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Minerals
Social climber
The Deli
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Dec 15, 2008 - 09:38pm PT
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Glad to see that this thread is still going and that some of you took a stab at the rock types in the outcrop photo. Cool.
I wasn’t looking for anything fancy, just the basic rock types. And yeah, maybe it’s not so easy if you don’t have the actual rock in front of you!
Here’s what I was thinking:
Oldest
 Metamorphic rock (phyllite/schist), the dark-gray rock, lower center, and the greenish-colored rock in the upper right.
Intrusive igneous rock (granodiorite), medium-gray/tan-colored, left side of outcrop.
Intrusive igneous rock (aplite/pegmatite dike), light-colored band of rock that cuts the older rock types.
Sedimentary rock (tufa – leave it to the hydro guys…), tan-colored surface choss on the far upper left that is sharper than schist!
Youngest
Back to basic mineralogy…
Here’s a photo of the surface of a boulder of granodiorite from the good ol’ Nevada desert. This rock is similar in age to the granitic rocks of the Sierra Nevada Batholith (mostly Cretaceous) and has good examples of some of the common accessory minerals found in granitic rock.
H = hornblende
B = biotie (mica)
T = titanite
Hornblende is easily recognized by its rectangular shape and black color. The largest hornblende crystal in this photo (lower section of photo) contains smaller crystals of feldspar. A mineral that contains smaller grains of another mineral is said to have a poikilitic texture. The smaller mineral grains within are referred to as poikilitic inclusions. Biotite is also black but is much softer than hornblende (can be scratched with a knife) and occurs in stacks or clots of platy hexagonal sheets. Biotite often weathers from black to a dark-greenish-color. The titanite grains in this rock are yellowish-brown, however the titanite grains seen in Yosemite are more brownish in color, sometimes resembling a “root beer” brown color.
The small white grains are plagioclase, the small tan grains with a tinge of pink are potassium feldspar, and the small light-gray grains are quartz.
Tami, I haven’t forgotten about your question – just need to finish typing…
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Darwin
Trad climber
Seattle, WA
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Dec 16, 2008 - 01:26am PT
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With little or no basis whatsoever, I've always thought of columnar basalt
as granite that cooled much more quickly. I'm sure that's wrong, but:
can someone give examples of distinct rock types that have similar/same composition,
but differ in cooling rates at the time of formation?
Darwin
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Minerals
Social climber
The Deli
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Dec 16, 2008 - 07:53pm PT
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Good stuff, Wes.
Increase in H20 content decreases magma viscosity.
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Minerals
Social climber
The Deli
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Dec 16, 2008 - 07:54pm PT
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Well, I hope this all makes sense. I may not have described some things in the best way possible but it’s an attempt. Please correct me if you notice any mistakes/errors.
Tami wrote:
“Have you seen The Stawmus Chief ? It's got an almighty basalt dyke ( the Black Dyke ) splitting the entire cliff. Better still, this basalt dyke on the face of the Chief is only a visible piece of a massive basalt dyke - you can see where it comes out of the sea , crosses through the Malemute cliff, crosses the highway, cuts through the main face of the Chief , over to the backside and so on ( I guess ! ) . It's an astonishing geologic feature.
How does it get there as a "dyke" of rock in the larger granite whole ?”
Tami, I’ve never been up there but that Black Dike sounds really neat! A pretty major feature, eh? Based on its extent, maybe it is some sort of feeder dike (system?) that fueled volcanic activity far above the granitic rock that it cut(s) through. I dunno. Are there other basaltic dikes nearby or just the one?
Hey BASE104, lots of good info in your post! Thanks for explaining how dikes form, and yeah, the “Wild Dikes” on the East Ledges descent are wicked cool. I need to read that section of the McPhee book on zircons and the craton, but it’s wrapped up and stored under lock and key. Thanks again for that!
