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ORV Detail - Geology

Additional Geology Documents

Geology Report - PDF from Management Plan Appendices

The following text is an excerpt from the Watershed Management Plan Chapter VI.

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1. The Unique Geology of the Eightmile River Watershed

Many aspects of the geology in the Eightmile River Watershed stand out as being regionally and locally significant. Following is a summary of the components of the geologic features of the Eightmile River Watershed which make geology stand out as an outstanding resource value. A glossary of select terms is provided for your convenience.

An Exceptional Assemblage of Bedrock

Bedrock is the solid rock that underlies soil or other unconsolidated surface material. In 1966, Lundgren described the bedrock of the Eightmile River Watershed as “an exceptionally varied suite of rocks that includes representatives of nearly all of the major stratigraphic and granitic units known in eastern Connecticut”.

The exceptional variation in the Eightmile’s bedrock assemblage has its origin in New England’s plate tectonic history. This history includes the closing of the Iapetos Ocean between 480 and 250 million years ago when the African and North American tectonic plates converged and ultimately collided. Eleven bedrock units representing the remnants of the Iapetos Ocean were crushed together with units once part of western Morocco (North Africa). Heating and metamorphosis then formed what is now the bedrock foundation of the Eightmile River Watershed.

For most of New England the closing of the Iapetos Ocean resulted in a general north-south alignment of terrane boundaries and their attendant bedrock units. This is not the case for a small area of Southeastern Connecticut, including the Eightmile. In this region, a small crinkle in the bedrock fabric produced an anomalous east-west alignment. As a result, rocks from two major players in the New England-wide plate tectonic history are found in the watershed. The east-west trending Honey Hill fault is a terrane boundary that delineates the contact of oceanic affinity Iapetos-Terrane bedrock units to the north, and African affinity Avalonian-Terrane bedrock units to the south.

Also unique to the Eightmile is the occurrence of basic or “sweet” soils. Most metamorphic bedrock in Connecticut is acidic, which over time breaks down (weathers) to acidic soil. Five of the eleven metamorphic bedrock units underlying the Eightmile have basic, calc-silicate or marble members. Mapped in the Eightmile near Cedar Lake and at the south end of Moulsons Pond, the basic bedrock members will likely weather to basic soils. These basic soils are ecologically significant to a region generally dominated by acidic soils.

Atypical Local Topography

Stream erosion over the past 250 million years coupled with the erosive power of two known Pleistocene glaciations has sculpted the bedrock surface into New England’s rolling, north-south orientated topography. The north-south landscape pattern in Connecticut was caused by selective weathering of less resistant bedrock units as well as north-south aligned fault/fracture zones that developed when rifting formed the Atlantic Ocean. Ridge systems running north-south drained by south-flowing streams are typical throughout most of Southern New England – except for a small area in and around the Eightmile.

The anomalous alignment of bedrock units in the Eightmile creates a series of east-west trending strike ridges (bedrock alignments) which are cut by valleys that mirror the regional pattern of north-south fractures. The result is a rectangular or “blocky” local topography that is uncharacteristic in Connecticut and the region as a whole. The drainage pattern of the Eightmile River and its tributaries reflects the east-west bias of the strike ridges and the north-south bias of the crosscutting fractures.

Glacial Evidence Remains

The pattern of glacial deposition in the Eightmile is similar to other areas of Southern New England that are underlain by metamorphic bedrock. Blanketed by thin till, uplands are punctuated by patches of thicker till, drumlins and bedrock outcrops. Associated with exposed upland bedrock are striations, polished surfaces, rouche moutonnee and evidence of relict glacial spillways. The valleys in the Eightmile are filled with stratified drift deposits of sands, gravels and lake/pond deposits left by the last glacier during its northward retreat. Five former ice positions are marked by ice-contact stratified drift deposits that lie in the valley between Hamburg Cove and Route 82. Eskers and Kettles occur in several locations, and there are exemplary examples of these passive ice features in the Pleasant Valley Preserve in Lyme. Open fields just north of Hamburg Cove, in the Pleasant Valley area, and in the North Plains area are nice example of an “eggs in basket” topography that compelled the Scottish to invent the very popular game of golf.

