The above photo was taken shortly after the road crews had cut
and banked the shoulders and drainage ditch.
Consolidated rock is a solid, coherent whole. We often refer to
it as "bedrock", and that's a good name for it. Between the soil
and bedrock is a layer of rock referred to as "saprolite"; rock
that is being altered by ground water, acidic compounds in the
water, and other things which leach out of the air and
soil. If the bedrock is very hard, such as granite or
gneiss, the layer of saprolite is quite thin. In areas where the
bedrock is sedimentary and not quite as hard, the saprolite layer
can be thick. That's the case here, as you are looking at a rock
known as the Floyd Church Formation mudstone. The bedrock mudstone
is a bluish gray color, is quite hard, and is used as aggregate for
gravel roads and in building foundations. The rock
you're looking at is still pretty hard, but definitely not hard
enough to use as a road surface. While slow, low level
deconsolidation of the mudstone had been taking place for thousands
of years, in the summer of 2006, this rock began to be attacked in
many new and significant ways.
When previously buried rocks are exposed, they are subject to
immediate, often serious, erosional elements. The rocks here were
exposed because of human actions, but rockslides, earthquakes,
forest and grass fires, excessive rains and flooding, volcanic
flows and ejected materials, and other events can naturally expose
many, many areas of the crust to "the elements". The elements are
deconstructors.
1. Running Water. You
might think that the saprolite would be real happy -- after all, it
now has sunshine and fresh air to experience. It might
be happy until the first rain storm. Walk back and forth along the
road and look for newly forming "gullies" that are being
created. The road cut here is steep, and you can already
see a few little places where gulleys are being created from the
top of the cut to the bottom. Water running steeply
downhill has much more force than shallow run-off, and will erode
the rock much more quickly. The more force, the more
erosion.
2. Frozen Water. Look at the
rocks. There are numerous straight line cracks and
fractures. In cold weather, water can run/seep into
these cracks, and then freeze at night. Water freezes to
about 110% of its liquid volume (size); at the molecular level,
this will break apart some of the consolidated rock. The
more cracks and water -- the more cold weather -- the more
erosion.
3. Plants. Plants form
where water collects or is present. No water; no
plants. While plants may slow down the velocity of
running water in some places, they also offer a place where water
can collect and freeze overnight. As the years go by and small
seedlings turn into trees, the expanding roots of the trees will
further serve to crack the saprolite and hasten erosion.
4. Slumps. When a block of
rock is suddenly exposed, there is usually a great variety of rock,
soil, and "angles" involved. Commonly, you'll see a fairly steeply
angled block of saprolite overlain by soil, or saprolite that is
more weathered than that under it. What's holding that
more weathered block up? Often, very little. Walk about
30-40 yards uphill to the vicinity of N 35º 15.621'
W 80º 17.941'. You'll be looking at
a small slump, whereby the "dirt" overlying the rocks beneath them
gave way and slumped down into the ditch. Note how nice and
clean the rocks above look, as opposed to the slumped
soil. In nature, slumps can be miniscule like this one,
or can be hundreds of yards across. Running water down
the ditch will very shortly erode this slump material away.
5. Bugs. Usually, you
can't see much evidence of how bugs affect erosion. Depending on
the time of year you're here, you might see some big fire ant
hives, or the remains of them at the top of the cut.* Insects are
burrowing into the saprolite and adjacent areas, and are weakening
the resistance of the rock to the forces of erosion. Since fire
ants typically burrow at the top of a slope, they will
inadvertantly help running water be an effective erosional element
as it runs down the slope. Also, burrowing bees and other insects
will provide entrances for water, which will later freeze and open
up more and more of the saprolite to erosion.
Usually, when those of us that create EarthCaches speak about
geological events, we talk about things that take place over
millions and millions of years. In looking at this road cut, it's
obvious that we're talking about events on a human time scale.
It's easy to see that only several years after the road cut
was made, significant erosion has begun. Take your human-time hat
off, and look at the exposed bank in geological time: If all things
stayed the same here for a hundred, a thousand, or many thousands
of years, imagine how much saprolite and soil would be eroded
away! Thousands and thousands of tons of material would
have flowed off towards the Atlantic Ocean.
Exposure, whether by earthquake scarp, fires, rockslides, floods
or any other reason, is a catalyst for extremely accelerated
erosion.
Other Educational Information:
Logging
Questions:
Send me an e-mail – not part of your log
– responding to the following:
1. Make the first line of the e-mail “GC2305C, The
Deconsolidators: Exposure”
2. How many people were in your party?
3. Describe the differences you note between the 2006
photo and what you see today.
4. Based on what you can see today, which of the five
agents of erosion mentioned above do you feel is the most
significant? Why?
5. Please DO NOT post a photo of
your party at the coordinates given.
Platinum EarthCache Master