there is a very cool rock at a beautiful beach in sunset, on deer
isle. it is best accessed at half-tide or low tide.
i want to thank my friend, northwoods explorer, for his help on
this earthcache and for sending me information about many other
geologic features of deer isle. his description of the rock in this
earthcache is as follows: The story of this earthcache begins
during the Cambrian Period when the sediments formed from the
rifting of Avalonia from Gondwana and the associated volcanism on
Gondwana accumulate in the adjacent seas. These sediments would
become Ellsworth Schist when they were metamorphosed by the
colliding land masses during the Acadian orogeny. The Acadian
orogeny is the name of a long-lasting mountain building disturbance
that most greatly affected the Northern Appalachian region (New
England northeastward into the Gaspé region of Canada). The
"climax" of this orogeny is dated as early in the Late Devonian,
but deformation, plutonism, and metamorphism related to this
orogeny continued well into the Mississippian Period. The cause of
this great period of deformation is a result of the plate docking
of a small continental landmass called Avalonia (named after the
Avalon Peninsula of Newfoundland). The docking of Avalonia onto the
margin of Laurentia resulted in the closing of a portion of the
Iapetus Ocean. The Acadian Orogeny spanned a period of about 50
million years (beginning roughly 375 million years ago). During the
course of the orogeny, older rocks were deformed and metamorphosed,
and new faults formed and older faults were reactivated.
A schist is a rock that has been so strongly metamorphosed, or
altered by intense heat and pressure, that platy minerals called
micas have formed. The micas in the schist allows it to break
readily along closely-spaced irregular surfaces. The Ellsworth
Schist started out as a sediment composed of sand grains derived
from volcanic and granitic rocks. In this area the original
sedimentary rock here were probably interlaminated beds of
quartz-rich sand (lighter bands) and silt (darker bands). Viewed in
the right direction, you will see that these laminations have been
intensively deformed into very small folds. This is the type of
deformation that occurs in subduction zones on the edge of the
plates.
These plates are on the surface and part of the lithosphere. The
lithosphere includes the crust and the uppermost mantle, which is
joined to the crust across the mantle. The lithosphere is underlain
by the asthenosphere, the weaker, hotter, deeper, and more plastic
part of the upper mantle. The boundary between the lithosphere and
the underlying asthenosphere is defined by a difference in response
to stress: the lithosphere remains rigid for long periods of
geologic time, whereas the asthenosphere flows much more readily.
In addition, the colder rocks in the lithosphere are denser than
those in the underlying asthenosphere, and thus tend to sink, given
the opportunity. Thus when, for example, a section of the
lithosphere is bowed down and breaks, perhaps as a result of
loading by sediment eroded off a continent, the material on one
side of the break may begin to sink into the asthenosphere. This
process is called subduction. The swirls are the same as when you
add cream to coffee except it takes a much longer time for it to
take place. At the posted coordinates you will see evidence of this
subduction in the small folds in the metamorphosed rocks. The event
that formed these rocks took place 375 million years ago.
remember that because this is an earthcache, there is no
physical container. to log the cache, you must post a photo of you
(or your hand) with your gps, showing a view of one of the layers
of the rock. please email me through my profile and tell me the
thickness of the various layers of this rock. please include the
name of the earthcache in your email.
THIS CACHE IS BEST DONE AT HALF-TIDE OR LOW TIDE.
