|Rocky Zone||Rocky Zones (RZ) are rugged areas of extreme bathymetric relief ranging from nearly vertical rock cliffs to relatively flat, gravel-covered plains littered with boulders up to 4 m in diameter.|
|Nearshore Basin||Nearshore Basins (NB) are areas of shallow, low-relief seafloor adjacent to the mainland and separated from offshore areas by islands and shoals.|
|Nearshore Ramp||Nearshore Ramps (NR) are areas of gently-sloping seafloor with generally shore-parallel bathymetric contours. This zone is primarily covered with sand-rich sediment, although small exposures of ledge, cobbles and boulders locally crop out on the seafloor.|
|Shelf Valley||Shelf Valleys (SV) are elongate depressions that extend offshore more or less perpendicular to the trend of the coastline, and slope gently seaward.|
|Ebb Tidal Delta||Ebb-tidal Deltas are lobate sandy shoals found on the seaward side of tidal inlets that form through the interaction of waves and ebbing tidal currents|
|Outer Basin||Outer Basins are generally found in water depths greater than 40 meters, and are finer grained, but may contain occasional rock outcrops.|
|Hard-Bottom Plains||Hard-bottom Plains tend to have low bathymetric relief, with a coarse sediment texture consisting of primarily gravel, sand, and rock.|
|Sand Waves||Sand Waves are features developed by currents over the sea floor.|
|Dredged Channel||Dredged Channels are anthropogenic features where the sea floor has been modified to accommodate navigation.|
|Range of values|
These sea floor physiographic zones were created from geophysical and sample data collected from Nahant to Northern Cape Cod Bay, and are used to characterize the sea floor in the area. Physiographic zone maps are important data layers for marine resource managers charged with protecting fish habitat, delineating marine boundaries, and assessing environmental change due to natural or human impacts.
Are there legal restrictions on access or use of the data?
Use_Constraints:Public domain data from the U.S. Government are freely redistributable with proper metadata and source attribution. Please recognize the U.S. Geological Survey (USGS) as the source of this information. Additionally, there are limitations associated with qualitative seafloor interpretations. Because of the scale of the source geophysical data and the spacing of samples, not all changes in sea floor texture are captured. The data were mapped between 1:8,000 and 1:25,000, but the recommended scale for application of these data is 1:25,000. Features below 5,000 m2 or less than 50 m wide were not digitized due to positional uncertainty, lack of sample information, and the often ephemeral nature of small-scale sea floor features. Not all digitized sea floor features contained sample information, so often the physiographic zone is characterized by the nearest similar feature that contains a sample. Conversely, sometimes a digitized feature contained multiple samples and not all of the samples within the feature were in agreement. In these cases the dominant sample texture was chosen to represent the primary sediment type for the polygon. Samples from rocky areas often only consist of bottom photographs, because large particle size often prevents the recovery of a sediment sample. Bottom photo classification can be subjective, such that determining the sediment type that is greater than 50% of the view frame is estimated by the interpreter and may differ among interpreters. Bottom photo transects often reveal changes in the sea floor over distances of less than 100 m and these changes are often not observable in acoustic data. Heterogeneous sea floor texture can change very quickly, and many small-scale changes will not be detectable or mappable at a scale of 1:25,000. The boundaries of polygons are often inferred based on sediment samples, and even boundaries that are traced based on amplitude changes in geophysical data are subject to migration. Polygon boundaries should be considered an approximation of the location of a change in physiographic zone.
Neither the U.S. Government, the Department of the Interior, nor the USGS, nor any of their employees, contractors, or subcontractors, make any warranty, express or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe on privately owned rights. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the U.S. Geological Survey in the use of these data or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
|Data format:||WinZip (v. 14.5) file contains qualitatively derived polygons that define physiographic zones from Nahant to Northern Cape Cod Bay, MA and the associated metadata in format Shapefile (version ArcGIS 9.3.1) Esri Polygon Shapefile Size: 0.8|
These data are available in Environmental Systems Research Institute (Esri) Shapefile format. The user must have software capable of importing and processing the data file.