The Thickness of Glacial Sediments in Connecticut and the Long Island Sound Basin layer contains interpretive polygon data representing the thickness of glacial sediments in Connecticut and the Long Island Sound Basin. Compiled as part of the Quaternary Geologic Map of Connecticut and Long Island Sound Basin, Stone and others (1998, 2005). Includes thickness of glacial sediments in 50 foot (0-50, 50-100) and then 100 foot (100-200, 200-300, etc) intervals.

Data derived from original mapping at 1:80,000 scale for the Long Island Sound basin and at 1:125,000 scale on the land in Connecticut. The data are NOT recommended for use at higher resolution scales.

The purpose is to show the thickness of glacial sediments on land in Connecticut and in the Long Island Sound Basin. This layer can be used other geologic information shown on the Quaternary Geologic Map of Connecticut and Long Island Sound Basin (USGS publication SIM-2784) as well as other geologic GIS data layers for Long Island Sound derived from the U.S. Geological Survey (USGS) / Connecticut Department of Environmental Protection (DEP) Marine Geology Cooperative Program. Complimentary data layers include the marine transgressive surface (marinet) and thickness of postglacial deposits (pgthkpol) found on Georeferenced Sea-floor Mapping and Bottom Photography in Long Island Sound (USGS Openfile report 00-304).

These data appear as figure 2 on sheet 2 of the Quaternary Geologic Map of Connecticut and Long Island Sound Basin, Stone and others, 2005.

Glacial and postglacial geologic deposits have long been studied on the land in Connecticut. Similar deposits are also present beneath modern marine sediments in Long Island Sound. Offshore geologic units in Long Island Sound are mapped largely from analysis of seismic-reflection profile data instead of from direct observation. 

The wasting away of the late Wisconsinan ice sheet, and the recession of its margin from south to north across Long Island Sound and Connecticut, was initiated when the rate of melting along the glacial margin exceeded the rate at which ice was flowing in from the north. Newly exposed lowlands and valleys were immediately filled with meltwater in the form of glacial lakes and ponds and meltwater streams.  In these meltwater settings, much of the unsorted glacial debris (till) that melted out of the ice was sorted into layers of gravel, sand, silt, and clay.

Glacial Lake Connecticut was impounded behind the Harbor Hill-Fishers Island- Charlestown moraine. At its maximum extent (when the ice front stood at a position near the present Connecticut shoreline) this glacial lake occupied all of the Long Island Sound Basin and extended into the mouths of the surrounding river valleys. The extensive glacial lake deposits that accumulated in Glacial Lake Connecticut often exceed 100m (328 ft) in thickness, and at one time very nearly completely filled in the Long Island Sound Basin. Today, these deposits have been locally extensively scoured by tidal currents (primarily in the eastern Sound) and generally lie beneath a blanket of Holocene marine sediment that is a few to 15m (45 ft) in thickness. 

To the north, Glacial Lake Hitchcock occupied the Central Valley north of Middletown, and smaller lakes and ponds occupied most river valleys. As meltwater streams carried sediment away from the receding ice front, these lakes and ponds were filled with sequences of stratified glacial deposits (most often deltaic), which choked the valleys. In upland areas, thick till deposits (e.g. drumlins) are locally scattered across the bedrock controlled topography which is generally mantled by a thin blanket of till (generally less than 5m or 15ft thick).

Mary DiGiacomo-Cohen (Long Island Sound Resource Center) for compiling and editing the digital data.  Original research conducted by USGS and Connecticut Geological and Natural History Survey Cooperative Mapping Program; contributing authors include Janet R. Stone, Sally Needell, Ralph S. Lewis, Nancy F. Neff, and Steven M. Colman.

publication date

None. The data are in the public domain and may be redistributed.

Data derived from original mapping at 1:80,000 scale for the Long Island Sound basin and at 1:125,000 scale on the land in Connecticut. These data are NOT recommended for use at higher resolution scales. Please note that the on-land portion of these data was developed at 1:125,000 scale, and even though it was developed as part of the Quaternary Map and by the same author as the Quaternary Map and the Surficial Materials Map (J.R. Stone) that those digital data layers (quaternary geology and surficial materials) were compiled at 1:24,000 scale - because of the difference in scale and compilation methods these data sets may be noticibly incompatible at inappropriate scales.

