Bob,
You have hinted several times for me to look at the geology of
MTSF. Here is a basic summary of what I found. There is much more
data I looked at, but much of it is pretty esoteric. Attached is a
geologic map of the Mohawk Trails Area - Todd Mountain
Moretown Formation Om
http://tin.er.usgs.gov/geology/state/sgmc-unit.php?unit=MAOm;0
This correlates with the Beauceville Formation: The Beauceville
Formation of the Magog Group is mainly composed of graptolitebearing
pelagic mudstone and volcaniclastic rocks, which were deposited in a
fore-arc basin during the Ordovician. The volcaniclastic rocks of
the Beauceville area, their transport and depositional mechanisms
are not well known. http://theses.uqac.ca/resume_these.php?idnotice=10982977&lang=en
Simlarly it is correlated with the Missisquoi Formation of VT which
is described as a metasedimentary sequence that has undergone
multiple episodes of deformation.
Gneiss at Hallockville Pond Ohpg http://tin.er.usgs.gov/geology/state/sgmc-unit.php?unit=MAOhpg;0
Hawley Formation Oh http://tin.er.usgs.gov/geology/state/sgmc-unit.php?unit=MAOh;0
Geologic Map http://mrdata.usgs.gov/sgmc/ma.html
The rocks on Clark and Todd Mountains are metasedimentary rocks
that have undergone several episodes of deformation, The original
rocks that formed the sediment may have been of volcanic origin, but
it was eroded, transported, and deposited as sediment before it
lithified into the original bedock before deformation. There is a
possibility of some igneous intrusions into the metasedimentary
rocks, but none are mapped ion the area of Clark or Todd Mountians
Ed
----- Original Message -----
From: Bob
To: ENTS
Sent: Wednesday, August 12, 2009 10:12 PM
Subject: [ENTS] Sun, Mike, Joe, and Me
ENTS
Today
the above named hiked in MTSF. We'll likely all post
pictures, but just a few observations sent from my trusty iPhone.
Joe examined rocks on
Todd and Clark Mtns and observed
complex geology. Heavily metamorphized rocks that I loosely called
schist appear to have a volcanic origin. I'll let Joe address the
subject, but if Joe is correct and I suspect he is, then the mineral
content of the rocks is probably very rich.
Mike,
Would you mind sharing your
thoughts on what you saw today?
Bob
Sent from my iPhone
ENTS,
Determining what the original rock was in a metamorphic rock
sequence is often very difficult to downright impossible. If it is
only slightly metamorphosed then some of the original structure may
still e detected. If it has been heavily metamorphosed any original
structure is long gone. In metamorphic rocks the larger the grain
the higher the degree of metamorphism. When sandstone is
metamorphosed you will always get quartzite - the quartz in the
sandstone will simply reform to other sizes of quartz grains under
metamorphosis. Similarly limestone will form marble when
metamorphosed as it consists of just the mineral calcite. Since
limestone layers may be relatively thin under metamorphosis they may
intermix with layers above and below and not form a pure marble.
When you have rocks with mixed minerals in them - like shale and
siltstone, or many of the igneous rock you get a variety of
different metamorphic rocks. Shale is the most common sedimentary
rock making up 70% of the total mass of sedimentary rocks followed
by siltstone.
During metamorphosis the rocks are subject to heat and pressure.
The pore space space is lost. The minerals in the rocks or
sediments melt under the pressure and recrystalize in a different
orientation, or combine with other minerals originally to form new
minerals. The more heat and pressure the more dramatic the changes,
and the larger the crystal grains that form. The specific minerals
present are a function of the original mineral composition of the
rock and the the heat and pressure to which it was subjected. In
lightly metamorphosed rock the grains are impossible to see - these
are slates and greenstones. In the next greater stage you can
almost see the individual grains - you can convince yourself you can
see them - they are often shinny metallic looking muscovite mica -
these are called phylites. The next greater degree of metamorphoses
forms schist. These have grains easily seen by the naked eye. The
minerals in slates, phyllites and schists form layers that dependant
of any original bedding called cleavage. The layers in slate for
example, are not the same orientation as the layers in shale that
likely was the original source. The grain orientation is dependant
on the directions of stress and strain in the folding process.
Under the greatest degree of metamorphosis Gneiss is formed, It is
coarse grained rock and the layers are folded and contorted within
the rock pieces.
In general the minerals in a schist or gneiss may be exactly the
same as those found in a granite. The difference is that in a
schist and gneiss the minerals are arranged in layers, while in
igneous rocks the minerals are randomly distributed and oriented.
Greenstones are a metamorphic rock that are derived by low grade
metamorphosis of basalt (lava) and associated massive clay deposits.
Remember that water is often incorporated into the basalt material
when exposed at or near the surface ad prior to metamorphosis. The
only good way to determine the original rock material of most
metamorphic rocks is to correlate or trace the deposits to areas
outside the region of metamorphism where they have not be altered
(or altered as much) by heat and pressure and see what they were.
The Hawley Formation at Mohawk Trails State Forest are described
as: Hawley Formation - Interbedded amphibolite, greenstone,
feldspathic schist and granofels. Coarse plagioclase in some
amphibolite near top; local coarse hornblende blades or sprays.
Sparse coticule (Emerson, 1917, p. 43). As used here the Hawley
includes amphibolite, sulfidic rusty schists, abundant coticules,
silvery schists, quartzites and quartz conglomerates, and quartz,
feldspar, biotite granulites. The quartzites and quartz
conglomerates occur at two positions in rocks here assigned to the
Hawley. Amphibolite is technically really an igneous rock comprised
primarily of the mineral amphibole, but as used here I believe to
refer to a metamorphic rock with amphibole as the primary mineral
The schist, greenstone, gneiss and granofels are all metamorphic
rocks. (In granofels the layeres are indistinct because of the near
uniformity of the mineral composition).
Ed
Continued
at:
http://groups.google.com/group/entstrees/browse_thread/thread/350fe83f77e17b9c?hl=en
|