PLEASE NOTE AND COMPLY: USGS
Security has indicated that they want this EarthCache to be hunted
only during visitor/business hours. That seems to indicate that the
cache cannot be hunted on weekends or holidays. I am working with
the Visitor Center Manager to get a clear reading on this. In the
meantime, please hunt this EarthCache during weekday business hours
only.
The USGS Campus contains many geologic informational displays, this
is a short 1.5 mile circuit hike that will bring you to 10
different "stations" where you will find rock and mineral exhibits
from throughout the United States. Not only will you be able to see
and feel these rock formations, but you will also come away with a
better understanding of geology and the forces that impact the
Earth you live upon. There is plenty of on-street parking
available, but NOT on USGS Drive. You may park on either side of
South Lakes Drive, or Sanibel Drive, closest to "Stop #1" on the
tour. Additional parking is available in the Sunrise Valley
Technical Park, just off Sunrise Valley Drive, that parking
location is closest to "Stop #5" (additional parking waypoints are
listed). If available, you might also request USGS Visitor Parking
in a designated Visitor lot…follow the appropriate signage. Mid-way
through your walking tour you will arrive at the entrance to the
USGS Headquarters building, you can enter through the security
checkpoint and visit the USGS Map Store. DO NOT hunt this Earth
Cache at night. This is a Federal facility video surveillance
cameras are in use, and the area is patrolled by roving security
guards - DO NOT hunt this cache at night! This Earth Cache was
published with support and permission of the USGS Visitors Center
Manager.
STOP #1. N38 56.640 W77 21.986 Your first stop on this Tour de
Geology. Here you will find an example of Barre Granite. Granite is
an extremely hard natural igneous rock formation of visibly
crystalline texture formed essentially of quartz and orthoclase or
microcline and used for building and for monuments, it has
unyielding firmness or endurance. The granite you are viewing was
formed during the Upper Devonian Period, approximately 417 to 354
million years ago (type locale is Barre, Vermont). There are
examples of Barre granite in almost every hamlet, village, town and
city in America, commemorating the resting-places of those who have
been loved and lost. Long centuries after other products have
passed into oblivion, these Barre granite memorials in churchyards,
cemeteries, battlefields, parks and town squares will permanently
designate and commemorate the ideals, the tradition, the sentiment
and the devotion of the American people.
STOP #2. N38 56.751 W77 22.205 Your second stop on the "tour".
You are looking at 3 "slices" of Columnar Basalt. Basalt is a
common gray to black volcanic rock. It is usually fine-grained due
to rapid cooling of lava on the Earth's surface. It may be
porphyritic containing larger crystals in a fine matrix, or
vesicular , or frothy scoria. Unweathered basalt is black or gray.
Basalt magmas form by decompression melting of peridotite in the
Earth's mantle. The crustal portions of oceanic tectonic plates are
comprised predominantly of basalt, produced from upwelling
peridotite in the mantle below ocean ridges. Columnar jointing
forms in lava flows, sills, dikes, ignimbrites (ashflow tuffs), and
shallow intrusions of all compositions. Most columns are straight
with parallel sides and diameters from a few centimeters to 3
meters. Some columns are curved and vary in width. Columns can
reach heights of 30 meters. The columns form due to stress as the
lava cools (Mallet, 1875; Iddings, 1886, 1909; Spry, 1962). The
lava contracts as it cools, forming cracks. Once the crack develops
it continues to grow. The growth is perpendicular to the surface of
the flow. This Columnar Basalt formed during the Miocene Period
approximately 23.8 to 5.3 million years ago.
STOP #3. N38 56.797 W77 22.237 This is your third "tour" stop.
Here you find two large examples of Diopside Crystals from the
Precambrian Period. Hey, this is some old stuff! These examples
formed approximately 4600-570 million years ago. Diopside is an
important rock forming mineral in several metamorphic and basic to
ultra basic igneous rocks, also found in meteorites. Diopside is a
part of an important solid solution series of the pyroxene group.
