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Brushbuffalo
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PostSun Feb 25, 2018 2:01 pm 
Hi texasbb! Thanks for the post. I have a couple of possibilities for that interesting rock. The material itself is almost certainly andesite of the extinct "Goat Rocks" volcano. The splintered fracture pattern in not unusual in andesite. Joints are a major reason that rock in volcanoes in the Cascades is notoriously chossy. I think the prominent flattened face on the right of the block in the images that is roughly perpendicular to the two sets of closely-spaced joints ( fractures) forming the splinters is another joint formed after the other two joint sets. The rusty color is due to chemical weathering across the exposed joint surface, and such weathering also is evident on surfaces of several exposed splinters in the lower left of the first picture. Another possibility is that the flattened surface is a glacially planed surface and the block was ripped out of and rotated from its bedrock location. Although glacially smoothed and polished rock surfaces are very common in the Cascades, overriding movement by ice on this thoroughly fractured rock would most likely leave a rougher surface due to splintered pieces being ripped out one by one to a much greater extent. Personally I favor the first explanation (jointing). A visit to the site to look around would easily resolve which of these ( or other) explanations is correct. I have been there or very near there, but it was in about 1964, so pardon my fuzzy memory. confused.gif

Passing rocks and trees like they were standing still
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texasbb
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PostSun Feb 25, 2018 4:02 pm 
Brushbuffalo wrote:
Personally I favor the first explanation (jointing). A visit to the site to look around would easily resolve which of these ( or other) explanations is correct.
Well, that's two geologists against one know-nothing (well, two know-nothings if mike above accepts that label smile.gif). My friend also said a joint, including mention of the chemical weathering. If I understand the (backwards IMHO) nomenclature, a joint is a fracture, right? One resulting from tension--i.e., the rock was pulled apart? I don't see how that could have happened without shredding or separating all those loose mini-slabs. I can, however, imagine it getting sanded flat by glacial movement without getting torn apart because there would be so much pressure holding it together from all that ice on top of and around it. Guess I better stick to other subjects! biggrin.gif I don't know if seeing a map would be helpful to you, but the rock is right here (the blue polygon).

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Brushbuffalo
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PostSun Feb 25, 2018 6:32 pm 
Yes, a joint is a fracture, just a crack. Joints are similar to faults except the latter involves differential movement on each side of the fault plane relative to the other. Joints can be caused by tension but also by compression or shear. They frequently occur in groups with more or less parallel alignment in what are called joint sets due to a specific application and orientation of stress. As mentioned, it would be a quite simple matter to figure out the cause by examining the surrounding geologic and geomorphic setting. However, without seeing the setting, I favor jointing...although without question that area was extensively glaciated during the Pleistocene.

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mike
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PostMon Feb 26, 2018 11:19 am 
texasbb wrote:
(well, two know-nothings if mike above accepts that label smile.gif)
Guilty smile.gif I stick around hoping to learn something.

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texasbb
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PostMon Feb 26, 2018 7:58 pm 
Brushbuffalo wrote:
As mentioned, it would be a quite simple matter to figure out the cause by examining the surrounding geologic and geomorphic setting.
Here's the best pic I got of the surroundings:

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Brushbuffalo
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PostMon Feb 26, 2018 9:53 pm 
Thanks for the picture. Inconclusive, however. If I was there I would be walking and looking over a wide area surrounding the boulder. I would examine lots of other boulders and especially look for outcrops of bedrock. Otherwise it's similar to finding the odd ponderosa pine in a Douglas fir forest.

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NorthBen
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PostTue Feb 27, 2018 11:31 am 
Here's another one, taken along the Ptarmigan Traverse on the climb up to the Dana Glacier, a little south of White Rock Lakes. My boot is a men's 11 for scale.
Centered on the general locale:
View larger size in new window

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Brushbuffalo
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PostTue Feb 27, 2018 1:23 pm 
NorthBen wrote:
This consists of many mostly angular clasts of igneous and metamorphic rock. I am calling it an intrusive breccia. These rocks form as magma intrudes from depth and incorporates fragments of the surrounding rock that was being intruded into. Substantial areas of exposed granitic and metamorphic rock are found in the area where you found this interesting and attractive-looking rock, so without doubt those rocks are also found at depth nearer the magma source. As magma intrudes into these surrounding " country rocks" (pre-intrusive), pieces get physically ripped from the wall rock surrounding the magma channels. If the inclusions have higher melting temperatures than the intruding magma the inclusions remain solid (otherwise they might melt). Usually an intrusive breccia is fairly limited in volume, but they aren't rare in areas of intrusions, including the North Cascades. The only curious thing about this example is that the intrusive breccias I've seen in the North Cascades have a few angular fragments contained within a more uniform-appearing cooled crystallized igneous rock that did the intruding. This sample is very short on that igneous rock but is just loaded with fragments.. Once again, if I was there I would look around a lot. Do readers see this last sentence as a common comment by your friendly geologist in Name That Rock? 😁 Other breccias* with different modes of origin include sedimentary breccias, volcanic breccias, and even tectonic breccias ( the latter formed by the forces of faulting. Tectonic breccia is usually fine-grained). I don't believe this example is any of those types. *The word has its origins in the Italian language, in which it means either "loose gravel" or "stone made by cemented gravel. (Wikipedia definition)

