In A Geological Stew EarthCache
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This EarthCache can be found on Route 603 approximately 1-½
miles west of Route 16 at Troutdale. Using the given coordinates,
park on opposite side of road from the outcrop.
The Pot for the Stew - The Outcrop
This Geological Stew of rocks is a good introduction
to the Mount Rogers Formation. As in the stews we eat, our
Geological Stew has a lot of visible ingredients. We will
get into what is in the Stew later. Actually, this outcrop
represents the ‘lower’ Mount Rogers Formation. The
Mount Rogers Formation is quite diverse. It contains members of all
three of the major rock groups. Here, igneous, metamorphic and
sedimentary rocks are easily found. Before looking at this outcrop,
let’s examine the origin of the Mount Rogers
Formation.
It is thought that the Mount Rogers Formation began after
the erosion of the ancient Grenville Mountains. Some of the
evidence of this is the presence Cranberry Gneiss. The Cranberry
Gneiss is dated to a billion years ago and was part of the
Grenville Mountains. Beginning at about 760 million years ago the
Grenville basement rocks were intruded by a new set of volcanic
rocks. These rocks are part of the Mount Rogers Formation. During
this time, the rifting of the Earth’s crust and volcanic
actions resulted the creation of rhyolite and basalt rocks. Valleys
were also created which was the lowland that was surrounded by the
new volcanic mountains. This lowland was at one time covered with
water and/or ice and thus the opportunity for the formation of
sedimentary rock.
The volcanic rocks of the Mount Rogers Formation are
layered with sediments that are from the uplifted basement. At the
later stages of the Mount Rogers Formation, rhyolite volcanoes
began to dominate and this resulted in a huge outpouring of lava.
It is thought that during these last stages in the development of
the Mount Rogers Formation, there was some glaciation . The evidence of this is found with
the dropstones that were carried by ice to eventually settle in the
layers of fine mud. There, they became the conglomerate rock that
is found at this outcrop. An alternate theory of how the cobbles
and/or dropstones were moved and deposited was from a debris flow.
With this theory, it is felt that sediment gradually filled in the
lakes left behind by the glaciers or ice
sheets.
Now you know what happened to form the rock at this
particular outcrop, let’s look at just what you will find at
this spot. As stated earlier, the rock is a conglomerate, which is
a sedimentary rock. It is a diamictite with a variety of large
dropstones or cobbles within the diamictite. You can find the
‘granitic’ cranberry gneiss, white quartz, greenstone,
sandstone and rhyolite cobbles all ‘suspended’ within
the diamictite.
It isn’t often that you can observe examples of the
three major rock groups at one spot. The cobbles are igneous,
metamorphic and sedimentary rocks. The gneiss and greenstone are
metamorphic while the rhyolite and sandstone are igneous and
sedimentary rocks respectively. Observe carefully because you will
need to identify and photograph at least two of these
cobbles.
A Quartz Cobble
Note: You cannot claim the dog ate my homework
because in order complete a find, you need to do the following:
1. Find, photograph and post pictures of two of the
following cobbles that are within the outcrop: a. white
quartz, b. cranberry gneiss or c. sandstone. The
white quartz is white. What else? The cranberry gneiss is pink and
the sandstone is tan/brown. Now we don’t expect you to become
geologists, especially since we are not geologists, so
‘spoiler’ photos are provided. The photos may be
upside down or side ways but they are spoilers none-the-less! You
can use our photos as a guide but you still have to find and
photograph the cobbles! In the photos, point to the cobble with
your GPSr. 2. Also using your GPSr, give the elevation at
the base of the rock. 3. Last, but not least as an option,
please post a photo of yourself or the GPSr pointing to the
outcrop. While it would be nice, it is not necessary to show your
face in the picture. We sincerely hope you enjoy this unusual
location and appreciate some of the Earth’s ancient history.
Looking at this rock will instantly make you feel young. We
guarantee it!
Bonus View, Across Road from
Outcrop!
This Earthcache was approved by the
Geological Society of America
| We have earned GSA's highest level: |
 |
Additional Hints
(Decrypt)
Qba'g pbooyr hc gur ebpxf!
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