Hmmmm… Well, I’ll give this a shot…
As far as water’s involvement in the formation of granite, Dingus is on the right track. Water (H2O) content affects the melting temperature of the upper mantle and the sediments riding on a subducting slab are saturated with seawater. As a slab dives downward, water is added to upper mantle material and magma is generated. The magma is very mafic in composition and is representative of its primitive mantle source. Compositionally, the magma is far from granitic.
As stated earlier, mafic minerals generally have a higher melting temperature than felsic minerals and thus, mafic magmas are generally hotter than felsic magmas. As mafic magma rises into the crust, it may partially or completely melt and digest crustal rock (assimilation), which changes its overall composition, making the magma more felsic. If the magma reaches the surface after little interaction with crustal material, then basaltic (mafic) volcanism results. If the magma (or a partial melt byproduct of the magma) reaches the surface after significant interaction with, and assimilation of crustal material, then rhyolitic (felsic) volcanism results. If magma does not make it to the surface, it cools and solidifies to form a pluton. Separate magmas of differing composition may mix to create hybrid magmas of intermediate composition. Felsic magma may also be produced by the partial melting of older plutons (mafic to felsic) during the emplacement of younger mafic magma.
Terminology side note:
Melt = molten rock in liquid form.
Magma = molten rock in liquid form that may or may not contain solid crystals in suspension.
Below is a series of simplified conceptual diagrams showing the development of a magmatic arc, from a passive continental margin to a convergent margin, and subduction of oceanic lithosphere beneath continental lithosphere. The dip angle of a subducting slab may vary greatly, from a shallow angle to a steeper angle as shown in the diagram. The modern-day Atlantic coast is an example of a passive margin and the modern-day Cascades in the Pacific Northwest and Andes in South America are examples of oceanic-continental plate convergence. The upper mantle is called the aesthenosphere and is shown in orange on the diagram. Magma is shown in red and the third stage diagram shows a cooled pluton in gray, beneath an inactive volcano.
(Image pirated from the net…)
Here are a few photo examples of dikes:
While not exactly the clean granitic rock of Squamish, this photo shows two basaltic (mafic) dikes cutting through Cretaceous granodiorite. These dikes are significantly younger than the granodiorite (probably Tertiary in age) and formed after the granodiorite cooled and crystallized completely. As BASE104 mentioned above, magma forces its way along fractures in solidified and (in this case) much cooler host rock, separating the two sides of the fracture, forming a roughly planar sheet of magma. This magma may cool and solidify in place or may be transported through the fracture opening to another location, either within the crust (intrusive) or at the surface (extrusive). The pre-existing fractures in the host granodiorite may result from internal cooling of the pluton (shrinkage) or from external localized or regional stress (tectonics). Fractures may develop in mostly-crystalline magma with melt still present; i.e. magma does not have to be completely solidified in order to fracture. (I forget the minimum percentage range of melt to crystals in which magma becomes rigid enough to deform in a brittle manner (shearing in a skeletal framework.))
This is an example of how rock types of differing composition weather at different rates. The rounded boulders in the foreground are pieces of granodiorite and the line of boulders of lighter-colored rock that looks like an old rock wall is what’s left of an aplite dike. Aplite is a fine-grained felsic rock that is composed mostly of quartz, potassium feldspar, and some plagioclase feldspar, which makes it more resistant to weathering than the host granodiorite; hence the “rock wall.”
Here’s a boulder of granodiorite that includes a section of an aplite dike, showing the 3-dimensional nature of a dike. The dike can be seen on the backside of the boulder as well, giving the appearance of a layer of white sandwiched between two chunks of darker rock. (The black spots in the granodiorite are hornblende and biotite.) When viewed along the plane of a dike, it appears as a straight line or linear band. When viewed from a sub-perpendicular to perpendicular orientation, a dike may appear as an irregular shape, such as the patch of aplite on the top left of the boulder. The slabs at the base of Upper Yosemite Falls are great for viewing aplite/pegmatite dikes in three dimensions.
