Counting the Stripes EarthCache
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This most unusual outcrop can be found on State Route 603.
You will be approximately halfway between Konnarock and Trout Dale.
Please park (see coordinates) in the gravel area on the opposite of
the road.
Count Here! - Lower Rock
This outcrop is of the middle to upper part of the
Konnarock Formation. Although it's age is not precisely known, it
rests above the Mount Rogers Formation, thus, it is younger than
760 million years old, and is overlain by the Unicoi Formation
which is more than 570 million years old.. With nearly 200 million
years in between, that leaves quite a bit of uncertainty for the
age of the Konnarock! One thing for certain, our Konnarock
Formation is between 570 and 760 million years old! I have
researched the subject and cannot find out why there have been no
radiometric studies to properly age the Konnarock Formation. We can
only approximate it’s age in comparison to the Mount Rogers
and Unicoi Formations.
NOTE: Closely observe the ‘lower’ striped
rock that is nearest to the road. What is of significance at
this outcrop of the Konnarock Formation are the maroon and
grayish-green bands seen in the rock. Yep, that’s where we
got the name of this EC. The bands (or stripes) alternate in a
cyclical way and are fairly even.. What is especially impressive to
is how well preserved these stripes are. Normally the Blue Ridge
area is extensively metamorphosed, destroying almost all of the
sedimentary layering that might be in the original rock. Thus, a
well-preserved outcrop like one this is a rare and a window into
the sedimentary history of the Blue Ridge
.
The maroon bands that you are observing are composed of a
mud, making this part of the rock a shale (or "argillite"). In
order to deposit such fine sediment quiet water is needed. Why is
quiet water needed? It is needed so that mud in suspension can
settle out slowly. A quiet lake is a distinct possibility. An
important bit of information is that all the sedimentary rocks
found in the Konnarock Formation are considered to be land
deposited. This is in contrast to ocean deposited rock found near
coastal areas. You are not looking for an environment like
"continental shelf" for this rock but definitely inland in
origin.
These rocks are what is known as "varves". Varves form in
lakes in cold climates. During the winter months, the lake freezes
and the water becomes very quiet. The mud in suspension settles. In
the warmer months there is a more active situation. Creeks and
small rivers bring silt and mud to the lake. The silt then settles
out. Because of the alternating seasons and each mud-silt pair of
stripes represents a one-year cycle. While the rock you are viewing
certainly is not a tree, this is one of the few instances in
geology where it is possible to physically count numbers of years
in a rock. The seasonal cycles are called “rhythmites”
for obvious reasons (the rhythm of the seasons). Each set of
stripes represents one year just like the stripes in a tree
trunk!
Part of the Upper Rocks
Let’s look at the rocks above OLD STRIPE. (See above
photo). The sediment here is much coarser than found lower striped
rock. Much of this is sandstone. Keeping in mind that this area was
a lake, the way to bring sandstone into a lake is by currents and
the turbidity of the currents. The grains were much larger and
stronger than ones that could have formed the silt layers in the
lower rocks. Perhaps sand accumulating at the margins of the lake
got kicked loose and that sent the sediment-water current flowing
into the center of the lake like an underwater landslide. The
isolated chips of red mud that appear to be suspended near the base
of the upper rock are "rip up clasts". The current that formed this
was so abrasive that it literally scoured the previously deposited
red mud, ripped it up, and incorporated the chips into the flow.
The bottom of the sandy layer literally scoured grooves into the
underlying varves. What an amazing process
!
In order for you to claim a find of this earthcache the
following tasks must be completed : 1. Answer the
following questions: A. How many red stripes do you count in
the lower rock? B. How many gray/green stripes do you count
in the same rock? C. What is the elevation at the lower
rock? 2. Post a photo of you and/or your GPSr pointing to
the lower striped rock. While it would be nice, it is not necessary
to show your face in the picture. We hope you have enjoyed this
little bit of geology as much as we did. We are certainly not
geologists………..far from it! We have found the
subject becoming more and more interesting so from a strictly
amateur point of view, we simply want to share these wonderful
local geological phenomena with you. We are learning as we go
along.
This Earthcache was approved by the
Geological Society of America
| We have earned GSA's highest level: |
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Additional Hints
(Decrypt)
Rira gubhtu vg unf fgevcrf qbrfa'g zrna vg'f n crgevsvrq gerr!