Fracture Characterization Mapping for Regional Geologic Studies: The Hydrostructural Domain Approach, Ayer Quadrangle, Massachusetts

Stephen B. Mabee And Joseph P. Kopera

Office of the Massachusetts State Geologist Geosciences Department University of Massachusetts 611 North Pleasant Street Amherst, MA 01003

Abstract

While traditional bedrock geologic maps contain valuable information, they commonly lack data on brittle fracture characteristics and distributions. The increased need for better understanding of groundwater flow behavior in bedrock aquifers has made this data critical. The concept of hydrostructural domains is used to redefine bedrock mapping units based on an assemblage of lithologic and fracture characteristics thought to be important controls on groundwater flow and recharge. These maps are constructed from detailed field observations and measurements of 2000-3000 fractures from 60-70 stations across a 7.5' quadrangle. Hydrostructural domains are displayed on the map as traditional lithologic units would be, with detailed descriptions and photos of the fracture systems contained in each hydrostructural “unit”. In the Ayer quadrangle, such domains closely correspond with bedrock lithology and ductile structural history. Steeply dipping metasedimentary rocks of the Merrimack Belt have pervasive, closely spaced, throughgoing fractures developed parallel to foliation, and therefore may provide excellent potential for vertical recharge and foliation-parallel flow. Where these rocks are intensely cut by a strong subhorizontal cleavage, a parallel fracture set dominates providing an opportunity for lateral flow. Massive granites generally have a well-developed, widely-spaced orthogonal network of fracture zones which may provide excellent local recharge. High-grade gneisses of the Nashoba formation have poorly developed fracture sets except near regional shear zones, where foliation parallel fractures and cross-joints may provide good vertical recharge and provide a strong northeast trending flow anisotropy. These maps are intended to provide regional-scale information to assist in site-specific groundwater investigations. We believe that such maps are an example of how new types of geologic maps can, and must, be developed to address changing societal needs. The next step is to determine if the qualitative descriptions provided by the hydrostructural domain can be translated into a quantitative measure of hydraulic properties.

169 to 186 Introduction

The use of fractured-bedrock aquifers to meet private, public and commercial water supply needs is increasing in the New England region. Municipalities and water suppliers are finding it increasingly difficult to locate and develop water supplies in overburden aquifers because of contamination and a lack of suitable sites. As a result, water suppliers are going deeper into bedrock aquifers (Drew et al., 2001). Yet information on the factors that influence the availability and recharge characteristics of fractured bedrock aquifers in highly deformed crystalline metamorphic rocks is limited.

The availability of water in fractured rock aquifers is particularly critical in eastern Massachusetts because growth and development along the coast, major transportation corridors, and in rural communities adjacent to large metropolitan areas is extensive. The I-495 corridor, a circumferential highway 20 miles west of Boston, has become the focus of recent growth. Professional office buildings, research and development parks associated with the computer industry, warehouses and light industry are springing up along this corridor, as are housing and condominium developments. Municipalities and water suppliers are simply unprepared for this onslaught of development and need help in understanding the complex dynamics of the ground water system.

Since 2003, the Office of the Massachusetts State Geologist has been preparing a new suite of 1:24,000 scale mapping products referred to as fracture characterization maps (Mabee and Salamoff, 2004; Mabee, 2005; Kopera et al., 2006). These maps were created to fill a critical need for brittle fault and fracture information relevant to groundwater issues in bedrock environments that was not being met by traditional bedrock geologic mapping. This purpose of this paper is to describe briefly how these maps are constructed and utilized by the user community. We will provide an example of one of the fracture characterization maps from the 1:24,000-scale Ayer quadrangle in eastern Massachusetts.

Study Area

The Ayer quadrangle is located approximately 25 miles NW of Boston. Two major transportation routes traverse the quadrangle (Figure 1). These include Interstate 495 and Route 2. I-495 is the major transportation corridor connecting northern New England with the entire eastern seaboard megalopolis. Route 2 is the major east-west corridor that transits the northern half of Massachusetts connecting Boston with New York state. Both of these transportation routes and the communities adjoining them have been the focus of development and recent growth (Figure 1).