Pegmatite is generally similar in composition to aplite but is very coarse-grained. Pegmatite may contain more accessory minerals than aplite. This photo of the planar surface of a pegmatite dike shows (in this case) the difference in color between potassium feldspar (pink) and plagioclase feldspar (white). The very light-gray mineral is quartz and the thin black lines are sheets of biotite (mica). I’m not totally sure, but think that the yellow spots that are mainly seen between the edge of the pegmatite and the granodiorite are due to iron staining, mostly on/in quartz.
In addition to quartz and feldspar, the aplite/pegmatite in this example contains muscovite (mica – the fine- to medium-grained light-gray lines), garnet (the obvious red mineral), and a band of fine-grained magnetite (black) that stains the rock a rusty-yellow color as it weathers. The thing at the bottom of the photo with the shadow is a dead piece of cheat grass.
This is a section of the pegmatite dike shown in the “4-rock-outcrop” photo in my earlier post. The pegmatite is composed mostly of potassium feldspar and quartz. The small red dots are garnet and the jet-black mineral is tourmaline; there might also be a little muscovite in there as well. My finger points to a “graphic” texture of a quartz-tourmaline intergrowth (kinda neat…).
Aplite and pegmatite may also occur in the form of pods and irregular masses of varying size, in addition to dikes, as seen in some sections of the SE face of El Cap (“The Brain” and “The Cauliflower” etc.). Aplitic magma is the remaining fraction of a granitic magma that is left over after all/most of the mafic minerals have crystallized. Aplitic magma crystallizes last (the youngest unit) in the crystallization sequence of a pluton and aplite/pegmatite dikes can often be seen cutting all other rock types in an outcrop.
Here’s a texture that forms in dikes that is not terribly common. This texture is referred to as comb layering, because of the alignment of prismatic minerals (in this case mainly hornblende) that grow perpendicular to the walls of the dike. I have only seen this texture in outcrop in two locations – west of Lake Tahoe (pictured) and in Tuolumne.
Photo by Greg Stock, YNP Geologist
This is the spot on the East Ledges descent of El Cap that is labeled “Wild Dikes” in the Reid guide, the spot that BASE104 asked about. Note vegetation for scale.
Photo by Greg Stock, YNP Geologist
A closer view of this amazing feature shows the intricate geometric relations between highly fractured diorite/gabbro (dark-gray) and aplite/pegmatite (white). My guess (with emphasis on the word guess!) is that this batch of hot mafic magma was quickly emplaced into the surrounding El Cap or Taft granite, which was much, much cooler but still may have had melt present. The mafic intrusion may have been large enough in volume to contain the thermal energy necessary to partially melt the surrounding granite, but small enough in volume such that it quickly lost its thermal energy and was rapidly “quenched” within the granite. This rapid cooling may have caused the batch of mafic magma to quickly solidify and basically shatter. As an extreme example, think of it this way; if you slowly heat up a beer bottle in a campfire until it is red-hot and then toss it into a bucket of cold water, what happens? The partial melt from the granite (very felsic) was remobilized and injected into all of the weaknesses (fractures) in the diorite/gabbro, creating the angular mosaic of separated puzzle pieces that we see today. It appears that there were at least two stages of felsic diking, given the fact that some of the dikes cut other dikes.
This example of diking is similar to the relations and patterns seen in the Wild Dikes photos. This is (was) a boulder at the base of Lower Cathedral Rock, in the Mecca area, that has since been decimated by more-recent rockfall. The coarse-grained diorite/gabbro was shattered and intruded by felsic magma, creating a “breccia-like” texture. The rust-colored stains result from the weathering of pyrite (FeS2) to hematite (Fe2O3).
What happened here? This aplite dike has been offset by post-magmatic(?) left-lateral displacement along brittle joints in the granitic host rock. (left-lateral = sinistral; right-lateral = dextral.) One thing to remember when looking at a structural feature in two dimensions is that the structure may also be displacement in the 3rd-dimension (upward or downward in this case). Displacement that is seen in two dimensions is referred to as apparent offset. (Note blue pen for scale.)
