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.
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.
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.
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.