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Brushbuffalo
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PostTue Jan 19, 2021 11:42 am 
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As a long time geology professor (43  years but retired for the past 6 1/2), I believe I can speak with some authority  in highly recommending  Nick Zentner as an informative, accurate, and entertaining teacher.

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reststep
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PostFri Mar 19, 2021 9:25 am 
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I was hiking up Mt. Walker last week and noticed this large boulder just off the trail. It appears to me to be granite. Must be a glacial erratic.

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Brushbuffalo
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PostFri Mar 19, 2021 12:21 pm 
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reststep wrote:
Must be a glacial erratic.


Excellent. Although lithology isn't clear from the photo, it is probable that this is indeed a granitic boulder. The northern portion of the Olympic Peninsula is littered with clasts ranging  from sand and pebbles to cobbles and boulders that were transported  by continental glaciers from bedrock in British Columbia during the Pleistocene. Granite is abundant on mainland B.C. but absent as bedrock on the OP.
The erratic you found shows a couple of good facets , in erratics being the result of the rock being dragged along the base of the slowly moving ice and becoming flat, much as  you get by rubbing a potato across a grater.

Ice was about 4500 feet thick in the Mt. Walker area based on upper elevation of  erratics (4000' plus depth of Strait of Juan de Fuca (500'].

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klar
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PostFri Mar 26, 2021 2:13 pm 
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Looking at photos from last summer and remembered this thread.

Interesting rock found below Circle peak in the North Cascades.

Any insights into the geology?

vein
vein

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coldrain108
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PostFri Mar 26, 2021 2:42 pm 
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about 5500' in the Olympics.  Interesting scrape marks, the shadowing really brings it out.

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mike
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PostFri Mar 26, 2021 5:57 pm 
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I have those in my front yard smile.gif
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Brushbuffalo
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PostFri Mar 26, 2021 7:18 pm 
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klar wrote:
vein
vein

The geology of the area surrounding and including Circle Peak  is designated as the Nason Ridge Migmatitic Gneiss.Your picture appears to be an example of one of interlayered mica schist found in the Nason Ridge formation. Note the little 'ridges' and 'valleys' in your sample, which are the result of differential weathering between mica (valleys) and quartz and feldspar ( ridges).
The prominent white band is a vein that cross-cuts the host rock as a 3 dimensional  plane, not a  line as it appears in two dimensions if we look only at the front face. White veins like this are usually composed of quartz but in sedimentary rocks are often made of calcite. Since this vein cuts across the foliation of the mica schist, the vein is younger than the host rock.....possibly by a lot!

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Brushbuffalo
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PostFri Mar 26, 2021 7:26 pm 
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coldrain108 wrote:

Although I can't identify the rock ( other than guessing it is a  fine-grained grained sedimentary or low-grade metamorphic rock), the scratches are unmistakable  glacial striations.

Can you give us a  more precise location about where you took the picture?

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klar
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PostSat Mar 27, 2021 9:26 am 
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Cheers for the explanation! Did not realize how complex the geology was in that region.

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Anne Elk
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PostSat Mar 27, 2021 1:10 pm 
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Brushbuffalo - You and this thread are a NWH treasure.   wub.gif
Geology noobs such as myself looked at that Circle Peak sample that klar posted, and thought the "ridges and valleys" were sedimentary layers.  That was one of the more interesting samples offered up in quite a while.

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Brushbuffalo
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PostSat Mar 27, 2021 4:04 pm 
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Thanks for the enthusiasm, Anne Elk.
Concerning klar's Circle Peak rock, note carefully the portion of the vein in the 'top' of the sample. We see a slight ridge where the vein has withstood weathering better than the host rock. That is true in the ' front' too but the relief doesn't stand out when we look straight on. Different materials change differently in the same environmental conditions (i.e. differential weathering)

A mini-lecture could be made just from these two examples of differential weathering in the same rock.

Oh wait, I suppose you just got it!😊

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NorthBen
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PostFri Apr 09, 2021 6:29 pm 
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These interesting formations were seen on an attempt of Moapa Peak, north of Las Vegas, in what looks like limestone. Some kind of concretion? Or?

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Mike Collins
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PostFri Apr 09, 2021 6:41 pm 
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NorthBen wrote:

They look like chert nodules embedded in the limestone.  Here is more to read about their formation. https://pubs.geoscienceworld.org/gsa/geology/article-abstract/7/6/274/188865/A-model-for-the-origin-of-chert-in-limestone
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Brushbuffalo
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PostFri Apr 09, 2021 7:08 pm 
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Good find, Mike. I concur. The only word I would modify is 'embedded', which possibly  implies deposition of chert clasts as the limestone forms. Instead, the chert forms in situ ( 'in place') in limestone.  Small but important difference.

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Mike Collins
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PostFri Apr 09, 2021 7:14 pm 
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Would saying the chert is an "inclusion" be appropriate? I have seen red jasper occurring in a similar presentation in limestone. When I saw the red jasper I was canoeing the Green River in Utah. I knew immediately than someone would have made arrowheads nearby. So I went to a shady locale under a rock overhand and sure enough there was a pile of chips about 1/2 meter tall the result of knapping likely thousands of arrowheads.
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