Although this data set  has been used by the State of Connecticut, Department of Environmental Protection, no warranty, expressed or implied, is made by the State of Connecticut, Department of Environmental Protection as to the accuracy of the data and or related materials.  The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the State of Connecticut, Department of Environmental Protection 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 it in a software application, analysis, or report, please acknowledge the State of Connecticut, Department of Environmental Protection as the source for this information. For example, include the following data source description when printing this layer on a map: Glacial Thickness - From the Thickness of Glacial Sediments in Connecticut and Long Island Sound Basin layer, compiled and published by CT DEP and USGS. Source map scale is 1:80,000 (LIS) and 1:125,000 (CT).

The Long Island Sound Resource Center, CT DEP, and USGS must be referenced as the originator of the dataset in any future products or research derived from these data

Dataset shows the thickness of glacial sediments in Connecticut and Long Island Sound basin, in 50 foot (0-50, 50-100) and then 100 foot (100-200, 200-300, etc) intervals.

Line features are encoded for glacial sediment thickness in feet (50, 100, 200, 300, 400, 500, and 600). Line features that are encoded with sediment thickness values of 9999 define the (boundary) limits of the data and are not thickness isopach lines. They generally define the extremity of the data, following the Connecticut state boundary and southern boundary of the Long Island Sound basin. They also enclose areas where the glacial sediment thickness is unknown. 

Polygon features are encoded for glacial sediment thickness interval in feet (0-50, 50-100, 100-200, 200-300, 300-400, 400-500, 500-600, > 600, and unkown). Values of "unknown" represent areas where the presence of methane gas in the sediment makes the interpretation of underlying layers difficult or impossible.

Areas of equal glacial sediment thickness

State of Connecticut, Department of Environmental Protection and the U.S Geological Survey

All lines were digitized, edited and coded in a consistent maner. No additional checks for topological consistency were performed on this data set. Polygon features conform to the following topological rules. Polygons are single part. There are no duplicate polygons. Polygons do not self overlap. Polygons do not overlap other polygons. Lines are single part. Line features conform to the following topological rules. There are no duplicate lines. Lines do not self overlap. Lines do not overlap other lines. Lines intersect only at nodes, and nodes anchor the ends of all lines. Lines do not overshoot or undershoot other lines they are supposed to meet and intersect. In general, there are no duplicate features, unresolved intersections, overshooting lines, open polygons, sliver polygons, or unlabeled (unattributed) polygons. The tests of logical consistency were performed by the Long Island Sound Resource Center using ESRI ArcInfo software to maintain feature topology in ArcInfo coverage format. The data is topologically clean. The ArcInfo Clean function was repeatedly used following edits to verify topology and enforce a minimum distance between vertices of 10 feet (fuzzy tolerance) and a minimum allowed overshoot length of 0 feet (dangle length).

All available (previously published or compiled) data for the area has been presented. There are data gaps in central Long Island Sound where the presence of gas in the sediments makes the interpretation of underlying layers difficult or impossible. Data gaps are either unmapped areas or represented by polygon features having Thickness Interval  attribute (THICKI_FT) values equal to "unknown".

The THICKI_FT attribute is in feet and for the Long Island Sound area was derived from interpretation of analog seismic records generated by a Uniboom seismic system. The Uniboom employed for this project had a resolution of approximately 1-2 m; original contours were drawn at 10 meter intervals (and reinterpreted or converted to 50 and 100 foot intervals to agree with the onshore mapping interval). Considered accurate for use at appropriate scales.

For the Long Island Sound area, ship position was determined using Loran-C navigation (typical Loran accuracy for Long Island Sound is about 100 m). Additional horizontal error may have been introduced during the drafting of original compilation maps or the digitizing process. Data is considered accurate for use at 1:80,000 scale for Long Island Sound and 1:125,000 scale for Connecticut mainland.

Thickness of Glacial Sediments in Connecticut and the Long Island Sound Basin

Although this data set  has been used by the State of Connecticut, Department of Environmental Protection, no warranty, expressed or implied, is made by the State of Connecticut, Department of Environmental Protection as to the accuracy of the data and or related materials.  The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the State of Connecticut, Department of Environmental Protection in the use of these data or related materials. The user assumes the entire risk related to the use of these data.

Geographic information sytem (GIS), computer-aided drawing or other mapping software is necessary to display, view and access the information. The original format of this data is ArcGIS coverage format.  The user must have a ESRI compatible software to read and process the data file.  In lieu of an ESRI software, the user may utilize another GIS application package capable of importing the data.

The data distributor does not provide custom GIS analysis or mapping services. Data is available in a standard format and may be converted to other formats, projections, coordinate systems, or selected for specific geographic regions by the party receiving the data.

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