dominant 

foliation is locally mylonitic and characterized by extensive quartz 

ribbons, sericite-biotite-sillimanite±magnetite layers, and quartz-feldspar 

leucosomes. Sillimanite-K-feldspar assemblages show retrogression as 

sericite alteration of K-feldspar and minor alteration of sillimanite in 

sillimanite-magnetite bands surrounded by sericite. K-feldspar occurs as 

pre- to syntectonic porphyroblasts, as much as several cm across, that 

show retrogression to sericite. Unit is exposed in the northwestern part 

of the map, above the Assabet River fault. Contacts with the 

amphibolite unit (S|na) are sharp, but gradational with the sulfidic schist 

(S|ns) by intercalation over a few meters. The best exposures occur 

along the Massachusetts Turnpike. The age of the Nashoba Formation 

is interpreted as Cambrian to Silurian (Hepburn and others, 1995, 

2004; Wintsch and others, 2007; Loan and others, 2011)

Amphibolite—Dark-green to black, rusty-weathering, well-foliated, fine- to 

medium-grained, plagioclase-hornblende amphibolite gneiss and lesser 

banded dark-gray and light-gray well-layered calc-silicate gneiss. 

Amphibolite contains accessory sphene and quartz, and trace pyrite, 

chalcopyrite, magnetite, epidote, and apatite. Calc-silicate gneiss is 

gray to light-gray, well-foliated, locally sulfidic, fine- to medium-grained, 

sphene-biotite-chlorite-actinolite-quartz-hornblende-plagioclase rock. 

The best exposures occur in roadcuts along the Massachusetts Turnpike

Garnetiferous schist—Dark-gray, medium- to coarse-grained, rustyweathering, moderately foliated, kyanite-garnet-quartz-sillimanite-biotite 

schist to granofels. Contains trace plagioclase, K-feldspar, magnetite, 

monazite, sericite, and zircon. Biotite defines the foliation and occurs 

as post-tectonic porphyroblasts across the foliation. Sillimanite occurs 

as fibrolite, needles, and prisms in the plane of the foliation, and as 

porphyroblastic prisms and needles across the foliation. Microscopic 

kyanite (<1 mm) and megascopic garnet (<1 cm) occur as post-tectonic 

porphyroblasts. Garnet contains fibrolite inclusion trails that define the 

overgrown dominant foliation, especially along the rims of the 

porphyroblasts. The foliation is defined largely by biotite, quartz, and 

sillimanite. Contacts with the surrounding map unit are not exposed. 

Exposed at two outcrops in Millbury, west of the Assabet River fault

Sulfidic schist—Dark-gray, medium-grained, rusty-weathering, wellfoliated, sulfidic, K-feldspar-plagioclase-sillimanite-biotite-quartzmuscovite schist. Contains accessory graphite, sulfides, and hematite. 

Sericite and larger flakes of muscovite occur around K-feldspar. Larger 

flakes of muscovite occur both along and across the foliation. 

Sillimanite and K-feldspar show retrogression to sericite. Contains rare 

amphibolite mapped as S|na. K-feldspar occurs as pre- to syntectonic 

porphyroblasts as much as 1 cm across. Typical exposures occur along 

Rose Lane, west of Lordvale Avenue (actually Brigham Hill Road) in 

western Grafton

Lower Paleozoic Intrusive Rocks

Grafton Gneiss (Cambrian)—Biotite granite gneiss. Light-gray to 

light-pink, medium-grained, well-foliated, chlorite-biotite-quartzplagioclase-K-feldspar granite gneiss. Contains secondary accessory 

sericite, epidote-clinozoisite, sphene, apatite, and magnetite. Relict 

allotriomorphic texture of intergrown quartz and 1 to 5 mm long 

feldspar grains shows grain-size reduction along crystal boundaries. 

