OVERVIEW

This data layer is part of a comprehensive study by the U.S. Geological Survey to produce a statewide digital map of the surficial geology at a 1:24,000-scale. This compilation of surficial geologic materials defines the areas of exposed bedrock, and the boundaries between glacial till, glacial stratified deposits, and overlying early-postglacial and postglacial deposits.

Four separate polygon datasets were developed:

• Till and Bedrock - areas of Thin Till, Thick Till, End Moraine Deposits, Bedrock Outcrops, and Areas of Abundant Outcrop and Shallow Bedrock. Thin till and bedrock outcrops polygons are mutually exclusive; thick till polygons overlie areas of thin till, and abundant outcrop/shallow bedrock polygons overlie areas of thin till and bedrock outcrops. In ArcSDE the layer is named SURFGEO24K_TB_POLY.
• Stratified Deposits - areas of glacial stratified deposits, categorized as Coarse, Glaciolacustrine Fine, and Glaciomarine Fine. In ArcSDE the layer is named SURFGEO24K_SD_POLY.
• Early Postglacial - areas of early postglacial Inland Dune and Marine Regressive Deposits. In ArcSDE the layer is named SURFGEO24K_EPG_POLY.
• Postglacial - areas of Swamp Deposits, Floodplain Alluvium, Beach and Dune Deposits, and Artificial Fill. Swamp polygons may overlie alluvium polygons; artificial fill polygons may overlie areas of swamp or alluvium. In ArcSDE the layer is named SURFGEO24K_PG_POLY.

These layers revise previous digital surficial geologic maps (Stone and Beinikis, 1992, MassGIS Surficial Geology) that were compiled on base maps at regional scales of 1:250,000.

Multiple agencies and individuals contributed to this data layer: MassGIS scanned and georeferenced paper copies of published surficial geologic maps, on a quadrangle-by-quadrangle basis. 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 (Stephen Mabee and others, 2004). U.S. Geological Survey personnel compiled and on-screen digitized the basic units for unpublished quadrangles from scanned field maps, and then joined and edge-matched the quads in order to form seamless geologic layers. Work at the USGS was compiled by Byron D. Stone, Janet R. Stone, and Mary L. DiGiacomo-Cohen.

Each layer's polygon attribute table contains the items LEGEND and MAP_UNIT. Codes for each layer are as follows:

Layer Name		LEGEND		MAP_UNIT
SURFGEO24K_TB_POLY		bk		bedrock outcrop
		em		end moraine deposits
		sb		areas of abundant outcrop and shallow bedrock
		t		thin till
		tt		thick till
SURFGEO24K_SD_POLY		sd-c		glacial stratified deposits, coarse
		sd-f		glacial stratified deposits, glaciolacustrine fine
		sd-fm		glacial stratified deposits, glaciomarine fine
SURFGEO24K_EPG_POLY		d		early postglacial inland dune deposits
		mr		early postglacial marine regressive deposits
SURFGEO24K_PG_POLY		af		artificial fill
		al		floodplain alluvium
		bd		beach and dune deposits
		sm		salt-marsh deposits
		sw		swamp deposits

For full details on the map unit descriptions, see the FGDC-compliant metadata distributed with the data.

Because of overlapping polygons amongst the layers, and because some map units in each layer are not mutually exclusive, the order in which the layers are drawn is important in order to to correctly represent the physical nature of the geologic features. When displaying the data in ArcGIS, it is best to use the group layer file developed by USGS and modified by MassGIS for use with our SDE layers and exported shapefiles. The layer file includes definition queries and the preferred symbology developed by USGS. All layers should be displayed together and, for better context, overlaid with basemap features such as roads, elevation contours, shaded relief, hydrography, and geographic annotation (as indicated in the layer's sample image - geology with basemap features). At map scales of  1:15,000 or larger (i.e., more zoomed in), USGS recommends overlaying images of 1:24,000 topographic quadrangle basemap features (see sample image - geology with quad image). These images were scanned specifically for this surficial geology project and were used as the base for on-screen digitizing of geologic map units. See the USGS 1:24,000 Surficial Geology Topographic Base Map Images. Non-ArcGIS users should refer to the group layer file image for the proper symbology, datasets, and definition queries when displaying the data.

The USGS is actively developing these layers. New data will be made available when received by MassGIS. Please note that some data now available through MassGIS may be replaced with updated versions as USGS' data development project continues. See the Status Map for areas currently available (based on the Surficial Geology (1:24,000) Status Index layer).

For information on specific areas and access to interim data products and imagery, see http://www.geo.umass.edu/stategeologist/frame_maps.htm?USGS_surficial.htm.

Last Updated 8/28/2009
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