The watershed’s geology plays a critical role in overall watershed quality affecting resources from hydrology to biodiversity to the cultural landscape. The combination of an exceptional assemblage of bedrock, unique atypical local topography and exemplary glacial evidence remains all provide a distinct representation of geology in Connecticut and as such is considered an outstanding resource value for the Eightmile River Watershed.

See Map 6: Bedrock Geology

Glossary

ge•ol•o•gy (je-ol'?-je) n.
The scientific study of the origin, history, and structure of the earth, a specific region of the earth's crust, or the solid matter of a celestial body.

gran•ite (gran'it) n.
A common, coarse-grained, light-colored, hard igneous rock consisting chiefly of quartz, orthoclase or microcline, and mica, used in monuments and for building.

met•a•mor•phic (met'?-môr'fik) adj.
Changed in structure or composition as a result of metamorphism, a process by which rocks are altered in composition, texture, or internal structure by extreme heat, pressure, and the introduction of new chemical substances.
roche mou•ton•née (rôsh' mut'n-a', mu'tô-na') n..
An elongate mound of bedrock worn smooth and rounded by glacial abrasion.

stra•tig•ra•phy (str?-tig'r?-fe) n
The study of rock strata, especially the distribution, deposition, and age of sedimentary rocks.

stri•a•tion (stri-a'sh?n) n.
One of a number of parallel lines or scratches on the surface of a rock that were inscribed by rock fragments embedded in the base of a glacier as it moved across the rock.

tectonic plates (tek-ton-ik) n.
The dozen or so plates that are about 30 miles thick that makes up the surface of the Earth. Their motion is studied in the field of plate tectonics. The plates are not the same as the continents. The North American plate, for example, extends from the middle of the Atlantic Ocean to the west coast of the United States and Canada.

ter•rane also ter•rain (t?-ran', ter'an) n.
A series of related rock formations.

2. How Geology is Threatened

An assessment of threats to the geology of the Eightmile River Watershed identified two potential activities that could cause geological quality to degrade. Earth material extraction, such as gravel mining operations or quarrying, was identified as one threat that could alter the bedrock and surficial geological features within the watershed. In addition, the threat of development activities causing changes to the slope and topography of the landscape, including glacial features was also identified (e.g. development activities altering or removing distinct eskers or filling in kettle holes). It should be noted that both of these threats were ranked to have a small potential impact to the overall quality of the features that qualified geology as an outstanding resource value in the watershed.

3. Existing Protections and Gaps

In order to understand potential additional protection needs for geology a comprehensive review was completed of current protection measures planned and implemented at the local, state and federal level.

There were few tools found that would address the threats to geology. They include:

East Haddam and Lyme do not permit bedrock quarrying.
All three communities have gravel extraction regulations. Lyme’s have just been revised to further minimize impacts caused by gravel extraction operations.
East Haddam and Salem require new streets to follow natural contours wherever possible.

Gaps in protection were identified as a result of comparing known geology threats with existing protection measures. Primary gaps in protecting geological features include:

All three towns require net buildable area calculations for determining the number of lots allowed in a new subdivision. While the buildable area calculations require eliminating areas of steep slopes or exposed rock they do not prevent structures or activity from occurring in those areas.
Slope requirements for restoration of sites after gravel has been removed do exist: however, there is no mention of trying to restore a natural topographic condition.
Local regulations do not identify glacial features as important to conserve.
There is no comprehensive summary and map available to towns identifying significant geological features for use in land use and open space planning.

4. Geology Protection Goal

The geology goal for the Eightmile River Watershed is to promote the distinct geology of the Eightmile River Watershed and work to protect the features that best exemplify the area’s geologic uniqueness.

5. Geology Indicators

a. Natural Topographic Condition
b. Intact Bedrock Geology
c. Intact Glacial Features