This glacial erratic contains numerous augens, meaning "eyes", of large feldspar crystals. The rock also includes other minerals such as quartz, amphiboles and sphene. The Macintosh laptop provides scale, with its 15.25-inch screen measured diagonally, and also served to placemark the boulder with a GPS device and Google Earth.
Click the photo for its Flickr photo page.
The bedrock stratum where the rock originated somewhere to the north may be Pumpkin Ground Gneiss or Brookfield Gneiss, which have been identified Fairfield County in southwestern Connecticut. Long after the rock formed, it was carried to its present location by continental glaciation. The classification of these rock units by the United States Geological Survey is described in technical terms below. This rock formed and underwent metamorphism (physical and chemical alteration due to heat and pressure without melting) about 450 million years ago during the Taconian Orogeny, which lasted from about 485 to 440 million years ago. It was metamorphosed again during the Acadian Orogeny from about 420 to 360 million years ago. Metamorphism often includes shearing in which rock material slides along internal planes due to intense forces. The augens were formed during the Acadian Orogeny and tended to rotate during metamorphism so that their long diagonal axes became more closely parallel to the direction of the shearing. During the Alleghenian Orogeny, from about 350 to 300 million years ago, the rock may have undergone some additional alteration. Orogenies are episodes of mountain building that result from the collision of landmasses brought about by the motion of the tectonic plates on which they ride. The landmasses can include continents, like North America, and volcanic island arcs, like Japan.
Closeup of the boulder with part of a shear plane traced and labeled "A", and the long axis of an augen traced and marked "B". The long axis is nearly parallel to the shear plane. The instrument on the rock is a Brunton compass.
Click the photo for its Flickr photo page.
Technical Descriptions of Rock UnitsFrom
USGS: Harrison Gneiss:
Age: Middle? Ordovician
Harrison Gneiss (including Prospect Gneiss) - Interlayered dark- and light-gray, medium-grained, well-foliated gneiss, composed of andesine, quartz, hornblende, and biotite (also locally K-feldspar as megacrysts 1 to 5 cm long). Thought to be metavolcanic equivalent of unit Ob.
(Here, Ob refers to Brookfield Gneiss, described below. The megacrysts are augens.)
From
USGS: Pumpkin Ground Member [of Harrison (Prospect) Gneiss]:
Age: Middle? Ordovician
Description Pumpkin Ground (porphyritic) Member [of Harrison (Prospect) Gneiss] - Medium- to light-gray, medium- to coarse-grained, well-layered and foliated gneiss, composed of oligoclase, microcline, quartz, and biotite; some layers have numerous microcline megacrysts 1 to 5 cm across; others have hornblende. Minor layers of garnetiferous schist and gneiss. Pumpkin Ground and Beardsley Members of Harrison Gneiss, formerly considered conformable metavolcanic members, are here recognized as juxtaposed metaplutonic units and are renamed the Beardsley and Pumpkin Ground orthogneisses. Isotopic dating yields ages of 428+/-2 Ma (Early Silurian) for the Pumpkin Ground and 446+/-2 Ma (Late Ordovician) for the Beardsley, accepted by authors as crystallization ages. Pumpkin Ground intrudes the Trap Falls Formation. The Harrison Gneiss as described by Rodgers (1985) has no stratigraphic significance and cannot be correlated regionally (Sevigny and Hanson, 1993).
(Porphyritic means that it has some large crystals, such as augens, and metaplutonic means that it was a large mass of molten material deep below the surface that cooled and solidified, and later become metamorphosed.)
From
USGS: Brookfield Gneiss:
Age: Middle? Ordovician
Brookfield (dioritic and granodioritic) Gneiss (including Newtown Gneiss of Crowley, 1968) - Dark and light, commonly speckled or banded, medium- to coarse-grained, massive to poorly foliated gneiss, composed of plagioclase, biotite, and hornblende, generally with quartz and K-feldspar, the latter commonly as megacrysts 1 to 3 cm across (also plagioclase megacrysts in darker rocks), locally associated with amphibolite or hornblende schist.
(Megacrysts are augens.)
Related Google Earth Community Posts
Connecticut Bedrock and Long Island BouldersConnecticut Geologic Map Overlay
Previous Topic
Index