The foliation is defined by recrystallized quartz and feldspar and aligned 

biotite. Plagioclase shows relict polysynthetic twinning, local myrmikitic 

texture, and significant saussuritization and sericitization in a 

microcrystalline sieve texture. K-feldspar shows relict braided albite 

lamellae and mesoperthitic texture, and sericitization. Biotite, 2 to 5 

percent of the rock, shows static retrogression to chlorite. Locally a 

dark- to medium-gray variety of gneiss contains 5 to 10 percent biotite. 

The interior of the map unit contains a somewhat heterogeneous 

distribution of biotite-rich and K-feldspar-rich layers but is relatively 

more homogeneous than the margins, which appear migmatitic and are 

characterized by abundant foliation-parallel granitic layers injected into 

the Marlboro Formation. Locally contains xenoliths of amphibolite 

derived from the Marlboro Formation. A SHRIMP U-Pb zircon age of 

515±4 Ma (sample GR–203) indicates a Cambrian age. Good 

exposures occur in Grafton on Brigham Hill, on Potter Hill, and in 

roadcuts along the Massachusetts Turnpike

Lower Paleozoic Metasedimentary and Metavolcanic Rocks

Marlboro Formation (Cambrian)

Amphibolite gneiss and layered gneiss—Dark-green to black, rustyweathering, well-foliated, fine- to medium-grained, plagioclasehornblende amphibolite gneiss containing banded dark-gray and 

light-gray well-layered felsic gneiss and calc-silicate gneiss. Amphibolite 

gneiss contains accessory sphene and quartz and trace pyrite, 

chalcopyrite, magnetite, epidote, and apatite. Layered gneiss consists 

of alternating bands of dark amphibolite gneiss and biotite amphibolite 

gneiss with lighter biotite granite gneiss, foliated pegmatite, and minor 

dioritic gneiss and calc-silicate gneiss. Banded gneiss with injected 

granitic material is migmatitic in the eastern part of the Marlboro 

Formation and along the contact with the Grafton Gneiss. Dioritic 

gneiss is dark-gray, medium- to coarse-grained, weakly to moderately 

foliated quartz-biotite-hornblende-plagioclase gneiss that somewhat 

resembles deformed Silurian(?) diorite (Sd), but unequivocal crosscutting 

relations were not observed, and it is unclear if the dioritic layers are 

intrusive. Calc-silicate gneiss is gray to light-gray, well-foliated, locally 

sulfidic, fine- to medium-grained biotite-chlorite-actinolite-quartzhornblende-plagioclase rock. Age is Cambrian or older on the basis of 

the age of the Grafton Gneiss, which intrudes it

Granofels—Gray, fine- to medium-grained, massive and blocky 

weathering biotite-quartz-feldspar granofels. May occur as a banded 

gneiss with as much as 50 percent of light-colored granitic and 

pegmatitic leucocratic layers. Contains thin layers of amphibolite that 

resemble rocks elsewhere in the Marlboro Formation. Unit is 

approximately 3 to 5 m thick, so its size is exaggerated on the map. 

Individual granofels layers, interpreted as beds, are 10 to 20 cm thick. 

Granofels consists of approximately 40 to 50 percent feldspar, most of 

which is plagioclase, 40 to 45 percent quartz, 3 to 10 percent biotite, 1 

to 3 percent epidote-clinozoisite, and trace to 1 percent magnetite, 

sphene, hematite, apatite, and tourmaline. Microscopically the rock 

contains an inequigranular granofelsic texture with weakly developed 

foliation defined by aligned biotite. Subangular to subrounded clasts, 

interpreted as detrital grains, of quartz and feldspar range in size from 

approximately 1.0 to 2.0 mm and occur in a matrix of quartz and 

feldspar that ranges in size from approximately 0.08 to 0.1 mm. 

Feldspar grains consist mostly of recrystallized, partly zoned plagioclase 

with relict polysynthetic twins and lesser microcline K-feldspar indicated 

by cross-hatched twinning. Contains rare detrital grains of 

quartz-feldspar rock fragments with hypidiomorphic texture, suggesting 

a possible granitoid source. The rock is interpreted as a metawacke. 

Contacts with the enclosing map unit are not well exposed, but adjacent 

rocks include banded amphibolite gneiss of the Marlboro Formation. 