More deformed dikes… This granitic dike was emplaced into metamorphic rock (phyllite/schist) while the metamorphic rock was undergoing tectonic-related ductile (plastic) deformation. Sections of the dike have been squished apart, parallel to the foliation (planar alignment of minerals and structural weakness) in the metamorphic rock, and now appear as discontinuous ellipsoidal blobs of granitic material. This “pinch and swell” structure is referred to as boudinage and the individual blobs are called boudins. Boudinage forms due to the difference in competence between separate layers of rock when deformed under stress.
Uggg… That’s enough for now. I have other photos to post - maybe more at some point… Magma mixing?
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Dr. Rock
Ice climber
http://tinyurl.com/4oa5br
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Dec 16, 2008 - 09:58pm PT
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Awesome!
Howabout some Au intrusion models?
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Minerals
Social climber
The Deli
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Dec 17, 2008 - 04:59pm PT
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Thanks, Dr.
Don’t know anything about gold. Never studied ore deposits. You?
The gold polish in Tuolumne is pretty nice...
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UncleDoug
Social climber
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Dec 17, 2008 - 07:49pm PT
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Minerals,
I'm an avid crystal hunter and in regards to pegmatites I have a couple of questions.
It seems that most pegmatites in granite originate as an intrusive element, except for the case of "gas-pocket" pegmatites.
How common are GPP's and is there any rhyme or reason to their formation?
And if you know much about GPP's could you elaborate on them a bit?
I've pulled some stupidly-sized crystals from GPP's in N. Nevada and want to learn more abut what I've found.
Thanks much!
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mongrel
Trad climber
Truckee, CA
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Dec 17, 2008 - 10:40pm PT
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Minerals, I expect I speak for a legion of tacoists in thanking you for fantastic, informative posts on this thread. One of the huge joys in rock climbing is interacting with geology closely, which is enhanced greatly by learning about it. Thanks!
As for your pothole, one thought to consider is a spring, as superficially unlikely as that may seem. This fits with the Fe-staining; O2-depleted groundwater conducting more-soluble ferrous then it comes out of solution when oxidized to ferric. These little springs come and go a good deal in the Great Basin, I've seen ones that winked out due to no external cause other than maybe a slight near-surface fault shift, within the past 10 or 20 yrs. Another one in S. Tahoe area that has recently substantially increased in flow, drowning a whole big patch of pine forest. Again, virtually 100 percent certainty it's seismic. Admittedly, this doesn't seem like enough flow to account for the amount of erosion of the boulder seen in your photo, but it's an idea.
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Dr. Rock
Ice climber
http://tinyurl.com/4oa5br
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Dec 17, 2008 - 11:48pm PT
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re:AU
all i got is a cooling and cracking of rock, then water hitting molten rock, creating a steam engine to drive the heavy gold into the fissures.
quartz is the host rock.
happened during geo period of Ca. that had the rivers running N to S, like the Sac.
Then sierra uplifting forced the rivers east to west.
where the east to west rivers bisect the north to south tertiary rivers/deposits, you have a burned out gold town.
or is it primary deposits?
primary in the N to S rivers, Tertiary in the E to W river biescts.
grey/blue gravel is a good sign.
Hwy 80 runs thru a red rock open pit gold mine near Cisco.
Flat rocks in the river means good gold hunting.
low pressure inside bends.
freeze your ass for nothing, don't go there.
unless hit hits 3000 a Oz that is...
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Double D
climber
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Dec 18, 2008 - 06:07pm PT
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It's important to understand dikes. There are regular dikes...
and then there are bull-dikes
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Double D
climber
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Dec 18, 2008 - 06:10pm PT
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Sandstone dikes?
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Minerals
Social climber
The Deli
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Dec 18, 2008 - 10:49pm PT
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This thread is now officially climbing-related, complete with historical significance. Nice.
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Minerals
Social climber
The Deli
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Dec 18, 2008 - 10:58pm PT
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There is actually more info on the web than I thought… it just takes time to find.
