CTgeo

Granitic Rocks and Granite of the Westwoods Area in Guilford

 The area that includes the southeast corner of Branford and the southwest corner of Guilford is mapped by geologists as a gneiss dome. Although the topography is not dome-shaped, there is other evidence that this is a dome. The rocks in the center are older than the rocks around the edge. And the foliation of the rocks is flat lying in the center and steepens gradually toward the margins.  

The geologic map below covers the Westwoods and Stony Creek area. 

Westwoods and Stony Creek

 Symbols used on the map 

Pn - Narragansett Pier Granite

Zsc - Stony Creek Granite Gneiss

Zw - Waterford Group

Zp - Plainfield Formation

Zpq - Quartzite unit in Plainfield Formation 

The red circle is the approximate area we will observe on this trip 

The above map and the following unit descriptions are from John Rodgers Bedrock Geological Map of Connecticut, 1985, scale 1:125,000, published by the Connecticut Geological and Natural History Survey.

The above map and the following unit descriptions are from John Rodgers Bedrock Geological Map of Connecticut, 1985, scale 1:125,000, published by the Connecticut Geological and Natural History Survey. 

 

 

granite (right) and granite gneiss (left) 

 Figure 1. Coarse-grained granite on right, foliated granitic gneiss on left.

 Pn - Narragansett Pier Granite (Permian): Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, plagioclase, quartz, and biotite with accessory muscovite and magnetite. Considerable associated pegmatite. 

Zsc - Stony Creek Granite Gneiss (Precambrian): Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of plagioclase, K-feldspar, and quartz with minor biotite and magnetite, sporadic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. 

Zw - Waterford Group (Precambrian): Light to dark, generally medium grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others. Some layers of amphibolite. 

Zp - Plainfield Formation (Precambrian): Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite, light- to medium-gray gneiss composed of quartz, plagioclase, and biotite (rarely microcline), medium- to dark-gray schist composed of quartz, plagioclase, biotite, sillimanite, and garnet, dark-gray or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside), amphibolite, diopside-bearing quartzite, and calc-silicate rock. In places contains quartz-sillimanite nodules. 

Zpq - Quartzite unit in Plainfield Formation (Precambrian): Light-gray, glassy, generally thin bedded quartzite, also feldspathic and micaceous quartzite containing quartz-sillimanite nodules. 

As you can see from the above map, these units are mapped as mixed in the various parts of the area.  

1.     Follow the Green Rectangle trail from Moose Hill Road to its intersection with the Orange Circle trail. Along the way collect samples or write descriptions of the different granitic gneisses you see.  

2.     When you reach the Orange Circle trail turn left (north). In about 100 yards you will come to the large outcrop numbered "4 Massive Rock Forms" on the map. Also look at those rocks to see if they differ from the others you have seen.

 

Gathering Point at 4

    Figure 2. Gathering point at "4" on Orange Circle trail. You can squeeze through a narrow cleft in the rocks or walk around them on the right to get a good look at the entire outcrop.

 

 3.   Discuss with the other class members the various rocks you have found and where. Which of the above rock descriptions do they match? 

4.   On the walk back to the parking area, point out the sites of your different rock types.

 

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