Hanging Rock: A Relic
of Ancient Seas
In the Great Lakes region of North
America, a belt of marine reefs was developed in the Silurian seas
more than 400 million years ago. Many of these reefs are 100 to 400
feet thick and as much as a mile in diameter.
Hanging Rock, towering above the Wabash
River, about a mile southeast of Largo, is a remnant of a huge
Silurian Limestone Coral Reef . Constant undercutting by the Wabash
River has given this limestone pinnacle a “hanging” appearance.
This rock is a National Natural Landmark.
THIS REEF-LIKE ROCK IS PART OF THE
"KLINTAR" OF THE WABASH VALLEY. IT IS COMPOSED OF THICK-BEDDED,
HARD DOLOMITE (LIMESTONE WITH MAGNESIUM) FORMED FROM THE FOSSILIZED
REMAINS OF CORALS AND OTHER REEF-DWELLING ORGANISMS. REEFS LIKE
THIS ONE ARE READILY RECOGNIZED BY THE FACT THAT THE BEDDING DIPS
STRONGLY AWAY FROM A CENTRAL CORE, WHICH IS IN SHARP CONTRAST TO
THE FLAT LAYERS OF SEDIMENTARY ROCK THAT USUALLY CHARACTERIZE THE
MIDWEST. THIS AREA WAS PART OF A SHALLOW SUBTROPICAL SEA DURING THE
SILURIAN PERIOD, 400 MILLION YEARS AGO.
Reefs in several Hoosier counties in the
Wabash Valley contain the only rock suitable for high-quality
aggregate. In many places outside Indiana where buried reefs are
uncased in impermeable strata, oil and gas are found within the
porous reef rock, but in Indiana all petroleum produced to date
from reef-induced traps has been found in permeable strata above
the reefs.
In order to log a find on this cache, you
must do two fun things:
1. Hike up the short path to the top of
Hanging Rock, and create a photograph of yourself, including your
face, holding your GPSr, with the river in the background, and post
it with your log. You will be rewarded with a magnificent view of
the Wabash River and its valley.
2. Hike down the path to the riverside.
Estimate the height of Hanging Rock from the path. And then email
us with your estimate of how long in years you think it might have
taken the River to erode the rock away, using the data supplied
below.
Scientists have determined through
experimentation that the Erosion Rate of Silurian Limestone exposed
to flowing water is approximately 1.0 millimeters per year, more or
less. Therefore, it would take about 300 years to erode away a foot
of limestone.
Estimating Height - The Pencil Method
To use the pencil method, you need a friend who knows his or her
height and a pencil. Then do the following:
Choose an object that you want to measure-maybe a tree in the
backyard.
Have a friend who knows his or her height stand close to the
tree.
Hold a pencil in your hand and extend your arm fully so that your
elbow is locked.
Line up the top of the pencil with your friend's head.
Place your thumb on the pencil to line up with your friend's
feet.
Now the pencil represents the scaled height of your friend
standing against the object.
See how many pencil lengths it takes to reach the top of the
tree.
If the person standing next to the tree is six-feet-tall and it
takes six pencil lengths to reach the top of the tree, the tree is
approximately 36 feet high.
11/25/2006: Buddaman,Team Shydog,DougSmiley,Team
GCHound,Doobies
***********************************
Winter 2006: Che K, Lead Dog, Superstutz, Kodiak Cowboy,
WCNutt
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Winter 2006: Auntie Nae, The Tapps, Lord Elwood, Butterfly
Angel & ThaBigBird, The Graysons
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2007:Master of my Domaine, Are_we_there_yet?
NOTE: Failure to accomplish both tasks
completely, or giving away the answers in your log will result in
deletion of found log. Thank you for your consideration.
--Professor
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Hanging Rock was designated by the National Parks Service as
a National Natural Landmark in 1986 because it is an impressive
natural exposure of an exhumed reef. The Wabash Reef is one of the
best known fossil reefs in the world, because it has been the
subject of numerous studies responsible for the development of
modern reef theory.
GAG PHOTOS WILL NOT BE ACCEPTED