Occurs as a single map unit exposed on the hilltop between Marble 

Avenue and Sibley Road, west of State Route 146, in the southeastern 

part of the Marlboro Formation

ROCKS OF THE AVALON ZONE

Intrusive Rocks

Metadiorite (Neoproterozoic)—Dark-gray to grayish-greenish-black, 

medium- to coarse-grained, weakly foliated quartz-plagioclasehornblende metadiorite to amphibolite. The rock contains 

approximately 3 percent K-feldspar, 5 percent epidote, 10 percent 

quartz, 35 percent plagioclase, 40 percent hornblende, and minor to 

trace amounts of biotite, apatite, and opaque minerals. Observed at 

one location between Estabrook Avenue and the Massachusetts 

Turnpike where it is in contact with megacrystic biotite granite gneiss 

(Zpg); a thin zone of unmapped gray biotite granodioritic gneiss was 

observed between the diorite and granite gneiss along the eastern 

contact of the metadiorite. Interpreted as a dike or sill

Amphibolite gneiss (Neoproterozoic)—Dark-green to black, 

well-foliated, fine- to medium-grained, plagioclase-hornblende 

amphibolite gneiss. Contains accessory sphene and quartz, and trace 

chlorite, pyrite, chalcopyrite, epidote, and apatite. Hornblende 

porphyroblasts are as much as 4 cm long. Hornblende forms both 

syntectonic aligned mineral grain lineations and post-tectonic random 

crystals oriented in the plane of the foliation. Occurs as 

foliation-parallel layers as much as 2 m thick. Observed at three 

locations and are shown as small units within the granitic gneisses 

northwest of Farnumsville, southwest of Fisherville, and along State 

Route 146 in Sutton. Contacts with the adjacent rocks are sharp. May 

be deformed dikes or sills, or restitic material

Hope Valley Alaskite Gneiss (Neoproterozoic)—Light-pink, tan- to 

rusty-weathering or light-gray- to white-weathering, fine- to 

medium-grained, equigranular, well-foliated, plagioclase-quartz-Kfeldspar alkali granite gneiss. Contains approximately 25 percent 

plagioclase, 30 to 40 percent quartz, and 30 to 40 percent K-feldspar, 

and trace to no biotite (Zhv) and trace to 3 percent biotite (Zhvb). 

Contains less than 2 percent muscovite and less than 1 percent 

magnetite. Zhvb contains up to 3 percent chlorite from retrograded 

biotite. Contains accessory garnet, apatite, and epidote-clinozoisite. 

Recrystallized quartz and feldspar define a faintly visible mineral 

aggregate lineation. Contacts are sharp where observed. The alaskite 

intrudes the Ponaganset Gneiss and the Northbridge Granite Gneiss. 

Four locations show strike and dip symbols for sill-like bodies that are 

subparallel to the foliation and as much as 3 m thick. Other thinner 

aplitic alaskite dikes or sills occur within the Ponaganset Gneiss and the 

Northbridge Granite Gneiss, but they were not mapped separately. A 

SHRIMP U-Pb zircon age of 606±5 Ma (Zhvb, sample GR–512) 

indicates a Neoproterozoic age. Good exposures occur along State 

Route 146, along the Massachusetts Turnpike, and on the cliffs east of 

Rockdale Pond just north of downtown Northbridge

Northbridge Granite Gneiss (Neoproterozoic)

Coarse-grained biotite granite gneiss—Light-gray to light-pink, very light 

gray to tan-weathering, coarse-grained and, locally, medium- to 

coarse-grained, equigranular, weakly to moderately well-foliated 

biotite-plagioclase-quartz-K-feldspar granite gneiss. Contains 

approximately 25 percent plagioclase, 35 percent quartz, 35 percent 

K-feldspar, and 3 percent biotite. Contains less than 1 percent 

muscovite and magnetite. Contains accessory chlorite, apatite, allanite, 

zircon, and sphene. Distinguished from the Ponaganset Gneiss by the 

equigranularity due to the lack of alkali feldspar megacrysts. 