The Ayer quadrangle encompasses parts of six communities, Groton, Ayer, Shirley, Harvard, Boxborough and Littleton (Figure 2). The population has grown by an average of 22% from 1980 to 2000 with Boxborough and Groton growing at 53% and 42%, respectively. An average of 283 acres per year are being converted from undeveloped to developed land in the six towns combined. Nearly 15% of the total land area in Groton has become developed in the 28 years extending from 1971 to 1999. Growth is expected to continue in the coming years.

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Rt 2

Figure 1. Map showing the percentage of land area, by town, converted from undeveloped land (crops, pastures, forests, open space) to developed land (residential, commercial, industrial land uses, etc.) from 1977 to 1999 (from MassGIS). Note correlation between growth patterns and major state routes and interstates. The Ayer quadrangle is labeled Project 1.

All of the communities rely primarily on groundwater to meet their potable water supply demands despite being within 30 miles of Boston. All water is supplied either through private domestic wells or limited municipal water supply systems. Ayer, Groton, Littleton and Shirley have municipal water supply systems comprised of gravel pack wells but most individuals not connected to public water rely on bedrock wells to meet demand. Harvard and Boxborough have no public water supply systems and must rely predominantly on bedrock wells.

Geology

In the broadest terms, Massachusetts is the amalgamation of rocks from three tectonic plates (Figure 3). These include rocks associated with the margin of Figure 2. Close-up view of the Ayer Quadrangle. Approximate location of Fort Devens shown in blue shaded area.

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Laurentia, medial New England, which includes Ordovician-age intrusive and extrusive rocks associated with an island arc system and Proterozoic rocks of uncertain origin, and the Avalon plate. These plates were active during the early to middle Paleozoic (Robinson et al., 1993).

Figure 3. Generalized map of Massachusetts showing the geographic distribution of lithotectonic packages and terranes into which the rocks of the State have been grouped (modified from Hatch, 1991). Regions discussed in this proposal are bold and all caps.

Structural and metamorphic features in Massachusetts were produced during three orogenic events. These include the late Ordovician Taconian Orogeny (the docking of medial New England with Laurentia, affecting central and western Massachusetts), the Devonian Acadian orogeny (the docking of Avalon with amalgamated Laurentia and medial New England, affecting eastern and central Massachusetts predominantly) and the Pennsylvanian-Permian Alleghenian orogeny (metamorphism and reactivation of faults produced by the collision of Africa with North America). All structures were later modified and reactivated by extension in the Mesozoic during opening of the present day Atlantic Ocean (Robinson et al., 1993).

Sandwiched between medial New England and Avalon is an enigmatic terrane referred to as the Nashoba terrane (Figure 3). Acceptance as a separate terrane did not take place until the early 1980’s following identification of the large terrane-bounding fault zones (Castle et al., 1976; Bell and Alvord, 1976). The Clinton Newbury fault zone delineates the western edge of the Nashoba terrane and separates it from the eastern portion of medial New England, an area known as the

4 Merrimack Belt (the Merrimack Belt lies within the Merrimack Synclinorium and forms the eastern half of medial New England) (Figure 3).

Rocks of the Merrimack Belt are comprised of calcareous metasiltstones, phyllite, metasandstones and quartzites of Silurian and Ordovician age (Robinson and Goldsmith, 1991). The rocks immediately west of the Clinton-Newbury fault are metamorphosed to the lower greenschist facies and progressively rise in grade toward the northwest (Hepburn, 2004). In contrast, the rocks of the Nashoba terrane, east of the Clinton-Newbury fault, are multiply deformed and metamorphosed middle to upper amphibolite facies rocks (sillimanite and sillimanite-K feldspar zones) (Hepburn, 2004) consisting of largely metavolcanic materials to the east and metasedimentary rocks to the west (Goldsmith, 1991).

