To earn a smiley, the EarthCacher
will:
.......Use given data and
the GPS altitude feature to calculate strata
.............. thickness
.......Identify, measure,
count and speculate on the origins of unique
..............features on
the outcrop
.......Determine, using
simple tools, the dip and strike angles and
..............directions of
selected
layers
.......At the posted coordinates, you will find the best and
almost the only measurable outcroping of the upper Waynesboro
Formation. Near Pond Bank, a short distance south of U.S. Rt. 30,
the formation consists of a belt about 2000’ in width which
widens to a mile NE of Waynesboro, PA. It extends south toward
Cavetown, MD and east, passing onto the South Mountain
anticlinorium. There is a reasonably distinct three-part division
to the Waynesboro. A double ridge characterizes the upper and lower
portions with the middle limestone forming the valley
between.
.......The base
layers consist of buff dolomite and sandy dolomite. The base is
exposed for about 100’ near the Waynesboro Country Club
opposite the dam. The middle limestone layer is the thickest and is
limited to scattered outcroppings. Many buildings in town,
especially those along Main Street, were built over limestone
bedrock. It is difficult to tell the difference between the Elbrook
and Waynesboro limestones. Caves and pockets in the formations
sometimes opened into basements and were used by residents as
coolers for produce, milk and other perishables. It is thought,
though not confirmed, that tunnels interconnect from one end of
town to the other. Sink holes have occasionally opened on both
private and business properties, exposing parts of the labyrinth. A
noteable example is the cave discovered during the construction of
the now abandoned Sheetz store under what had been Knepper’s
gas station at the corner of Main and Walnut Streets only a few
blocks
west.
.......The upper layer (that which is
before you) is composed of flaggy dark red to purple sandy shale
and siltstone. Some parts of the formation also evidence
ripple-marked sandstone, but not those at this location. By walking
west almost to the driveway, a change in the color and texture of
the upper levels of this layer can be
observed.
.......The Elbrook Formation overlies the Waynesboro to the west
and throughout most of the town. The Waynesboro overlies the
Tomstown Formation to the east. The Waynesboro has been dated to
the lower to mid-Cambrian Period. Fossils in the beds are almost
nonexistent.
.......Notice that
the rock layers here along the sidewalk are essentially horizontal.
Owing to poor exposure throughout the formation, the exact
thickness of the bed is difficult to estimate, but is thought to be
about 100’. Topographic maps show the highest elevations in
proximity to the exposure to be about
800’.
1. Based on your current elevation and the above estimates,
how much of the layer continues below sidewalk level?
.......Walk westerly until you reach the exposure between the two
heavy black street poles (see waypoint). There are some symmetrical
features on the rock that are not typical of
outcrops.
2. Describe these features including
their approximate size, shape, number and your best guess for their
origins.
For the next
questions, you will need some basic
tools:
....... A small amount of water
....... Carpenter’s
level (recommended) or laser level
....... Either a protractor
or a drawing compass
....... Directional
compass
....... A one foot ruler
might also be handy but is not necessary
....... Or, if you have one
and know how to use it, a Brunton compass,
...............which makes
all of the measurements at
once.
....... Still
between the poles and facing the rock wall, observe to your left,
that some of the bands have tilted away from the horizontal layers
at original ground zero. Choose one of these flatish inclined rocks
as your “work site”. “Dip” and
“Strike” are measurements taken of rock formations that
allow for comparison of strata and orientation of layers to other
formations.
Determining Dip:
....... Simply put,
the “dip” of a rock layer is its slope away from the
horizontal. Place enough water on the tilted rock surface so that
it runs down hill in a trickle. The direction it runs is the dip
direction.
Now align your
level against the
flat surface that the water just
ran down, and find a plane that is
horizontal. Use your protractor or
drawing compass to determine the
angle between horizontal and the
rock surface. This is the angle
of dip.
Determining
Strike:
.......
“Strike” is the imaginary line that
is made where a horizontal plane intersects the rock surface.
“Strike” is always perpendicular to the direction of
dip. Use the protractor (or any other object showing 90 degree
angles such as the corner of a sheet of paper) to find a line at
right angles to the direction the water ran. Note in degrees the
compass direction of the line. A strike at 90º from north is an
east-west strike. A strike at 0º is a north-south
strike.
....... Locate
magnetic north. Measure the angle between the north-south axis and
the strike line. This is the angle of
strike.
3. What is the direction and angle of dip of your chosen
rock?
4. What is the direction and angle of strike of your chosen
rock?
To get credit for the EarthCache, simply
E-mail answers to the cache owner. Please do not post any photos
that would compromise answers to the cache
questions.
References
Search “Strike
and dip”. There are many Online references available, some
with
....... interactive
practice.
Root, Samuel L. 1968. Geology and Mineral Resources of
Southeastern Franklin
.......County. PA
Geological Survey, Harrisburg: 9.
Fauth, John L. 1968. Geology of Caledonia Park Quadrangle Area:
South Mountain,
.......PA. PA
Geological Survey, Harrisburg:
41.