Recrystallized quartz and feldspar, and aligned biotite flakes define the 

foliation and mineral aggregate lineations. A SHRIMP U-Pb zircon age 

of 607±5 Ma (Zncg, sample GR–352) indicates a Neoproterozoic age. 

Good exposures occur in Northbridge along School Street and at Shining 

Rock in Northbridge, and in Grafton along Milford Road, especially at 

the junction with Bay Farm Lane (street name not on base map)

Lineated coarse-grained biotite granite gneiss—Light-gray to light-pink, 

very light gray to tan-weathering, coarse-grained and, locally, mediumto coarse-grained, equigranular, weakly to moderately well-foliated and 

well-lineated biotite-plagioclase-quartz-K-feldspar granite gneiss. 

Recrystallized quartz and feldspar, and aligned biotite flakes define the 

foliation. Similar to Zncg, except for a distinctive mineral lineation on 

foliation surfaces defined by elongate spots and streaks of aggregated 

flakes of biotite as much as 2 cm wide and 7 cm long that locally form 

distinct mineral lineations, along with recrystallized quartz and feldspar. 

Good exposures occur under the power lines near Keith Hill Road, 

Salisbury Street, and at the southern end of George Hill in Grafton and 

Upton

Ponaganset Gneiss (Neoproterozoic)

Megacrystic biotite granite gneiss—Light-gray to light-pinkish-gray, very 

light gray weathering, medium- to coarse-grained, inequigranular, 

moderately to well-foliated biotite-K-feldspar-plagioclase-quartz granite 

gneiss with distinct white-weathering, light-pink perthitic microcline 

megacrysts or augen. Megacrysts are anhedral to subhedral 

semi-rectangular grains as much as 5 cm long, and their presence is a 

diagnostic mapping tool. Rock is similar in composition, age, and 

chemistry to the Northbridge Granite Gneiss (Zncg), and the contact 

with it is based on the presence of megacrysts in Zpg. Contains 

approximately 33 percent quartz, 33 percent plagioclase, 25 percent 

K-feldspar, 5 percent biotite, and minor to trace amounts of magnetite, 

muscovite, chlorite, epidote, and apatite. Recrystallized quartz and 

feldspar and aligned biotite flakes define the foliation and mineral 

aggregate lineations. A SHRIMP U-Pb zircon age of 612±5 Ma 

(sample GR–23) indicates a Neoproterozoic age. Good exposures 

occur at Purgatory Chasm and along State Route 146 in Sutton

Megacrystic biotite-hornblende granite gneiss—Medium-gray to pinkishgray, light-gray-weathering, medium- to coarse-grained, inequigranular, 

well-foliated biotite-hornblende-plagioclase-quartz-K-feldspar granite 

gneiss with distinct white-weathering, light-pink perthitic microcline 

megacrysts or augen. Megacrysts are anhedral to subhedral 

semi-rectangular grains as much as 5 cm long, and their presence along 

with hornblende are diagnostic features for mapping this rock. 

Contains approximately 30 percent plagioclase, 30 percent quartz, 25 

percent K-feldspar, 10 percent hornblende, 2 percent biotite, and 

minor to trace amounts of epidote, magnetite, titanite, clinozoisite, 

chlorite, and apatite. Recrystallized quartz and feldspar, and aligned 

biotite flakes define the foliation, and mineral aggregate lineations. 

Hornblende forms both aligned syntectonic mineral grain lineations, 

and post-tectonic random crystals oriented in the plane of the foliation. 