The Ayer quadrangle is located at the junction between the Merrimack belt and the Nashoba terrane (Figure 4). The Clinton-Newbury fault passes through the southeast corner of the

Figure 4. Generalized bedrock geologic map of the Ayer quadrangle). Modified from Zen et al. (1983).

5 quadrangle (Zen et al., 1983). Northwest of the fault the rocks consist of metasiltstones and calcareous phyllite of the Silurian Oakdale formation, quartzites and phyllites of the Silurian Tower Hill Quartzite, calcareous metasiltstone, biotitic metasiltstone and metasandstone of the Silurian Berwick formation and the Pennsylvanian Harvard conglomerate (Robinson and Goldsmith, 1991). Three plutonic rocks ranging in age from Ordovician to Devonian intrude the metasediments. These include the granites of the Ayer Formation, the Chelmsford granite and diorites and tonalities.

Southeast of the Clinton-Newbury fault are the Tadmuck Brook schist and Nashoba formations. The Tadmuck Brook schist is a rusty weathering, sulfidic schist whereas the Nashoba formation is composed largely of biotite-feldspar-quartz gneisses and schists, amphibolite and calc-silicate rocks (Zen et al., 1983). The Clinton-Newbury fault trends northeast and dips 50° to 70° to the northwest. Motion history has been complex with evidence of thrusting and strike-slip displacements followed by normal motion (Goldstein, 1998).

Surficial deposits in the quadrangle consist of glacial till and/or bedrock and sand and gravel deposits associated with major stream valleys (Jahns, 1953). The tills are exposed predominantly in the higher elevations of the project area. The sand and gravel deposits generally occur in the lower elevations and river valleys.

Making the Fracture Characterization Map

Characterization of fractures is performed using the station approach. Approximately 40 to 70 outcrop stations are established across the quadrangle in as uniform a distribution as practicable. Most of the work is performed along roadcuts rather than pavements in order to get a better three-dimensional view of the fractures. Outcrops with various orientations are selected also to reduce orientation bias in the data. At each station, the following information is recorded: 1) rock type; 2) orientation of foliation, through-going fractures, faults, unloading (sheeting) joints and fracture zones; 3) fracture spacing and length distributions; 4) intersection relationships; 5) style and type of mineralization; 6) type of overburden; 7) the number of subveritical fracture sets; and, 8) presence or absence and degree of development of partings parallel to foliation. The data are entered into an Access database, queried and summarized in the standard way using stereonets and rose diagrams.

The fracture characterization map actually consists of four individual panels and two databases. The first panel is the Hydro-Structural Domain panel. The units and overlay zones shown on the map define regions that contain attributes thought to be important in influencing groundwater availability and flow in the bedrock. These attributes include bedrock type, the presence or absence of layering (foliation) in the rocks, the degree of development of transmissive partings parallel to the layering, the intensity of sub-horizontal sheeting development, the number and distribution of regional joint systems and outcrop-scale faults, and the distribution of permeable surface materials. Hydro-Structural Domain map units do not always follow lithologic contacts but can be independent polygons grouping more than one formation or unit.

As an example, one domain might consist of massive, unfoliated rocks with two ubiquitous orthogonal vertical fracture sets and well developed sheeting joints. The vertical fractures likely

6 provide recharge potential and the sheeting joints afford lateral connectivity. In another domain, there may be a highly foliated rock that exhibits a strong tendency to part parallel to foliation. Cross fractures may be present but are weakly developed and there are no visible sheeting joints. In this latter situation, it is expected that the penetrative foliation fabric will provide excellent recharge capability but also will impart a strong flow anisotropy parallel with the foliation. Thus, fracture mapping redefines mapping units on the basis of an assemblage of characteristics thought to be important for influencing groundwater flow. These mapping units are referred to as hydro-structural domains (Allen et al., 2003; Mackie, 2000) and the assumption is that the unique physical characteristics inherent in each domain reflect different hydraulic properties.