The series includes the minerals hedenbergite, CaFeSi2 O6, and
augite, (Ca, Na)(Fe, Mg, Al)(Al, Si)2 O6. A series occurs when ions
(in this case iron and magnesium) can freely substitute between
each other. Diopside is the magnesium rich end member of the
series. Diopside has several varieties, including a chromium-rich
gem variety called chrome diopside. Violan is rare blue variety
found in some localities in Italy. There is also a green "cat's
eye" variety that contains minute inclusions, probably of rutile,
that reflect light in such a way as to produce a lively linear
luminscence within the crystal. Still another variety is quite
dark, with included rutile needles aligned so as to produce a
4-rayed star, hence the name star diopside. Ordinary diopside is
typically white or green and can have a nice glassy luster. Mineral
specimens of diopside can be very striking in appearance, and of
interest to mineral collectors.
STOP #4. N38 56.853 W77 22.209 You're making good progress, here
is your fourth stop on the "tour". This is Cockeysville Marble,
showing a classic visual example of recumbent isoclinal folding. It
too is from the Precambrian Period between 4600-570 million years
old. In its simplest form, a geologic structure marked by the
folding of originally horizontal rock layers into a systematically
curved, concave upward profile geometry. A syncline is convex in
the direction of the oldest beds in the folded sequence, concave in
the direction of the youngest beds. Although typically upright, a
syncline may be overturned, recumbent, or upside down. Synclines
occur in all sizes, from microscopic to regional. Profile forms may
be curved smoothly to sharply angular. Fold tightness of a
syncline, as measured by the angle at which the limbs of the
syncline join, may be so gentle that the fold is barely
discernible, to so tight that the limbs are virtually parallel to
one another. The orientation of the axis of folding is horizontal
to shallowly plunging, but synclines may plunge as steeply as
vertical. Synclines are products of the layer-parallel compression
that arises commonly during mountain building. The final profile
form of the fold reflects the mechanical properties of the rock
sequence under the temperature-pressure conditions of folding, and
the percentage of shortening required by the deformation.
STOP #5. N38 56.887 W77 22.171 Halfway point on your rocky walk.
This brings you to a display of Cordierite Hornfels. A dark to
medium gray hard, compact rock that breaks into splintery
fragments. Mineral content is variable. (Cordierite hornfels, which
contains crystals of cordierite; andalusite hornfels, which
contains crystals of andalusite; pyroxene hornfels, which contains
crystals of pyroxenel and sillimanite hornfels, which contains
crystals of sillimanite. Hornfelsic and fine-grained, sometimes
with porphyroblastic crystals. Found in zones of contact
metamorphism. Cordierite Hornfels was formed during the Triassic
Period, approximately 248 to 206 million years ago. It is primarily
used today as aggregate in construction projects. Locally there are
numerous formations of Cordierite Hornfels (siltstone), in fact one
of the local Regional Battlefield Parks is completely underlain
with it, savvy cachers will recognize the Balls Bluff location.
STOP #6. N38 56.883 W77 22.129 Leesburg Conglomerate is a
rounded clastic rock with cobbles to granules. (The Capitol Rotunda
Statuary Room contains columns carved from Leesburg Conglomerate.)
The source area was the western border of the Cotoctin basin. It
was an active border fault. Paleozoic limestones were shedding
immature particles from the kilometer higher mountains. Silt and
clay were at the center of the basin. This was at the side of an
alluvial fan. There were distributory lobes in the alluvial fan
(fingers of the fan). Even though the deposit was close to the
source area, the cobbles were well rounded due to the limestone
composition of the rocks. The matrix of the conglomerate rocks is
sand (Manassas sandstone). The Cotoctins all eroded, from 1 billion
to 700 million years ago. The main rock was basalt and greenstone
(a metamorphosed basalt). Karstification and dissolution of the
limestone conglomerate between Cotoctin Mountains and the Potomac
has riddled the area with sinkholes and caves. (Special Note: There
is a U.S. Department of the Interior, Geological Survey Benchmark
"NC-2" adjacent to this rock display. Look at the stone walkway
near the flag pole).
STOP #7. NOTE: Satellite reception is very poor this close to
the building. From where you are at STOP #6, walk a bearing of 67
degrees True North for approximately 137 feet to find this display.
When is a "rock" not a "rock"?…why, when it's Petrified Wood!
Petrified wood is a type of fossil: it consists of fossil wood
where all the organic materials have been replaced with minerals
(most often a silicate, such as quartz), while retaining the
original structure of the wood. The petrifaction process occurs
underground, when wood becomes buried under sediment. Mineral-rich
water flowing through the sediment deposits minerals in the plant's
cells and as the plant's lignin and cellulose decay away, a stone
mold forms in its place. The wood is preserved due to a lack of
oxygen. Minerals such as manganese, iron, and copper in the
water/mud during the petrification process give petrified wood a
variety of color ranges. Quartz crystals are colorless, but when
iron is added to the process the crystals become stained with a
yellow or red tint. So in reality…when is "wood" not "wood"?…why,
when it's a "rock".