The unit is exposed along State Route 146 in Sutton, on unnamed hills 

north of Carpenter Reservoir, and in Northbridge Center 

Granodiorite gneiss at Nourse Farm (Neoproterozoic)— Medium-gray, 

weakly to strongly foliated, medium- to coarse-grained, equigranular 

K-feldspar-biotite-epidote-quartz-plagioclase granodiorite gneiss with 

distinctive green spots, laminations, and pods of epidote as much as 40 

cm across. Non-mylonitic samples contain approximately 30 to 50 

percent plagioclase, 10 to 30 percent quartz, 10 to 15 percent epidote, 

5 to 20 percent biotite, and 1 to 8 percent K-feldspar. Contains 

accessory muscovite, chlorite, clinozoisite, and sphene, and trace 

amounts of magnetite, apatite, zoisite, and hematite. Plagioclase shows 

greater saussuritization in the cores indicating chemical zoning from 

calcic cores to sodic rims. Secondary muscovite defines the foliation 

and is a replacement product of K-feldspar and plagioclase alteration. 

Named informally for exposures at the Nourse Farm located along the 

Grafton-Shrewsbury quadrangle border (Markwort, 2007). Unit extends 

into the Marlborough quadrangle to the north (Kopera and others, 

2006). Varies from medium to coarse grained where weakly foliated, to 

mylonitic and fine to medium grained at southernmost exposures on the 

Massachusetts Turnpike. Unit is exposed on the Massachusetts 

Turnpike, on the western side of the pond at the Nourse Farm off 

Jasper Street, and on the unnamed hill between Glenn Street and 

Jasper Street

Metasedimentary and Metavolcanic Rocks

Westboro Formation (Neoproterozoic)

Interlayered schist, phyllite, quartzite, amphibolite, and greenstone—A 

heterogeneous unit, variably interlayered or bedded, consisting of the 

following rocks: Silvery-gray to pale-green or gray, locally rustyweathering quartz-rich chlorite-muscovite-quartz phyllite and schist and 

calc-silicate phyllite and schist; pale-green to very light gray or white, 

tan- to rusty-weathering quartzite; dark-gray to pale-green calc-silicate 

granofels; dark-green to black amphibolite and greenstone; green 

chlorite-rich phyllite or schist; dark-green to black chlorite-biotite schist. 

Locally contains randomly oriented post-tectonic hornblende fascicles as 

much as 5 cm long. Calc-silicate phyllite to schist is a light-gray to 

gray-green, rusty- or tan-weathering, fine-grained, chlorite-calcitebiotite-plagioclase-quartz-tremolite rock that contains accessory epidote 

and sphene and trace apatite, hematite, pyrite, and zircon. Tremolite 

occurs as colorless to very pale green, very fine grained, elongate, 

parallel prisms aligned parallel to the foliation. Rarely observed bedding 

is generally less than 30 cm thick, but locally as much a 1 m thick, 

especially in quartzite. Mafic rocks (Zwa) and quartzite (Zwq) locally 

mapped separately. Unit is poorly exposed, especially in the 

southwestern part of the map. Best exposures occur in the vicinity of 

the Massachusetts Turnpike and Old Westboro Road

Interlayered amphibolite, greenstone, quartzite, and calc-silicate rock—A 

heterogeneous unit consisting of the following mafic metavolcanic, 

metavolcaniclastic, or calcareous metasedimentary rocks: Darkgrayish-green to black or green, fine-grained magnetite-actinolite-quartzepidote-hornblende-plagioclase-chlorite amphibolite, greenstone, or 

schist; calcite-quartz-epidote-biotite plagioclase schist to phyllite; 

sphene-epidote-clinozoisite-biotite schist; and quartzite. Actinolite 

shows pale-yellow to pale-green pleochroism and fine acicular habit 

generally parallel to the foliation. Hornblende occurs as larger 

post-tectonic porphyroblasts with green to bluish-green pleochroism 

and forms stubby to elongate fascicles as much as 5 cm long randomly 

oriented across the foliation. Rocks contain variable amounts of minor 

or accessory chlorite, muscovite, quartz, plagioclase, sphene, and 

apatite where they are not major minerals. Contains trace ilmenite, 

calcite, dolomite, and hematite. Epidote-clinozoisite and apatite locally 

occur as abundant post-tectonic porphyroblasts as much as 0.25 mm 

across. Locally contains secondary dolomite in vugs as much as several 

centimeters across. Best exposed along the Massachusetts Turnpike 

and along Brendan Drive (name not on base map) south of Millbury 

Street in Grafton

Quartzite—Pale-green to very light gray or white, gray- or tan- to rustyweathering massive quartzite, laminated quartzite, muscovite-quartz 

