Allsbury Formation
A
cache
by Northwoods Explorer
Hidden:
10/1/2007
Size:
 (Not chosen)
Difficulty: 
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(1 is easiest, 5 is hardest)
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This rock cut is an excellent example of the dip of the rock. What
is most interesting about this site is that it clearly shows
cleavage of the rock, nearly vertically dipping beds, a fault and
the possibility of seeing graptolites.
The Allsbury Formation is a deep-water formation that underlies
much of the region where the Central Maine and Aroostook-Matapedia
basins merge. It was formed during the Ordovician/Silurian period
some 430 million years ago. The rocks are light brown in color,
which is caused by the rare mineral siderite, a carbonate of iron.
What is most interesting about this site is that it clearly shows
cleavage of the rock, nearly vertically dipping beds, a fault and
has the possibility of seeing graptolites.
Cleavage is when the rock always breaks in certain manner. In
some rocks minerals, bonds between layers of atoms aligned in
certain directions are weaker than bonds between different layers.
In these cases, breakage occurs along smooth, flat surfaces
parallel to those zones of weakness. In some minerals, a single
direction of weakness exists, but in others, two, three, four, or
as many as six may be present. Here the rock breaks along two very
different planes. If you look at the rocks along the bottom of the
rock face you will see the planes. This tells scientists something
about the atomic structure of the rock.
The dip gives the angle below the horizontal of a tilted stratum
or feature. Typically the angle of dip is included on a geologic
map. This tells scientist what has happened to the rock in the
area. Once, long ago this rock was on the bottom of the ocean,
forces caused the land to be pushed up so that what was once flat
is now vertical. Dip, along with strike are a method of describing
the orientation of a plane in three-dimensional space. It is
usually applied to the orientation of tilted layers of rock.
Remember, dip is the angle of tilt, measured from the horizontal.
Think of the direction of dip as the direction that a ball would
roll if placed on the surface. The angle of dip is measured in
degrees.
There is evidence of a small fault at this location. A fault is
a planar rock fracture, which shows evidence of relative movement.
There are two parts to this simple thrust fault. The hanging wall
occurs above the fault and the footwall occurs below the fault.
This tells scientist how forces were applied to the rocks in the
past.
Graptolites, the fossil colonial animal, commonly appear as
feathery black tracing on the bedding surfaces. The name graptolite
comes from the Greek graptos, meaning "written", and lithos,
meaning "rock", as many graptolite fossils resemble hieroglyphics
written on the rock. They were originally regarded them as
'pictures resembling fossils rather than true fossils'. Graptolites
are common fossils and have a worldwide distribution. Scientists
use these important index fossils for dating rocks as they evolved
rapidly with time and formed many different species.
To log this Earthcache: You must post a photo of you and your
GPS with backgound in your log and then send an email to me through
my profile with the following information: At the above coordinates
you will see a thrust fault. As you are looking directly at the
fault from the roadside tell me if the direction that the hanging
wall moved was to the right or left? Make sure to check out for
bedding dip or perhaps fossils while you are at the location. You
should be able to view this earthcache from your car on a rainy
day. Please begin your email with the name of the earthcache and
make sure your log includes the number of people in your
group.
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