STOP #8. N38 56.839 W77 22.175 This brings you to an excellent
example of Conestoga Formation (limestone). This sample was formed
between 505 to 438 million years ago (the Lower Ordovician Epoch).
This micaceous, shaly limestone extends in the relatively wide belt
across Pennsylvania, Maryland, and Virginia. Shallow seas spread
for most of the time over much of North America. The Conestoga
Formation rocks are chiefly sedimentary. Because of the restricted
area and low elevation of the solid land, which set limits to
erosion, marine sediments that make up a large part of the
limestone; shale and sandstone are less conspicuous. During the
epoch in which this sample was formed, the waters spread over the
Appalachian area, then withdrew generally, only to return again.
Among the economic resources of the Ordovician strata are oil,
natural gas, lead and zinc , marble, and the calcium phosphate. The
seas were rich in animal life. The most characteristic
invertebrates were minute graptolites, other numerous forms being
brachiopods, bryozoans, and trilobites. Some cystoids and crinoids
appeared; there were a few corals and many cephalopods. Especially
noteworthy was the appearance of a few Primitive, fishlike
vertebrates (jawless fishes) and tiny land plants resembling
liverworts.
STOP #9 N38 56.818 W77 22.090 This brings you to a display of
Cannel Coal. Cannel coal consists of micrinites, macerals of the
exinite group, and certain inorganic materials. Cannel coal usually
occurs at the top or bottom of other coals. The excess of hydrogen
in a coal, above the amount necessary to combine with its oxygen to
form water, is known as disposable hydrogen, and is a measure of
the fitness of the coal for use in gas-making. This excess is
greatest in what is known as cannel coal, the Lancashire kennel or
candle coal, so named from the bright light it gives out when
burning. This, although of very small value as fuel, commands a
specially high price for gas-making. Cannel is more compact and
duller than ordinary coal, and can be wrought in the lathe and
polished. Cannel coal was probably formed in lakes and pools where
floating spores, transported by wind and water, accumulated in mud
mixed with plant debris. During the 19th century cannel coal was
used in the manufacture of illuminating gas and as fireplace coal.
The example you are looking at was formed between 318 and 311
million years ago.
STOP #10. N38 56.775 W77 22.084 The end of your journey, you are
viewing an example of Coarse Diabase. Coarse Diabase is a
fine-grained intrusive igneous rock of a composition similar to
basalt, but is slightly more coarse-grained than basalt. The rock
consists of the minerals calc-plagioclase (bytownite to
labradorite) and pyroxene (commonly augite, pigeonite, and
hypersthene) with apatite, magnetite, and olivine commonly present.
Modal rock composition of mafic minerals is less than 90% of the
rock by volume. Diabase is associated with sill and dike intrusions
in the hypabyssal (upper few km of the Earth's crust). The term
diabase is the North American word for dolerite. This sample is
from the Lower Jurassic Period, approximately, between 140 and 150
million years ago. In order to claim this EarthCache as a find
please e-mail me the answers to all of the following questions,
including the name of the "hardness scale" used to identify a
specific stone/rock property. Photos are welcome, but if you post
one please do not include any portion of the informational
sign(s).
STOP #1. Why are there holes in the Barre Granite?
STOP #2. What city and state are listed on the informational
sign?
STOP #3. Fill in the blank "in thin metamorphosed bed of
__________ Marble".
STOP #4. What quarry did this exhibit rock come from?
STOP #5. What city and state are listed on the informational
sign?
STOP #6. In what basin was this conglomerate deposited?
STOP #7. Where was this Petrified Wood from/found?
STOP #8. What city and state are listed on the informational
sign?
STOP #9. Fill in the blanks, "High in ________ and ________
content."?
STOP #10. What is the name of the quarry where this sample was
removed?
FINALLY: The hardness of a stone is one of the properties that
contribute to identification. Hardness is also an attribute which
is important to be aware of, because it may determine what a stone
may be used for (jewelry, carving, faceting, handling, storage,
etc.) What is the name of the scale used to identify the hardness
of stones/rocks, AND which of the examples do you think ranks the
lowest on that scale?