schist, and minor calc-silicate rock. Contacts are sharp or gradational 

by intercalation. Best exposed at a roadcut on the northern side of the 

Massachusetts Turnpike, just west of the Old Westboro Road overpass

EXPLANATION OF MAP SYMBOLS

Contact—Approximately located; dotted where concealed by water

Outcrops—Areas of exposed bedrock or closely spaced contiguous 

bedrock exposures examined in this study; dashed outcrops consist 

ofclosely spaced loose blocks where rock type was identified, but 

nostructural measurements were made

FAULTS

Inferred mylonitic thrust fault—Parallel to metamorphic foliation; 

sawteeth on upper plate show dip direction; locally reactivated as a 

normal brittle fault along the Bloody Bluff fault. Dotted where 

concealed

Mylonitic fault in the Burlington mylonite zone—May be parallel to, or 

postdate, metamorphic foliation; relative motion indicated where 

known; U, upthrown side; D, downthrown side

Brittle fault—Dashed where inferred; U, upthrown side; D, downthrown 

side

FOLDS

[Showing trace of axial surface, direction of dip of limbs, and direction of plunge

where known or inferred]

Inferred trace of F1 fold in the Nashoba Formation (Salinic 

orogeny?)—Box shows dip direction of axial surface; arrow shows 

plunge direction

Upright folds of the Milford antiform in the Avalon zone 

(Alleghanian orogeny)—Arrow shows plunge direction

Antiform

Synform

PLANAR FEATURES

[Symbols may be combined; point of intersection shows location of measurement]

Strike and dip of diabase dike (Jurassic)

Strike and dip of pegmatite dike (Permian)

Inclined

Vertical

Strike and dip of quartz vein (Permian)

Inclined

Vertical

Strike and dip of alaskite sill (Neoproterozoic)

Strike and dip of deformed relict foliation (S1)—Parallel to bedding in 

the Westboro Formation or compositional layering in the Nashoba and 

Marlboro Formations 

Strike and dip of compositional banding in intrusive rocks—Observed 

at the contact between Zncg and Zpg in the Avalon zone, where it 

predates the dominant foliation; also observed in intrusive rocks within 

the Marlboro Formation

Strike and dip of dominant foliation (S2)—In the Avalon zone, a 

schistosity in the Westboro Formation and a gneissosity in the intrusive 

rocks; in the Nashoba zone, a schistosity to gneissosity in the 

metasedimentary and metavolcanic rocks and a gneissosity in the 

intrusive rocks

Inclined

Vertical

Strike and dip of mylonitic or phyllonitic S2 foliation—Observed 

especially along the Bloody Bluff fault and in the Burlington mylonite 

zone

Strike and dip of axial surface of F2 minor fold parallel to S2

foliation—Tight to isoclinal, locally rootless folds in the Avalon zone, 

especially in the Westboro Formation and rootless isoclinal folds in the 

Nashoba Formation 

Inclined

Vertical

Strike and dip of axial surface of F3 minor fold—Open to tight, late 

fold observed in the Westboro, Marlboro, and Nashoba Formations; 

probably Alleghanian and correlative across structural domains 

Inclined

Vertical

Strike and dip of F3 minor kink bands—Late kink bands locally 

associated with extensional veins or pegmatites; probably Alleghanian 

and correlative across structural domains 

Inclined

Vertical

LINEAR FEATURES

[Symbols may be combined; point of intersection shows location of measurement]

