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What is This?


STNolan
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While recently out scouting a possible new EC location I came across this interesting formation and some initial googling didn't turn anything up, so I'm turning to you all to see if any of you know anything about this?

The exposed outcropping in the photo is in an area heavily influenced by glacial erosion. The average length of the darker sections of rock is about 12-15 cm long and the average width is about 1-2 cm wide. Each nodule sticks out from the surrounding rock by approximately 3-5 cm. There is a certain lustrous quality to the exposed nodules. They aren't shiny per-se but they aren't quite matte black either. 

 

Any thoughts? They vaguely remind me of "puddingstone" but I can't find a good match.

 

Thanks for your feedback!

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1.  Are the nodules generally aligned in the direction of the glacial flow?

 

2.  Are the nodules a harder material than the matrix?

 

It could be unequal glacial polish of the two rock types.  Any broken exposures nearby for comparison of bulk vs surface?

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4 hours ago, egroeg said:

1.  Are the nodules generally aligned in the direction of the glacial flow?

 

2.  Are the nodules a harder material than the matrix?

 

It could be unequal glacial polish of the two rock types.  Any broken exposures nearby for comparison of bulk vs surface?

 

1) yes the nodules are almost perfectly aligned with the flow of the glacier.

 

2) The nodules do seem to be harder than the surrounding rock based on a Mohs hardness “scratch” test. 

 

I have another rock surface nearby (about 100’ away)

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I would generally refer to these as inclusions.  The follow up would be how they got there.  I can't readily tell from the photos whether this is more of an igneous, granitic rock, or sedimentary.

 

If it's granitic, those could be inclusions, which can be more resistant to weathering than the surrounding rock.  Or they could be xenoliths.

 

If it's sedimentary, then this could be conglomerate, like a puddingstone, or just examples of extraneous pebbles that were included in the sediment when it compressed into rock.

 

My two, non-geologist cents.

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I'm thinking along the same lines as hzoi.  

The nodules might just be a harder material that weathered more slowly than the matrix.

There's one large nodule in the bottom right that looks like it is ready to pop out of the matrix.  Are there any holes where this might have happened?

Is the surface shown horizontal or vertical?  It seems odd that the nodules would line up like that, unless this is a vertical surface and we are seeing bedding.

 

 

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1 hour ago, egroeg said:

I'm thinking along the same lines as hzoi.  

The nodules might just be a harder material that weathered more slowly than the matrix.

There's one large nodule in the bottom right that looks like it is ready to pop out of the matrix.  Are there any holes where this might have happened?

Is the surface shown horizontal or vertical?  It seems odd that the nodules would line up like that, unless this is a vertical surface and we are seeing bedding.

 

 

 

The surface is at approximately a 45 degree angle. No visible holes that I could determine. 

I'm thinking I may go back for a better look and see if I can determine a rough level of hardness on the Moh's scale based on the 'scratch' test. At the very least it's peaked my geological curiosities, even if it doesn't eventually become an EC site.

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