This data is the result of an update to the pre-existing GRANIT surficial materials data layer.
Surficial geologic mapping represents a creative process of applying accepted models of glacial erosion and deposition to the interpretation of direct observations of landscapes and landforms in the field and representations of such features on topographic maps or aerial photographs. Exposures of undisturbed material in active or abandoned gravel pits, road cuts, excavations for building foundations, etc. provide critical data for mapping surficial geologic deposits and material textures. Geologists supplement subsurface observations at these sites by digging shallow holes with a shovel or extracting samples with a soil auger in areas that otherwise have limited exposures. Interpretation of mapped glacial features is aided by additional sources of information, including hydrography datasets and remotely sensed data such as aerial photographs. The locations of bedrock outcrops are also mapped to identify the extent of thin surficial materials, and to observe and record erosional features such as striations that indicate the direction of ice movement. All data localities and map unit boundaries are plotted as accurately as possible on a 1:24,000-scale topographic base map. Georeferenced water well data maintained by the NH Geological Survey, mostly representing private domestic water wells drilled in bedrock, are compiled for each map area. In particular, reported depths to bedrock and generalized stratigraphic descriptions serve as a valuable adjunct to field observations.
Although the exact procedures may differ between individual mappers, the process generally begins with an inspection of existing topographic maps and aerial photographs of the map area and surficial geologic maps in adjacent areas. A reconnaissance or "windshield survey" of the landscape is then conducted by driving as much of the road network in the map area as possible. Subsequent mapping is conducted on foot, preferably taking advantage of improved visibility during 'leaf off" conditions in the early spring and late fall. Map compilation is an ongoing process of refining and confirming preliminary observations and fitting the field evidence within a conceptual framework of glacial deposition.
Some surface features exhibit a morphology that geologists readily recognize as diagnostic of a specific mode of deposition. More commonly, however, depositional history must be reconstructed by interpreting numerous observations within a local study area. This effort is guided by a conceptual framework, generally known as the "morphosequence concept", that has been clearly articulated by Koteff and Pessl (1981) in the now classic publication "Systematic Ice Retreat in New England". To summarize:
"In the concept, a single sequence specifically refers to a continuum of landforms composed of melt-water deposits, from more collapsed forms due to melting of ice blocks at the head or upstream parts of outwash, to progressively less collapsed forms downstream. A sequence can thus be viewed as a body of stratified drift laid down, layer upon layer, by melt water at and beyond the margin of a glacier, while deposition was controlled by a specific base level. The complexity of the morphologic features depends on the relative number, size, and distribution of detached ice blocks around and over which the sequence was deposited."
The completed surficial geologic map thus represents a synthesis of all the field observations and other site-specific data subject to the best professional judgment of the individual mapper. Ultimately, the finished geologic map is the best representation of the surficial geology that can be compiled from all of the data available at the time of publication. Limitations always exist as a function of the scale of the map. Any map can be refined through the acquisition of new data, especially data collected at a higher resolution that supports presentation at a larger scale. In a sense, each surficial geologic map is a "progress report" on the evolving state of scientific understanding of earth processes and events during the last deglaciation.