The second panel in the Fracture Characterization Map is the Raw Fracture panel and includes a summary of all the raw data that was collected in the quadrangle. It includes individual rose diagrams at each station overlain on the geologic map, summary plots of all faults, foliation, vertical fractures, sheeting joints and identifies the orientation of major fracture sets including their spacing and length distributions. The third panel is the Spatial Analysis panel. This panel shows the spatial distribution of each major fracture family across the quadrangle. It also includes the distribution of all outcrop-scale faults and fracture zones.

The fourth panel is the Water Resources Panel. One of the byproducts of our bedrock and fracture mapping effort is the collection of well data from various sources to improve the quality of our map. We build a stand-alone, ArcView 3.x well inventory for each quadrangle that provides stratigraphic information or scanned boreholes logs, depending on what is available, of individual wells. The user opens the well shapefile, links the tables and the log or tabular data pops up on the screen when individual wells are selected. From the well data we construct a generalized structure contour map of the bedrock surface, isopach map of overburden thickness, contour map of groundwater elevations in the bedrock and a map showing the extent of permeable overburden lying over the bedrock. These maps are constructed at 1:72,000 scale and assembled on one sheet to make the Water Resources Panel.

Finally, all of the fracture data is assembled in a stand-alone, ArcView 3.x database and supplied with the fracture map. This allows consultants and other users the flexibility to extract data and augment their own site-specific data.

Results: The Ayer Fracture Characterization Map

The Fracture Characterization Map of the Ayer quadrangle represents the synthesis of 2980 brittle fracture measurements collected at 61 outcrops distributed across the quadrangle. The development, intensity, and orientation of fractures in the Ayer quadrangle is strongly influenced by bedrock lithology and structure. Four hydro-structural domains have been defined within the Ayer quadrangle based on mapped lithology (Kopera, 2006) and observed fracture characteristics. A brief description of the characteristics of each domain and their possible hydrologic significance is discussed below.

Domain 1 – Steeply Dipping Layered Rocks of the Merrimack Belt - Moderately to steeply dipping well-layered quartzites, phyllites, and schists of the Merrimack Group comprise Domain 1 (Figure 5). The dominant fracture sets in this domain are well-developed northeast-trending

7 near-vertical partings parallel to foliation/bedding (236° set) and cross-joints orthogonal to bedding (158°), which provide excellent potential for vertical recharge. A separate subdomain (1B) has been defined where a strong east-northeast to northeast trending subhorizontal anisotropy exists in the rock. The anisotropy is caused by well developed subhorizontal fold axes (F2) and a strong parting parallel to a subhorizontal axial planar cleavage (S2), providing good northeast-southwest lateral flow and connectivity between vertical fractures. Moderately to shallowly dipping, northwest trending fracture sets (303° and 331°) are also well developed in domain 1, and tend to have shorter trace lengths and/or truncate against steeply dipping fractures, providing good hydrologic connection between individual vertical fractures. East-west trending fracture zones in the north-central and northwest portion of domain 1A may serve as areas of local vertical recharge, and form lineaments on aerial photographs and topographic maps. Faults and fracture zones in domain 1 occur in the southernmost central portion of the quadrangle, along Pin Hill, and may be the most hydrologically significant features in that immediate area.

Figure 5. Summary data describing the characteristics of hydro-structural domain 1.

