EXCERPTS
FROM THE
U.S. GEOLOGICAL SURVEY
SURFICIAL MATERIALS MAP OF CT
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.)
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