Excerpts From The U.S. Geological Survey
Surficial Materials Map of CT

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Sandwiched between the Region’s soil layer (see local soil constraint maps) and its bedrock is an important middle layer of glacially deposited material known as "surficial materials."

Mapping of these glacially deposited surficial materials by municipality is available from the U.S. Geological Survey’s Surficial Materials Map of Connecticut and the CT DEP Map web site below:

• DEP-USGS SURFICIAL MATERIALS MAP OF BETHEL CT
• DEP-USGS SURFICIAL MATERIALS MAP OF BRIDGEWATER CT
• DEP-USGS SURFICIAL MATERIALS MAP OF BROOKFIELD CT
• DEP-USGS SURFICIAL MATERIALS MAP OF DANBURY CT
• DEP-USGS SURFICIAL MATERIALS MAP OF NEW FAIRFIELD CT
• DEP-USGS SURFICIAL MATERIALS MAP OF NEW MILFORD CT
• USGS SURFICIAL MATERIALS MAP OF NEWTOWN CT
• USGS SURFICIAL MATERIALS MAP OF REDDING CT
• USGS SURFICIAL MATERIALS MAP OF RIDGEFIELD CT
• USGS SURFICIAL MATERIALS MAP OF SHERMAN CT

These glacial deposits under the soil are divided into two broad categories, glacial till and glacial stratified meltwater deposits.

Glacial till deposit - an unsorted mix

Till is the most widespread glacial deposit. It was laid down directly by glacial ice and is characterized by unsorted sand, silt and clay with variable amounts of stones and large boulders mixed in.

The second category, glacially stratified meltwater deposits, were concentrated in valleys by streams and lakes in front of the retreating ice.

Glacial meltwater deposit - sorted and usable

These water related deposits are in layers of sorted gravel, sand, silt and clay. The well sorted gravel areas are the major groundwater aquifer well sites, sometimes in competition with sand and gravel mining activity found in these same deposits.

A few small post glacial sediments are also included on the map. These are primarily floodplain alluvium and swamp deposits. Alluvium is largely reworked from glacial materials and has similar physical characteristics.

These maps have the following uses in local planning and development:

1. Provide guidance for planning, zoning and inland wetland commissions in identifying geologically significant soil and water resources to assist in flagging potential site development issues.

2. Assist the designated “aquifer protection” commission of municipalities in identifying areas that may be added to their adopted aquifer protection maps in the future and to consider secondary regulatory restrictions for new developments in areas of potentially high yield.

3. Assist municipal water utility planners in identifying potential sites for large
water supply well production, keying into USGS maps that are dominated by dense layers of coarse drift materials. The maps can be used as guidance for more site-specific water yield information in the sane and gravel deposits area on the USGS maps.

4. Provide assistance in contaminant flow investigations, identifying those
stratified drift areas that may accelerate pollutant migration and unsorted till areas that may retard the flow of contaminants, and the receptor areas (e.g., private and public wells or area that may have potential for water supplies) that may be ultimate targets.

5.Municipalities can use USGS surficial maps in site plan review to identify locations that are dominated by deposits of compacted fine glacial tills. Such area will be a “red flag” for large construction and land clearing projects, since they are extremely sensitive to soil erosion.

Often such locations, once cleared and opened in a construction phase, will cause a degree of erosion that is unanticipated and can be difficult to rectify. Hence, in the plan review stage those areas of fine glacial tills (as identified by the USGS maps) should be a red flag that could induce one or more of the following measures:

a. Extensive soil testing to ascertain the site-specific degree of erosion potential due to fine glacial till deposits on the parcel of concern:

b. Scale down of the level of development due to the presence of extremely erodable soils.

c. Determine where to require extraordinary levels of sedimentation and erosion controls, including a phased development strategy.

6. The USGS maps should not be used to evaluate the suitability of sites for individual subsurface waste disposal systems (i.e., septic systems). Septic systems evaluations should be based solely on site-specific percolation and deep system tests on the parcel of concern.

(The preparation of the six points above for HVCEO was by consultant Jack Kozuchowski, with input to him from Todd Bobowick, U.S. Natural Resource and Conservation Service; George Benson, Director of Land Use, Town of Newtown; Sean Hayden, Director of the Northwest District Conservation District; and Rich Jackson, Town of New Fairfield Connecticut Sanitarian.)