8 Domain 2 – Massive Granite and Foliated Gneisses - The massive Ayer and Chelmsford granites and foliated granitoidal gneisses of the Devens gneiss complex comprise Domain 2 (Figure 6). Conjugate sets of north-south trending, steep to moderately dipping fracture sets (198° and 162°) and an east-west trending set (274°) are the dominant fracture sets in these rocks. Northeast trending, moderately to steeply dipping partings parallel to foliation (238°) locally dominate near the Clinton-Newbury Fault Zone. Rocks with pervasive partings parallel to foliation have been separated into Subdomain B. All of these sets have excellent potential for vertical recharge. The north-south trending conjugate sets tend to occur in conjunction with each other, forming fracture zones that may locally focus vertical recharge. Sheeting is well developed in these rocks, and has been observed in the field to be a major conduit for lateral groundwater flow near the surface. The more massive Chelmsford and Ayer granites (subdomain A) contain sheetlike xenoliths of presumed Merrimack Belt metasediments. These xenoliths are typically less than 1 meter wide, but are several meters long, and have well developed partings parallel to foliation. In the Ayer granite they are commonly near vertical, while in the Chelmsford granite they are moderately to shallowly dipping. They generally strike northeast, and may provide good potential for localized vertical recharge and subhorizontal communication.

Figure 6. Summary data describing the characteristics of hydro-structural domain 2.

9 Domain 3 – Tadmuck Brook Schist - Domain 3 is defined by the highly sulfidic Tadmuck Brook Schist (Figure 7). Strongly developed partings parallel to a northeast trending, moderate to steeply dipping foliation (222°) and orthogonal fractures developed perpendicular to foliation (316°) provide excellent vertical recharge and a strong regional flow anisotropy. An extensive, northeast trending moderately dipping fracture set (028°) provides good lateral communication and may feed more steeply dipping fracture sets in addition to subordinate, moderately and shallowly dipping fracture sets (110° and 147°, respectively). The 028° and 110° sets generally parallel the steep hillslope on the southeastern side of domain 3. These sets may locally dominate groundwater flow and recharge a potential highly transmissive fault to the southeast that is the boundary between domains 3 and 4. The 110° and 147° sets tend to have shorter lengths and truncate against the other sets. Weakly developed sheeting joints may also provide good lateral communication near the surface. Water quality in this domain may be generally poor, and may contain iron, manganese, and sulfur, in addition to other mineral constituents.

Figure 7. Summary data describing the characteristics of hydro-structural domain 3.

Domain 4 – Gneisses of the Nashoba Formation - The gneisses and schists of the Nashoba Formation comprise Domain 3 (Figure 8). Although the Nashoba Formation is well foliated, partings parallel to foliation are generally poorly to moderately developed, except locally near the Clinton-Newbury fault zone. Northwest trending fractures sets (327°) oriented perpendicular to foliation (222°) are dominant and provide good potential for vertical recharge. Subordinate, moderately dipping fracture sets (36°, 89°) may locally provide good recharge potential. The highly foliated character of the rock may form a strong northeast-southwest flow anisotropy, but partings parallel to foliation decrease in frequency away from the Clinton-Newbury fault zone.

Small-scale versions of the Hydro-Structural Domain, Raw Fracture Data, Spatial Analysis and Water Resources panels are provided (Figures 9 – 12). Although impossible to read at 8.5 x 11 inch size, they do provide the reader with an idea of what these maps look like. Full size maps

10 can be viewed at: ftp://eclogite.geo.umass.edu/pub/stategeologist/BedrockandFractureMaps/Ayer/fracture_map_Ayer/print_maps/

Figure 8. Summary data describing the characteristics of hydro-structural domain 4.

Benefits of Fracture Mapping

Preparation of recent bedrock geologic and fracture characterization maps (Kopera et al., 2004; Kopera, 2005; Castle et al., 2005a, 2005b, 2005c, 2005d; Mabee and Salamoff, 2004; Mabee, 2005; Kopera 2006; Kopera et al., 2006) is already helping users interpret the bedrock geology in a way that is more meaningful to those stakeholders grappling with water resource management issues. For example, consultants are using the bedrock and fracture characterization maps of the Marlborough quadrangle (Kopera et al., 2004; Mabee and Salamoff, 2004) to assist the town of Westborough with a bedrock groundwater exploration program. A strong correlation has been found between interpreted lineaments and outcrop level fracture data giving them more confidence as they plan the upcoming subsurface investigation program. Another consultant is using the maps to help plan a subsurface investigation program at a site contaminated by a perchlorate release. In both instances, the availability of the maps saved time and money.

