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Directions: First stop at the overlook into the open pit (FIG). Next drive to the entrance at the south end of the pit and turn around. Drive back north about half the distance to a red mailbox-size container and park.
The tan to brown weathering rocks (FIG) are serpentinized peridotites. Now composed of the green to black hydrous mineral serpentine, previously these rocks were composed primarily of the mineral olivine (or peridot). Two varieties of peridotite are present, dunite and harzburgite (FIG). The dunite, made of olivine and traces of black chromite, weathers smooth. The harzburgite, made of both olivine and resistant pyroxene, weathers rough.
Look for aligned chromite and/or pyroxene crystals which define a lineation formed at extreme temperature and pressure. Locally, rafts of harzburgite are enclosed within a network of dunite (FIG). Recent view has dunite network as plumbing system for crust. Harzburgite results from silica-enriching melt-rock reaction.
The peridotites were first pervasively serpentinized and later intensely serpentinized along brittle fractures (FIG). Fibers of chrysotile, a flexible form of serpentine, have grown across some of these fractures (FIG). The cross fibers are extracted and marketed as a variety of asbestos.
The peridotites in the Black Lake area are interpreted to be an actual slice of the earth's mantle. The lineation formed as hot, soft mantle flowed up and away from an oceanic spreading center (STAGE II). The pattern of dunite and harzburgite possibly resulted from partial melting of the mantle which yielded magrna to form the overlying oceanic crust.
Directions: This outcrop is located off Route 267 on a side road leading to Disraeli. From Route 267, drive about 1.6 miles and look for a broad ledge on the northeast side of the road. Park opposite the outcrop. A safe turnaround is located another half mile to the south.
This outcrop consists of pillowed basalt lavas (FIG) and several feeder intrusions (FIG). The pillows are cross sections through lava tubes, formed when lava erupts underwater and chills rapidly. The pillow tops are rounded while their bottoms conform to the underlying lavas, hence the top direction of the lava pile can be determined. The intrusions have parallel sides and are made of massive basalt. The largest one shows evidence of multiple intrusion. Magma rose up a crack and solidified. The crack widened and another batch rose and solidified and so on.
These lavas and their feeder intrusions represent the actual crust of the Iapetus Ocean (STAGE II). The orientation of the intrusions can be assumed to be parallel to the rift along the crest of the ancient spreading center.
Related links:
Thetford Open Pit
Mining
Sedimentary breccia (?)
Serpentine
Directions: From the Hotel de Ville in the center of Coleraine, take the road toward the town cemetery. A beautifully glacially polished knob is located on the south side of the cemetery.
The breccia (FIG) has a red shale matrix surrounding a variety of angular igneous and sedimentary fragments. The igneous fragments include specimens of basalt, diabase, and gabbro derived from the underlying crust (stop 10). The sedimentary fragments have irregular shapes and are indented by the hard igneous clasts, which suggests they were still soft sediment when the breccia formed.
This stop provides an example of sediment, albeit of an unusual type, that covered the Iapetan oceanic crust (STAGE II). The most common type of sediment blanketing ocean floors is an ooze made of plankton shells rained down from the surface waters. At Coleraine, the sedimentary breccia may have formed adjacent to a fault scarp cutting the oceanic crust.
Directions: A specific site has not been selected. Between Coleraine and Megantic on the way to Maine, you will pass several good roadcuts of layered sedimentary rocks.
A wide belt of sedimentary rocks extends through eastern Vermont and southern Quebec to the Gaspe Peninsula. These rocks weather relatively easily in comparison to the crystalline rocks of northern Maine (FIG). They span a wide age range, from Ordovician through Devonian.
During Ordovician time (STAGE III), this area was located between a trench to the northwest and an arc to the southeast. It may be compared to the Great Valley of California between the Coast Ranges accretionary prism and the Sierra arc. After the arc-continent collision, its setting changed. During Silurian time (STAGE III-B), limestones and quartzites record a shelf environment. During early Devonian time (STAGE IV-A), the region evolved into a topographic trench and accumulated a thick sequence of sand and mud.