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Ringangleclaw
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PostFri Mar 02, 2018 9:36 am 
Brushbuffalo wrote:
NorthBen wrote:
This consists of many mostly angular clasts of igneous and metamorphic rock. I am calling it an intrusive breccia. These rocks form as magma intrudes from depth and incorporates fragments of the surrounding rock that was being intruded into. Substantial areas of exposed granitic and metamorphic rock are found in the area where you found this interesting and attractive-looking rock, so without doubt those rocks are also found at depth nearer the magma source. As magma intrudes into these surrounding " country rocks" (pre-intrusive), pieces get physically ripped from the wall rock surrounding the magma channels. If the inclusions have higher melting temperatures than the intruding magma the inclusions remain solid (otherwise they might melt). Usually an intrusive breccia is fairly limited in volume, but they aren't rare in areas of intrusions, including the North Cascades. The only curious thing about this example is that the intrusive breccias I've seen in the North Cascades have a few angular fragments contained within a more uniform-appearing cooled crystallized igneous rock that did the intruding. This sample is very short on that igneous rock but is just loaded with fragments.. Once again, if I was there I would look around a lot.
It could also be a meta-clastic portion of the Cascade River schist. Looking at the map either hypothesis makes sence, although an arguement against mine is that Tabor in his PhD work speaks of igneaus and metamorphic cobbles shed from the Skagit River gneiss as composing a portion of the Cascade River Schist. He doesn't mention breccia or diamictite, but in my defence, neither would be incompatible with a mass wasting event from a crystal highlands into an adjacent basin which later was the protolith of the schist

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Brushbuffalo
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PostFri Mar 02, 2018 1:39 pm 
huh.gif Regarding NorthBen's image of a rock, here is a reply I got from my former classmate Dr. Ralph Haugerud, USGS geologist who has co-authored technical maps and papers and also a book on North Cascades geology with Rowland Tabor: Ralph wrote: "About that rock. It would be nice to have an approximate location. [Ralph evidently hadn't noted the location on NorthBen's map]. I see that (1) it is mostly not green (thus probably not Mesozoic), (2) contains granitic clasts, (3) has foliated clasts, (4) has clasts that for the most part are remarkably angular and unsorted, and (5) appears to have matrix that is as competent as clasts. My guess is a hornfelsed* caldera-wall collapse ** breccia, of which there are bits and pieces about in the North Cascades." * "hornfelsed" indicates a low grade of metamorphism. ** a caldera is a large volcanic crater frequently formed by a volcano collapsing above an intrusive body of magma. Ralph's interpretation and mine are similar. Intrusive breccia involves intruding magma breaking off fragments of wall rock in intrusive passageways, caldera-wall collapse breccia forms when substantial amounts of wall rock collapse into a magma chamber. His idea makes more sense to me. Similar rock is found associated with the Hannegan caldera near Hannegan Pass. Geologic interpretation can be little more than an educated guessing game sometimes, particularly ( to me) when the activity occurred long ago and far below! eek.gif I've contacted Rowland for his hundred dollars worth of input ( to say for "his 2 cents worth" would be a sham) and hope to hear back shortly (he might be traveling or something).

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Ringangleclaw
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PostFri Mar 02, 2018 1:56 pm 
Sounds good.

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Brushbuffalo
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PostSat Mar 03, 2018 9:38 am 
Regarding that fascinating image submitted by NorthBen, here is a response I received from North Cascades geology guru himself ( and my earliest mentor), Rowland Tabor. Rowland wrote: "Hi Doug: I think Ralph's explanation is as good as any, except that similar rocks seem to crop out in isolated patches, suggesting intrusion. But maybe Ralph's collapsed caldera wall gets mobilized and shoots off in a separate vent. I am at home now, but will take a look at my thesis next week when I am in the office. I'll see what I said back then. I have attached a photo of the Triplets breccia, which I recall also had quartz-lined cavities between clasts. But what do I remember from 60 years ago?" [ edit: I will try to attach Rowland's photo when at my main computer] Rowland also says this, after I mentioned that I am giving a slide show soon using real 35 mm slides about our summer of geologizing in the North Cascades in 1967: "You should scan your slides. Eventually bulbs for slide projectors will be hard or impossible to find. And your story of the student and little paper squares reminds me of a grade-school science teacher who told me that when she took a classic typewriter to school to show her students, one exclaimed, "Look, instant printing!!" My student had asked me how I got the images from my computer onto those little paper squares. He had never ever seen 35 mm slides!

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coldrain108
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PostThu Mar 29, 2018 1:38 pm 
Found in the moraine debris in upper Royal Basin. Looks like Grecian Urn style art work.

Since I have no expectations of forgiveness, I don't do it in the first place. That loop hole needs to be closed to everyone.
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Brushbuffalo
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PostThu Mar 29, 2018 1:45 pm 
Coldrain108, thanks for sending the picture of this beautiful rock. Can't recall when ( if) I've seen one quite like this. My first impression is that it is a shattered basalt ( or perhaps a low-grade metamorphic rock) with a light mineral, probably quartz, that incorporates the dark rock fragments. I'll investigate and give more info.

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coldrain108
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PostThu Mar 29, 2018 2:38 pm 
Here it is in a larger context:
And a nearby neighbor:

Since I have no expectations of forgiveness, I don't do it in the first place. That loop hole needs to be closed to everyone.
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