Another advantage of the ongoing fracture mapping is that it provides users with a regional geologic context for their site-specific studies. Many users working on a water supply problem or contamination site are dealing with relatively small sites of limited areal extent. With budget limitations, users want to take full advantage of any preexisting information before commencing with an expensive subsurface investigation program. In addition, outcrop and pavement exposures of the bedrock may be limited in the study area providing little insight into the geology or fracture pattern. Outcrops may be so small in the vicinity of the site that users may be able to observe foliation and perhaps one or two individual fractures. Without quadrangle scale bedrock and fracture characterization maps it is difficult to know how their data set fits into the regional geologic framework. Are the features they observe common and important to groundwater flow in this area? Do they tend to be transmissive? Are they

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Figure 9. Preliminary fracture characterizaton map of the Ayer quadrangle, sheet 1: Hydro- Structural Domain map. related to a nearby regional structure? The combination of bedrock and fracture mapping currently being developed in Massachusetts provides the user with a regional context that no other data source can offer.

In addition, lineament or fracture trace analysis is a common practice employed by the private sector to search for water or evaluate a contamination problem. The fracture characterization maps developed here provide a basis for ground truthing a lineament study. Traditional bedrock maps only show faults if they can be traced or significant offset is observed in the field. In contrast, outcrop scale faults that would not ordinarily be shown on a bedrock map can be shown on the fracture characterization map along with fracture zones.

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Figure 10. Preliminary fracture characterization map of the Ayer quadrangle, sheet 2: Raw Fracture Data.

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Figure 11. Preliminary fracture characterization map of the Ayer quadrangle, sheet 3: Spatial Analysis.

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Figure 12. Preliminary fracture characterization map of the Ayer quadrangle, sheet 4: Water Resources.

Summary and Future Work

At this point, the notion of hydro-structural domains as used on the fracture characterization maps presented in this paper is strictly a qualitative description. Although the interpretations appear intuitive, they should be considered tenuous. What is missing is the validation of the concept. Can the qualitative descriptions of domains shown on the map be tied to a quantitative measure of the hydraulic properties of the rock? Efforts are currently underway to address this issue and are being presented at this conference (see related papers in this volume by Manda et al. and Diggins et al.).

However, the bedrock and fracture maps produced to date do have value. They are being used for a variety of purposes and are reaching a broad audience that includes not only professional geologists but also students and non-geologist public interest groups. Thus, the maps are achieving their intended goals in general and are striking a chord with the consulting community

15 in particular. The key is to provide information that maintains flexibility for the user. The maps we provide are an interpretation only. However, by also providing the raw fracture data with the maps, users can augment their own data to create their own interpretations.

References Cited

Allen, D.M., E. Liteanu, D.C. Mackie. 2003. Geologic controls on the occurrence of saltwater intrusion in heterogeneous and fractured island aquifers, southwestern British Columbia, Canada. Second International Conference on Saltwater Intrusion and Coastal Aquifers – Monitoring, Modeling and Management. Merida, Yucatan, Mexico, March 27 – April 5, 2003, 12p.

Bell, K.G. and D.C. Alvord, 1976, Pre-Silurian stratigraphy of northeastern Massachusetts, Geological Society of America Memoir 148, p. 179-216 (Lowell, Lawrence, Ayer, Wilmington, Clinton, Maynard, Concord, Lexington, Hudson, Shrewsbury, Marlborough, Natick).

Castle, R.O., J.C. Hepburn and J.P. Kopera. 2005a. Bedrock geologic map of the Lawrence quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000, 1 sheet.

Castle, R.O., J.C. Hepburn and J.P. Kopera. 2005b. Bedrock geologic map of the South Groveland quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000, 1 sheet.