Bearing and plunge of L2 intersection lineation—Intersection between 

the dominant foliation (S2) and an older foliation that is parallel to 

compositional layering (S1) in the Westboro Formation; and intersection 

lineation between the dominant foliation (S2) and an older foliation that 

is parallel to compositional layering (S1) in the Nashoba Formation

Bearing and plunge of L2 mineral lineation—Aggregate lineation or 

grain lineation associated with the dominant foliation (S2); consists of 

quartz, biotite, amphibole, or K-feldspar in the Avalon zone and quartz, 

biotite, amphibole, sillimanite, or K-feldspar in the Nashoba zone

Bearing and plunge of L3 mineral lineation—Grain lineation of aligned 

hornblende that postdates the dominant lineation in the Avalon zone; 

observed in map units Zpgh and Za

Bearing and plunge of F2 minor fold axis—Fold axis of tight, isoclinal, 

or rootless fold associated with the dominant foliation (S2)

Bearing and plunge of F3 minor fold axis—Fold axis of late open to 

tight fold; observed primarily in the Westboro Formation

OTHER FEATURES

Geochronology sample location showing age

Quarry

Well—Cuttings used to identify rock type

Spring

Site of photograph—Tip of arrow at point of observation; number keyed 

to station in GIS database and figure(s) in explanatory pamphlet

Sn

Sd

Sna

Sng

Sns 

gg 

m

mg

Zd

Za

Zncg

Znclg

Zpg

Zpgh

Zhv

Zhvb

Zgdn

Zw

Zwa

Zwq

gg

mg m

Zhv

Zhvb

Zncg

Zpg

Zpgh

Westboro Formation

Ponaganset 

Gneiss

Hope Valley 

Alaskite Gneiss

MESOZOIC AND PALEOZOIC INTRUSIVE ROCKS

FOUND THROUGHOUT THE MAP

Marlboro Formation

Nashoba Formation

Northbridge 

Granite 

Gneiss

Grafton Gneiss

Sd

Sn Sng

Sna

Sna Sns

Spencer Brook fault

Assabet River fault

PALEOZOIC

METASEDIMENTARY AND 

METAVOLCANIC ROCKS

NEOPROTEROZOIC INTRUSIVE ROCKS

Zd

Za

Bloody Bluff fault

Zgdn

Zw

Zw Zwq

Zwa

Znclg

NEOPROTEROZIC 

METASEDIMENTARY AND

METAVOLCANIC ROCKS

PALEOZOIC INTRUSIVE ROCKS

CORRELATION OF MAP UNITS

NASHOBA

ZONE

AVALON

ZONE

SILURIAN(?)

JURASSIC

PERMIAN(?)

PERMIAN

CAMBRIAN

SILURIAN TO

CAMBRIAN

NEOPROTEROZOIC

Diabase dike

Quartz vein

Pegmatite dike

Pegmatite sill

Nourse 

Farm

Nashoba zone

Nashoba zone

Base from U.S. Geological Survey, 1:24,000

Grafton,1969 (photorevised 1979)

Polyconic projection. 1927 North American Datum

10,000-foot grid ticks based on Massachusetts coordinate 

systems. 1,000-meter Universal Transverse Mercator 

grid ticks, zone 19, shown in blue

Geology mapped in 2005–06 by Walsh

Geochronology in 2005–08 by Aleinikoff

Geochemistry in 2007 by Dorais

Digital compilation by Walsh

Printed on recycled paper 

This map is available for sale by U.S. Geological Survey, Information Services, 

Box 25286, Denver Federal Center, Denver, CO 80225

For product and ordering information:

World Wide Web: http://www.usgs.gov; Telephone: 1–888–ASK–USGS

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

SCIENTIFIC INVESTIGATIONS MAP 3171

Explanatory pamphlet accompanies map

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY 

Prepared in cooperation with the

COMMONWEALTH OF MASSACHUSETTS

MASSACHUSETTS GEOLOGICAL SURVEY

BEDROCK GEOLOGIC MAP OF THE GRAFTON QUADRANGLE, WORCESTER COUNTY, MASSACHUSETTS

By

Gregory J. Walsh,1 John N. Aleinikoff,2 and Michael J. Dorais3

1U.S. Geological Survey, Montpelier, VT 05601. 2U.S. Geological Survey, Denver, CO 80225. 3Brigham Young University, Provo, UT 84602.

2011

Avalon zone

Avalon zone