A Geological Walk In Time EarthCache
A Geological Walk In Time
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From Damascus, VA., via Hwy 58, turn right onto the Grassy
Ridge Rd (County 859), proceed about 3.3 miles to the parking area
(above coordinates) on right (look for the stage). Walk across the
road and cross the footbridge, which is to the left of the General
Store.
This bike/horse/hiking (take your pick!) trip will take you
from the lower Mount Rogers
Formation across the Konnarock
Formation and finally to the
Unicoi Formation along the
Virginia Creeper Trail. The trip is just slightly over one mile
(one way). This Trail is considered to be public right of way and
is available on a 24/7 basis. Everything is allowed on the Creeper
Trail with the exception of motorized vehicles. Several shops in
Damascus rent bikes and will cart you to the top of the Trail. The
Trail used to be a railroad bed, but after ceasing operations, the
railroad deeded the land back to the public. This short trip on the
Creeper Trail will span close to 200 million years.
First, lets talk a bit about the three previously mentioned
Formations.
Mount Rogers
Formation
The Mount Rogers Formation was developed some 760 million
+/- years ago. This was partially due to the erosion of the ancient
Grenville Mountains. Newer felsic and malfic rocks of the Blue
Ridge intruded the Grenville basement rocks. These rocks are part
of the Crossnore Complex as described by Rankin in 1970. This
concentration of igneous rocks denotes a cycle of rifting of the
ancient continent (Rodinia). The felsic (silicate along with
sodium, aluminum, or potassium) and malfic rock (rich in magnesium
and iron) intrusions broke through to the surface to create the
rhyolite that is often observed today. With this volcanic action
basalt was also created. The volcanic action and the up rifting of
the ancient mountains best characterize, the Mount Rogers
Formation.
Konnarock
Formation
The Konnarock Formation occurred between 760 and 570
million years ago. This formation can be described as best as a
period of some type of glaciation. Most don’t believe that large
continental ice sheets were here but some sort local alpine
glaciation occurred. During this Konnarock time, the rift valley
still existed creating a location for a huge lake. The upper
highlands were glaciated and those glaciers brought sediment to the
lake. It was during this time frame that the characteristic
sedimentary rocks were being formed.
Unicoi Formation
The Unicoi Formation took place some 570 million years ago
and naturally is the youngest of the three formations thus the
rocks of the Unicoi are the youngest of the rocks that you will
view. It was during this period that new rifting occurred. Pebble
stream deposits that are reminants of ancient stream gravel bars
represent course conglomerates of the lower Unicoi. The top of the
Unicoi contains white or clean quartz sands that were deposited in
a shoreline environment. Over the years, this sand settled and with
pressure was cemented into the fine white sandstone. Newer rock
from this period is the shale, which in millions of years could
form slate.
Before we begin the journey, lets
define the specific rocks you will be finding. Beginning from Stop
1 and continuing through Stop 6, here are the
rocks:
Stop 1. Basalt: is a
common igneous extrusive (meaning above ground) volcanic rock. It
is usually grey to black and fine-grained due to rapid cooling of
lava at the surface of a planet. Unweathered basalt is black or
grey. Pillow basalt is the lava which has more likely erupted into
a water environment. It is smoothed and rounded as it quickly cools
in the water to form a sort of large pillow. Basalt and rhyolite
are the oldest rocks found on our trip.
Stop 2. Rhyolite: is an
igneous, volcanic (extrusive) rock, of felsic (silicon -rich)
composition. The mineral content is usually quartz, alkali feldspar
and plagioclase. Sometimes, biotite and hornblende are additional
minerals that are found. Rhyolite can be considered as the
extrusive equivalent to the intrusive (meaning staying below
ground) granite rock, and consequently, outcroppings of it often
bear a resemblance to granite. It can be colored pink, light or
dark gray or near white or a combination depending on the mineral
content.
Stop 3. Diamictite: is a
poorly sorted conglomerate with a wide range of clasts, up to 25%
of them gravel sized (greater than 2 mm). Diamictites are composed
of coarse, angular to well rounded sedimentary clastic fragments,
or other type of fragments such as igneous and metamorphic rocks.
These clasts are supported by a typically argillaceous clay sized
matrix. Diamictites are formed by varing condidtions but it is
thought that our diamictites were fromed by glacial or ice sheet
and/or volcanic action. Look for the differing color pebbles
(cobbles) suspended within the larger rock. The ’pinkesh’, larger
cobbles are the metamorphic rock, cranberry gneiss, which is over 1
billion years old.
Can you find me? See Stop No. 4 below.
Stop 4. Conglomerate: is
a rock consisting of individual stones within a finer-grained
matrix that have become glued together. Conglomerates are
sedimentary rocks consisting of rounded fragments of other rocks.
Generally, conglomerates have a sandstone base but not always.
Those viewed on this trip are of the sandstone matrix type. The
individual stones are termed clasts and are classified based on
their size. Colors vary depending on weathering and the individual
clasts. Our conglomerate has predominately round, white quartz
stones. The conglomeratethat you will find is the ‘middle aged’
member of our five rocks.
Stop 5. Sandstone: is a
sedimentary rock composed mainly of very small, sand-size mineral
or rock grains. Most sandstone is composed of quartz and/or
feldspar because these are the most common minerals in the Earth's
crust. Like sand, sandstone may be any color, but the most common
colors are tan, brown, yellow, red, gray and white. Feldspar
composes as much as 60% of the Earth’s crust.
Look for me at Stop 5 above!
Stop 6. Shale: is also a
sedimentary rock that is fine frained whose original constituents
were clay minerals or muds. It is characterized by thin layers
breaking with an irregular curving fracture and is often splintery.
Shale is the most common sedimentary rock. The process in the rock
cycle which forms shale is compaction which is a process of the
original material settling to the bottom of water, accumulating
additional layers and under pressure becoming solidified or
compacted and hardened. Shales that are subjected to heat and
pressures form slate. The colors of shale vary from gray to almost
black and from tan to red. Generally speaking, shale is the
‘youngest’ of our rocks. Maybe in another million years or two, our
shale will become our slate! NOTE:
the final stop (no. 6) is on the Grassy Ridge road which is to the
right of the Creeper Trail at the bridge. Just follow the given
coordinates.
Going from Stop 1 to Stop 6, ALL rocks
are on the RIGHT hand side of the trail!
NOTE: In order to claim a smiley, use the coordinate
list below to take you to the 6 Stops. At each stop take a
picture with you and/or your GPSr in the foreground and the
specific rock in the background. Also, at each stop answer the
following questions (via email only): 1. What is/are the
color(s) of the rock?, 2. What is the elevation at the base
of the rock?, 3; Estimate the size (showing above ground) of
the rock. Enjoy your trip and take a walk back in time!
This Earthcache was approved by the
Geological Society of America
| We have earned GSA's highest
level: |
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Additional Hints
(Decrypt)
Vg jvyy or n ebpxl gevc!