Here’s a neat bit about intrusion-related gold deposits in the Yukon:
http://www.loganresources.ca/i/pdf/2006-6-tombstone.pdf
And a much more comprehensive discussion:
http://gsc.nrcan.gc.ca/mindep/synth_dep/gold/rirgs/index_e.php
And another:
http://www.ga.gov.au/image_cache/GA7241.pdf
UncleDoug, I’m guessing that the gas pockets that you refer to are miarolitic cavities. Here are a few links to articles on miarolitic cavities, which explain formation in more depth than I can. Miarolitic cavities are somewhat common in the youngest unit in the Tuolumne Intrusive Suite, the Johnson granite. Some of the landscape boulders around the Tuolumne Store parking lot contain small miarolitic cavities that are lined with smoky quartz. The smoky color in quartz comes from crystal damage due to the radioactive decay of traces of Uranium contained within the crystal.
What kinds of minerals have you been finding in your hunts?
Definition of miarolitic cavity:
http://en.wikipedia.org/wiki/Miarolitic_cavities
Short and understandable article:
http://www.geocities.com/oklahomamgs/London/Pegmatite2.html
A more comprehensive article:
http://www.minsocam.org/ammin/AM71/AM71_396.pdf
Pegmatite mining San Diego County abstract:
http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_79683.htm
Hey, thanks Mongrel. This has been a really good exercise for me – a refresher, a chance to learn more, and something to keep me busy out in the desert (without wasting ammo). If it helps climbers to learn more about what they are climbing on, then that’s cool. Geologists would be so psyched to be able to see what we climbers see.
Back to the pothole… I thought about a spring but figured that it wouldn’t produce enough flow over time to scour out such a void, as you say. There’s probably a typical normal fault running right along the base of the hill, though. Braintwister…
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Minerals
Social climber
The Deli
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Dec 18, 2008 - 11:15pm PT
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Holy ptygmatic folding! Meta-volcanic, Wes? Way cool.
Comb layering in your first image? Where? That one’s got a bit of a twist too…
Let’s see more!
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MisterE
Trad climber
Raising Arizona
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Dec 18, 2008 - 11:17pm PT
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Such a wealth of great information - bookmarked for further digestion.
Thanks, Erik
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Minerals
Social climber
The Deli
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Dec 18, 2008 - 11:32pm PT
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That sounds like fun. Where?
Toasters?
Old windows?
That reminds me... Lucho and I each have a cooking stone stashed at one of the bivies... Can't beat cooking on a slab of granite over an open fire.
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MisterE
Trad climber
Raising Arizona
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Dec 19, 2008 - 02:12am PT
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The high-country stone-cooked wrap is a taste of near-heaven, I can tell you.
Miss you guys,
Erik
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Jaybro
Social climber
wuz real!
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Dec 19, 2008 - 02:34am PT
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Man, you hard rocks guys are in a world of your own.
Am I the only Paleontologist (Quasi-emeritus) climber around?
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UncleDoug
Social climber
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Dec 19, 2008 - 10:32am PT
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Minerals,
Thanks for the info!
I've been mainly going after smoky quartz.
There is an article in Rocks & Minerals, Sept/Oct 1993 regarding "Nevada Smoky Quartz" that caught my eye and I've been hooked ever since.
The largest pieces I've managed to cary/drag back home are 120 & 147 lbs. respectively.
The 120 is fully formed and gradates from jet black smoky at the tip end to crystal clear near the base.
The 147 is oddly shaped with only 3 truly faceted sides. It sits on the ground like an anvil. I have found several miniature( about 1 oz.) versions of this expression in the same pit.
These are from gas-pocket pegs.
Got some crazy smoky-amethyst-citrine-rootbeer turkey heads and sceptres from the Petersen Mtn./Haleluja Jnctn. area. Nothing obscenely large but very beautiful.
Don't have any photos to post but will work on it.
Thanks again for your knowledge release!
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scuffy b
climber
heading slowly NNW
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Apr 13, 2012 - 02:58pm PT
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overburden bump
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SuperTopo on the Web
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Let us know!