Digital Data for Bedrock Geology
Geologic maps are the primary way that bedrock geologic information is portrayed. The Maine Geological Survey publishes bedrock geologic maps at various scales throughout the State as new information becomes available. Before downloading any digital data, please become familiar with Bedrock Geology Maps and Reading Detailed Bedrock Geology Maps.
A published bedrock map integrates a complex array of technical information collected and interpreted by geologists. For specialized purposes, it may be useful to extract only a portion of the information portrayed on the map. Furthermore, it is often the case that only a portion of the available data can be portrayed legibly on the map. So for convenience of digital re-purposing, and to provide access to all the data in the MGS bedrock geodatabase, we are providing the digital data associated with published bedrock geologic maps. It should be stressed that the published map represents an integrated view of the geology as understood by the author of the map. Providing digital data allows the user to select only part of the information, and it is the responsibility of the user to insure that any use of the data is consistent with the published geologic map. For more information on Maine's bedrock and obtaining bedrock maps, see our Maine's Bedrock Geology page.
Description of the Data
Bedrock geologic data is being provided in ESRI shapefiles, a common standard for map-based information. In order to view and use the data, the user will need software which can read shapefiles. Many such products are available.
The MGS bedrock shapefiles provide three basic types of geographically-located data: (1) polygons, or areas of the map with a certain geologic characteristic, (2) lines of various significance, and (3) individual points on the map at which information is provided. Each shapefile includes a description of the various features and gives the source of information. For most bedrock data, the source is the accompanying bedrock geologic map.
Available bedrock geology shapefiles for each quadrangle may include the following:
- Unit contacts - Contacts between geologic map units, and characteristics assigned to the contact. May include lines defined as non-plotting which are contacts that conflict with other symbols on the printed map.
- Unit polygons - Areas of the map assigned to various geologic map units, such as bedrock formations or plutonic rock bodies. An abbreviated explanation of each unit is included as part of the shapefile. In addition, a separate database file (Access format), named bedrock_explanation, is provided for each quadrangle. This file gives the complete explanation for each bedrock map unit, as displayed on the accompanying published geologic map.
- Features - Contains all lines which represent geologic features other than normal geologic contacts. These may include faults, shear zones, metamorphic isograds, axial traces of fold structures, or other features represented by a line.
- Data points - Points on the map for which information is available in the MGS bedrock geodatabase. The shapefile includes all the data attached to each point, including its geographic coordinates, source of the data, what type of information is available, and technical geologic details. In many cases there are more data points in the shapefile than are represented by symbols on the printed map.
- Overlay polygons - Areas of secondary importance. Overlay features might include, for example, regions of abundant xenoliths within a pluton, regions affected by contact metamorphism within a stratified unit, regions of abundant dikes or migmatite within a stratified unit, or areas of abundant, closely-spaced bedrock outcrops.
The digital data may not include information from the most recently published bedrock geology maps. This data will be updated periodically but may be out of step briefly with published maps. The user should check the source attributes associated with the bedrock geology data (points/lines/polygons) and verify that the digital data reflects the current published map for the quadrangle. The user can view the most recently published maps or check the source for the cited open-file map in the Maps and Publications Search by searching for the open-file number (e.g. 04-93) in the publication number field.
Digital bedrock data is available by quadrangle in zipped file format. Each zip file contains several shapefiles as described above, as well as an Access database file containing the complete explanation of bedrock units for the quadrangle, taken from the published map. Click on the links below to download the zip file (ZIP). A pdf file of the accompanying bedrock geologic map (PDF) may be found at Online Bedrock Geology Maps and Reports, Scale 1:24000