Abstract:

The stratified_deposits data layer is part of a comprehensive study to produce a statewide digital map of the surficial geology at 1:24,000 scale. This compilation of surficial geologic materials defines the boundaries between glacial till, glacial stratified deposits, and overlying postglacial deposits. The stratified_deposits layer shows areas of glacial stratified deposits for a twenty-four-quadrangle area in central Massachusetts. This data layer should be used in conjunction with the underlying till_bedrock data layer, and overlying postglacial data layers.

Purpose:

The purpose of this study is to provide fundamental geologic data for the evaluation of natural resources, hazards, and land information within the Commonwealth of Massachusetts. The stratified_deposits data layer is part of a comprehensive effort to produce a statewide digital map of the surficial geology at 1:24,000 scale. This compilation of surficial geologic materials defines the boundaries between glacial till, glacial stratified deposits, and overlying postglacial deposits. The stratified_deposits layer shows areas of glacial stratified deposits for a twenty-four-quadrangle area in central Massachusetts. This data layer should be used in conjunction with the underlying till_bedrock data layer, and overlying postglacial data layers. This map of twenty-four quadrangles revises previous digital surficial geologic maps (Stone and others, 1993; MassGIS, 1999) that were compiled on base maps at regional scales of 1:250,000.

No restrictions or legal prerequisites for using the data. The data are suitable for use at appropriate scale, and are not intended for maps printed at scales with higher resolution than 1:24,000 scale (1 inch = 2,000 feet). Although this data set has been used by the U.S. Geological Survey and the Commonwealth of Massachusetts, neither the U.S. Geological Survey nor the United States Government nor the Commonwealth of Massachusetts makes any warranty, expressed or implied, nor assumes any legal liability or responsibility for the accuracy of these data or related materials. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the U.S. Geological Survey, the United States Government, or the Commonwealth of Massachusetts in the use of these data or related materials. The user assumes the entire risk related to the use of these data. Once the data are distributed to the user, modifications made to the data by the user should be noted in the metadata. When printing these data on a map or using them in a software application, analysis, or report, please acknowledge the U.S. Geological Survey as the source for this information

Multiple agencies and individuals contributed to this data layer: MassGIS scanned and georeferenced paper copies of the published surficial geologic maps for fifteen quadrangles; the Office of the Massachusetts State Geologist vectorized the georeferenced images and compiled geologic units in order to digitally retain the original information on the published maps (Mabee and others, 2004); U.S. Geological Survey personnel compiled and digitized the basic units for nine unpublished quadrangles from scanned field maps, and then joined and edge-matched the twenty four quadrangles in order to form a seamless geologic map.

Polygon features conform to the following topological rules. There are no duplicate polygons. Polygons do not self overlap. Stratified_deposits polygons are exclusive and do not overlap other each other. No automated procedures or tests were performed to guarantee desired topology other than visual inspection.

The completeness of the data reflects the feature content of the basic units chosen for this interim product for all data sources. Additional information can be found on the data sources, which may be utilized in future compilations. The layer is complete in the sense that it accurately reflects the stratified deposits for this twenty-four-quadrangle area, noting that edits (including additional units) may occur to this data set when it is edge-matched to surrounding quadrangles during future compilations.

The horizontal positional accuracy of these data complies with the United States National Map Accuracy Standards for 1:24,000-scale maps. According to this standard, not more than 10 percent of the locations tested are to be in error by more than 1/50 inch (40 feet) measured on the publication scale of a USGS 7.5-minute topographic quadrangle map.

Paper copies of the published surficial geologic maps (9 quadrangles) were scanned at 500 dpi and georeferenced using lat/long coordinates (2nd-order polynomial transformation). Images were projected to UTM, zone 19, NAD27.

The Office of the Massachusetts State Geologist vectorized the georeferenced images in order to digitally retain the original line work of the published maps (Mabee and others, 2004). Digital geologic map units were compiled and grouped into basic units and attributed: artificial fill, swamp deposits, floodplain alluvial deposits, cranberry bogs, glacial stratified deposits, glacial till, and bedrock areas (outcrops and areas of shallow bedrock).

Basic unit coverages for each layer in each quadrangle were projected to Massachusetts state plane NAD83, then edited, edge-matched, and appended into a single layer for each geologic unit (covering all published quadrangles).

Unpublished field sheets were scanned and georeferenced. Published maps were consulted, and scanned and georeferenced if necessary. Geologic units were vectorized or created (on screen digiziting). Basic unit polygons for each layer in each quad were edited, edge-matched, and appended into a single layer for each geologic unit (covering all unpublished quadrangles).

Coverages were converted to shapefile format. Basic unit shapefiles for unpublished and published areas were edge-matched, edited as needed, and appended into layers for the twenty three quadrangle area, then combined into the three final layers.

GLACIAL STRATIFIED DEPOSITS - Sorted and stratified sediments composed of gravel, sand, silt, and clay (as defined in particle-size diagram) deposited in layers by glacial meltwater. These sediments occur as four basic textural units--gravel deposits, sand and gravel deposits, sand deposits, and fine deposits. On this interim map, gravel, sand and gravel, and sand deposits are not differentiated and are shown as coarse deposits where they occur at land surface.Textural changes occur both areally and vertically; however, subsurface textural variations are not shown on this interim map. COARSE deposits include Gravel deposits composed of at least 50 percent gravel-size clasts; cobbles and boulders predominate; minor amounts of sand occur within gravel beds, and sand comprises few separate layers. Gravel layers generally are poorly sorted, and bedding commonly is distorted and faulted due to postdepositional collapse related to melting of ice. Sand and gravel deposits occur as mixtures of gravel and sand within individual layers and as layers of sand alternating with layers of gravel. Sand and gravel layers generally range from 25 to 50 percent gravel particles and from 50 to 75 percent sand particles. Layers are well to poorly sorted; bedding may be distorted and faulted due to postdepositional collapse. Sand deposits are composed mainly of very coarse to fine sand, commonly in well sorted layers. Coarser layers may contain up to 25 percent gravel particles, generally granules and pebbles; finer layers may contain some very fine sand, silt, and clay

Stratified deposits of the Mount Grace-Ashburnham-Monson-Webster 24-Quadrangle Area in Central Massachusetts - stratified_deposits

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