Geologic maps contain information that is of significant value to society (USGS Circular). They help identify locations of valuable natural resources, such as sand and gravel deposits and groundwater in aquifers, which can be extracted and put to beneficial use. They also inform land use decisions in general, both helping to identify areas that are favorable for creating new infrastructure as well as those areas that should be avoided because of risks associated with natural hazards. In summary, geologic maps serve as indispensable tools for addressing many existing environmental problems and for proper planning to avoid other environmental problems in the future. Mapping is conducted on the basis of standard topographic quadrangles (tiles), historically at a less detailed scale of 1:62,500 (15-minute quadrangle) but now exclusively at a scale of 1:24,000 (7.5-minute quadrangle). All completed geologic maps may be ordered from the NH Department of Environmental Services Public Information Center.
What is a Geologic Map?
Geologic maps depict the underlying bedrock (solid rock at or below the land surface) as if the comparatively thin veneer of soil and vegetation has been stripped away. Surficial maps depict geologic materials, e.g., gravel, sand, silt, clay and till. Each map unit is identified and named based on distinctive characteristics that can be mapped over large distances. In addition to showing the distribution of bedrock and surficial deposits within a specific geographic area, geologic maps also provide information on the internal characteristics of each map unit and the boundaries between adjacent units (known as “contacts”), as though they were being viewed in cross-section. Points and lines are used to symbolize the three-dimensional aspects of the geology, including features of special interest.
Bedrock Geological Maps
Bedrock maps classify and describe the different types of rock exposed at the surface or present just below the overlying surficial material. In New Hampshire, bedrock consists of relatively hard, dense igneous and metamorphic (crystalline) rocks comprised of tightly interlocking mineral grains. (New Hampshire Bedrock Map ) Different rock types have different structural characteristics due to the conditions under which they formed and the major events of earth history (such as episodes of mountain building) that they have experienced. The relationships between different rock units, both in terms of age (New Hampshire rocks range in age from greater than 450 million years to 150 million years) and three-dimensional geometry, are important features represented by bedrock geologic maps. Most geologic maps include cross sections showing the rocks as they might appear in the subsurface.
Although one’s first impression of the bedrock is that it is essentially a solid mass, closer inspection (say where a thick section of rock is exposed in a road-cut) usually reveals that the rock is broken up to varying degrees by regularly spaced cracks that run in different directions and sometimes intersect at different angles. The extent to which the bedrock has been folded or sheared or fractured along zones of weakness is of special interest to hydrogeologists and geotechnical engineers. On the one hand these properties of the rock define its ability to yield groundwater to wells drilled into it, and on the other hand they determine its stability as a foundation for large buildings or its resistance to blasting. Fractures that extend over large distances and exhibit evidence of large-scale movement of rock on one side versus the other are known as geologic faults. These features are important to map because they provide information that is critical to understanding the geologic history of an area, but they also define zones that could experience movement in the future, with the potential for causing an earthquake.
A statewide bedrock geologic map is available at a scale of 1:250,000 (one inch represents four miles). A series of older bedrock quadrangles maps at a scale of 1:62,500 (one inch represents one mile) are also available, covering much of the state. Current mapping is at a scale of 1:24,000 (one inch represents 2000 feet).
Surficial Geologic Maps
Surficial geologic maps characterize the different earth materials of varying thicknesses that lie above the bedrock. All of these materials were at one time eroded and then transported by continental glaciers, eventually being deposited directly by the ice (glacial till) or by streams of melt-water as the glaciers thinned and retreated. These sediments typically have a layered appearance due to changes in the dominant size of the sediment particles from layer to layer. Because they are made up of particles that are not cemented together, these deposits are generically referred to as being “unconsolidated.” Where layering is evident due to sorting of the different grain sizes in and by water, either flowing in melt-water streams or ponded into lakes by temporary dams of glacial ice, the deposits are known to geologists as “stratified drift.” The size of the particles clearly reflects whether the water was flowing or not when deposition occurred - boulders, gravels, and coarser sands indicate high energy environments of deposition whereas fine sands, silts and clay indicate accumulation at the bottom of bodies of standing water. In some places these deposits cover the bedrock to a depth of hundreds of feet. The present-day network for rivers and streams has evolved by eroding and re-depositing these glacial sediments as “alluvium”. Surficial geologic maps record the imprint of man on the landscape by including “artificial fill” as a map unit.
Current mapping is at a scale of 1:24,000 (one inch represents 2000 feet). As of December 2008, NHGS has completed surficial geologic mapping in 95 of the 213 7.5-minute quadrangles that encompass the state, or 45 percent of the state.
Digital Geologic Maps
NHGS is committed to improving access to geologic map data developed under the cooperative STATEMAP program by systematically converting existing geology maps from conventional manuscript (paper) format to standardized digital (electronic) format so that the data can be readily accessed and used by geographic information system (GIS) software. Digital conversion also enables on-demand printing of map products for purchase and use by the public.
These data can be viewed and analyzed in combination with other New Hampshire geoscience data sets,
such as well records or hydrography, integrating subsurface information with other data about the surrounding landscape. Users with the access to GIS software can create their own geologic maps, customized for a specific area of interest.
Municipalities interested in exploring the possibility of creating geologic maps to meet special needs, whether for planning or zoning or some other purpose, should contact the NHGS using the contact information at the bottom of this page.
Map Benefits and Uses
Geologic maps are usually the starting point for any geologically related investigation. They are useful in construction and engineering projects, city and county planning, and in a variety of environmental assessments. Large projects (dams, roads, bridges, and buildings) require detailed geologic analysis because of monetary, health, and safety concerns. Smaller projects, such as excavation of detention basins or permanent surface water impoundments and construction of new homes with on-site water wells and septic systems, also benefit from an understanding of the surficial geology, and in some cases the underlying bedrock. For example, if a farm pond is located in porous glacial deposits (such as sand and gravel), it may have a limited the ability to retain water, whereas the same pond excavated in clay-rich glacial till or glacial lake bottom deposits is unlikely to leak. This basic information about the local geology can be ascertained from a geologic map. Other examples of how geologic maps can be used are listed below:
- Evaluation of geologic hazards (landslides, earthquakes, erosion potential, and land subsidence).
- Site selection for public facilities, such as groundwater supplies, landfills and other waste-disposal sites, treatment facilities, and public buildings.
- Development and protection of groundwater resources.
- Assessment of potential groundwater contamination from leaking underground storage tanks and landfills and development of remediation strategies for contaminated sites.
- Planning transportation and utility routes.
- Development of energy resources, e.g., geothermal.
- Siting of sand and gravel pits and bedrock crushing operations.
- Assessment of hydrologic and ecologic conditions of watersheds for water resources and land conservation planning.
- Development of local land use master plans and evaluation of land use proposals.
- Basic earth science education and research.
- Assistance with public policy decisions.
Geologic maps can be used to evaluate and predict the consequences of natural and human-induced activities on the environment. Using the information on geologic maps during a project’s planning and design stage produces long-term benefits and reduces problems that may develop after the project is completed.
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