Paleo Erosion EarthCache
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This series of earth caches is based on the publication “Roadside
Geology Along the Alexandria to Ashland (AA) Highway.” The road
logs were published by the Kentucky Geological Survey to give the
public an appreciation of the geologic world around them.
Many geologists have referred to the AA Highway as a “treasure
trove” and “an outdoor classroom” in which to study diverse and
significant features such faults, systemic rock boundaries, fossils
and ancient river markers.
Like modern-day sleuths, geologists interpret the clues they find
preserved in the rocks. These clues are of two main kinds: the
types of fossils contained in the rocks and the properties of the
rocks themselves. As geologists' knowledge of the Earth increases,
the record of its history has become clearer and more
meaningful.
Buckle your seat belts and head back in time and look for the clues
as you head down the AA from it’s intersection with US 23 in
Greenup County. Each cache in this series will stop at a unique
geologic formation and will seek answers to some basic questions
that should be easy to calculate. Sizeable pull off areas are
available at each stop in the series. Geology students frequent the
locations routinely. The calculations can be made from your car
even, making it handicap accessible!
The surface physiography of a region is shaped by its underlying
rock structure. The three main types of rock are sedimentary,
igneous, and metamorphic. The sedimentary rock is the most common.
Sedimentary Rock consists of successive layers of wind or
waterborne materials. Under pressure, these deposits turn into rock
such as sandstone, shale, limestone and coal. Travel to Mile Marker
7.6 to 8.1 to an outcrop of Carboniferous age sedimentary rock of
Sandstone from the Pennsylvanian Period and Limestone from the
Mississippian Period. The two periods of rock form an unconformity
as if some years of layers may be missing. This unique outcrop is
in stacked channel-fill sequences containing chert lag deposits
derived from PALEOEROSION. 
Stacked channel-fill sequences of Pennsylvanian and Mississippian
Period Rock of the Breathitt and Borden Formations here show the
base of each defined by a prominent erosion surface that cuts into
and through the underlying member resulting in erosion of channel
members. Lateral accretion surfaces like these are rare, but some
channel-fill members contain down current-dipping cross-strata that
are interpreted to represent times of shallow seas The scarcity of
lateral accretion surfaces and occasional presence of in-channel
macroforms may indicate that each channel member was deposited in a
low-sinuosity channel that was perhaps braided at low river stage.
This sequence of deposition is indicative of a flood plain in which
the main river-channels are meandering and prograding in an upper
deltaic environment. Channel cut and fill sequences are common in
such an environment.
Thick cherty limestone units weather to produce residual lag
deposits composed of angular chert fragments, which are highly
resistant to chemical breakdown. Cherts are hard, aphanitic rocks
made up of silt-sized quartz crystals that form radiating fibers
tens to hundreds of microns in length. The dark, organic-rich form
of chert is sometimes called flint. Chert generally occurs as
nodules or irregular beds, usually in limestones. The Chert
represent gravel deposited by ancient rivers that crossed the
region hundreds of thousands or even millions of years ago washed
from source areas hundreds of miles distant. Geologists attribute
these deposits to the earlier parts of Pleistocene geologic period,
which begins 1.8 million years ago, or even earlier, during the
Pliocene Geological era (5 to 1.8 million years ago). Since then,
these ancient waterways have been all but obliterated by geological
processes. The Paleoerosion and the continuous landscape remodeling
done by the drainage systems is on-going and can still be seen
today. 
It stretches the mind to believe that a stream or river once passed
over this now high and dry terrain. The changes that Earth has
undergone over geological time are very difficult for most of us to
grasp. At the time of deposition, the chert gravels occupied the
lowest topographic positions in the surrounding landscape- the
stream channels. The preservation of these chert lag deposits
demonstrates a wholesale inversion of topography. Considerable
erosion has taken place such that the former low points now occupy
the highest positions in the local landscape. The magnitude of
vertical erosion can be estimated from the elevations of the
highest chert lag deposits in relation to present stream-valley
floodplains in each drainage basin today.
Kentucky was a tropical paradise 300 million years ago. The fact
of Kentucky's tropical past is supported by today's mineable coal
resources. The thick coals in Eastern Kentucky needed a tropical
climate to form. But geologists say they have found evidence that
the climate wasn't continually wet. In fact, researchers believe at
times the climate may have been temperate or even semi-arid. In the
Carboniferous Era comprised of the Pennsylvania and Mississippian
Periods such tropical conditions prevailed as sea levels rose,
moving the coast more than 200 miles inland until it reached
Kentucky. Tides and waves reworked sandstones near the tops of
valleys. As ocean waters washed in and the sand-filled valleys
became shallow swamps with frequent rain, humidity, and abundant
plant life. These plants decayed forming peat, the precursor to
coal. Sometimes Kentucky was covered by shallow seas, sometimes it
was a swamp, and sometimes it was river plains. It just kept
cycling through time.
"
Paleoerosion is displacement of solids sediment, soil, rock and
other particles by the agents of currents such as, wind, water, or
ice that occur over millions of years. Researchers consider the
geomorphology, paleogeography, paleoclimate, paleotopography,
chemical and mechanical weathering, even CO2 levels in Carbon
Cycles when considering the on-going erosion process. Erosion is
distinguished from weathering, which is the process of chemical or
physical breakdown of the minerals in the rocks, although the two
processes act concurrently. Each climate has presented differing
erosional agents. Some of morphed the formations as they erode,
changing the topography. Each of these process can be considered
when discussing PaleoErosional impact.
To view the Paleo Erosion Process, follow your GPS to the
coordinates provided. The Northeast Side of the AA Highway is where
the greatest erosion is visible between the first and third benches
of the outcrop. The outcrop from bottom to top is roughly 180 feet
in height.
Your mission:
1.Estimate the amount of Pennsylvanian (newer)and Mississippian
Rock (older)exposed.
a. Miss 50 feet / Penn 130 feet
b. Miss 130 feet / Penn 50 feet
c. Miss 30 feet / Penn 150 feet
2.Estimate how many feet of erosion is visible between the first
and third benches.
a. Between 50-75 feet
b. Between 75-100 feet
c. Between 100-150 feet
3. Shoot an elevation for the outcrop location.
Email the answers and post a picture of the paleo erosion with your
log and then you too will be a geocaching geologic sleuth! Now
buckle up again and get ready for another new geologic adventure
along the AA Highway- Kentucky’s gateway to the past.
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