Castle, R.O., J.C. Hepburn and J.P. Kopera. 2005c. Bedrock geologic map of the Reading quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000, 1 sheet.

Castle, R.O., J.C. Hepburn and J.P. Kopera. 2005d. Bedrock geologic map of the Wilmington quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000, 1 sheet.

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Drew, L.J., Schuenemeyer, J.H., Armstrong, T.R., and Sutphin, D.M., 2001, Initial yield to depth relation for water wells drilled into crystalline bedrock-Pinardville quadrangle, New Hampshire: Ground Water, v. 39, no. 5, p. 676-684.

Goldsmith, R., 1991, Stratigraphy of the Nashoba zone, eastern Massachusetts: An enigmatic terrane, in Hatch, N.L., editor, The bedrock geology of Massachusetts, U.S. Geological Survey Professional Paper 1366-J, p. F1-F22.

16 Goldstein, A. 1998. Lake Char – Honey Hill – Clinton Newbury fault system from southern Massachusetts to southern Connecticut: Low angle normal faults in the northern Appalachians and their tectonic significance, in Murray, D.P., ed., Guidebook to field trips in Rhode Island and adjacent regions of Connecticut and Massachusetts. 1998 New England Intercollegiate Geological Conference, 90th Annual Meeting, Department of Geology, University of Rhode Island, pp.A1-1 to A1-20.

Hatch, N.L., ed., 1991, The bedrock map of Massachusetts, U.S. Geological Survey Professional Paper 1366A-J.

Hepburn, J.C., 2004, The peri-Gondwanan Nashoba terrane of eastern Massachusetts: An early Paleozoic arc-related complex and its accretionary history, in Hanson, L.S., ed., Guidebook to field trips from Boston, MA to Saco Bay, ME, 96th Annual Meeting of the New England Intercollegiate Geological Conference, Salem State College, Massachusetts, pp. A2-1 to A2-22.

Jahns, R.H., 1953, Surficial geology of the Ayer quadrangle, Massachusetts, U.S. Geological Survey Geological Quadrangle Map GQ-21.

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Kopera, J.P., S.B. Mabee and D.C. Powers. 2006. Preliminary fracture characterization map of the Ayer quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000 scale, 4 sheets.

Kopera, J.P. 2005. Preliminary bedrock geologic map of the Hudson quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000 scale, 1 sheet.

Kopera, J.P., R.G. DiNitto and J.C. Hepburn. 2004. Bedrock geologic map of the Marlborough quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000, 1 sheet.

Mabee, S. B. 2005. Fracture characterization map of the Hudson quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000 scale, 5 sheets.

Mabee, S.B. and S.A. Salamoff. 2004. Fracture characterization map of the Marlboro quadrangle, Massachusetts. Massachusetts Geological Survey, 1:24,000 scale, 5 sheets.

Mackie, D. 2000. An integrated structural and hydrogeologic interpretation of the fracture system in the upper Cretaceous Nanaimo Group, southern Gulf Islands, BC. Simon Fraser University: M.S. thesis.

17 Robinson, P., N.M. Ratcliffe, J.C. Hepburn. 1993. A tectonic-stratigraphic transect across the New England Caledonides of Massachusetts, in J.T. Cheney and J.C. Hepburn, eds.,Field Trip Guidebook for the Northeastern United States: 1993 Boston GSA, Volume I, Department of Geosciences, University of Massachusetts Contribution No. 67, pp. C-1 to C-47.

Robinson, P. and R. Goldsmith, 1991, Stratigraphy of the Merrimack Belt, central Massachusetts, in The bedrock geology of Massachusetts, U.S. Geological Survey Professional Paper 1366-G, pp. G1-G37.

Zen, E-an, editor, R. Goldsmith, N.M. Ratcliffe, P. Robinson, R.S. Stanley, compilers, 1983, Bedrock geologic map of Massachusetts, U.S. Geological Survey, 1:250,000 scale.

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