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?