Bedding planes in seemingly uniform sandstone

[SteveE]

Given a sample of sandstone that appears uniform to the naked eye, how can one determine where the bedding planes are?   Are there magnification/chemical or other tricks in our tool box?

[Kane]

Sometimes the fossils are simply scattered in a thickly bedded horizon (such as a micritic zone), and it becomes more a thing of luck to strike the rock in the hopes that it will split just above the fossil rather than through it. In particularly dense limestones and sandstones, that is fairly common. Think here of Moroccan Devonian trilobites where the rock will more often than not split through the trilobite necessitating gluing back together for preparation. 

 

At other times, the bedding planes are there, but not necessarily visible. In those cases, tapping along the edge might expose a tiny crack that can be exploited with more focused strikes, sometimes using a small chisel. 

 

And then there are times when the rock has been rounded by erosional forces and no clear indication of where the "side" of it is even inspecting the grain or other surface features that might lend a clue. In those cases, sometimes you just got to give it a whack and take your chances.  

[SteveE]

7 minutes ago, Kane said:

 even inspecting the grain or other surface features that might lend a clue....

 

Thanks!  I do plan to study this one with magnification.... Besides my hand lens I have access to microscopes of different styles.  Maybe I'll notice something, but really I have no idea what to look for.  Can you suggest some of the common clues you might see using optics, or alternatively a link to good reading material on the subject?

[Kane]

My method is fairly low-tech -- just my eyes while in the field. If there are fossiliferous features that seem to "bed" uniformly along the top or bottom, then I can obtain a better orientation (if they are jumbled everywhere, then all bets are off). In some rocks that were deposited near-shore, there may be wispy lines indicating a horizontal plane where tides were moving in and out, but that doesn't always mean the rock will break along those wispy lines if it is fairly compacted and dense, also pending the hardness of the fossil relative to the matrix or even volume (a higher volume of larger body fossils may result in structural weakness that can be exploited to split the rock). 

 

Most of the rocks in my immediate area are dense, thickly bedded limestones and dolostones, which make them ideal as armour stone for landscaping as they may almost never crack from exposure to the elements. In such cases, my method has been a kind of hail mary approach to discerning the "side" and bringing a hand sledge to strike around the middle of the side to see if a crack develops; where it does, I can either use the bevel-end of my sledge or hammer/chisel combination. 

 

I would think if the sandstone has relatively uniform grain size and compacted density that even a microscope wouldn't give a clue where a split can be made, but I'll defer to others who might have better insight on that score (possibly orientation of the grains themselves?). 

[FranzBernhard]

1 hour ago, SteveE said:

how can one determine where the bedding planes are?

As already pointed out by @Kane, this can be a tricky task!

 

Are you looking for planes of weakness or is it just scientifically interesting for you to find the bedding planes?

 

Concerning weakness, quarry workers were able to split nice regular blocks out of massive granite, exploiting the planes weakness that exist in massive granites. That was pure experience. It has nothing to do with bedding planes, of course, its just to show an example, how well hidden some important features of rocks are to the untrained eye.

Franz Bernhard

 

[SteveE]

10 minutes ago, FranzBernhard said:

Are you looking for planes of weakness or is it just scientifically interesting for you to find the bedding planes?

 

The latter.... pushing my nerd skills.   The immediate "play around and learn on" rock is the big boulder I posted the other day in the General Fossil Discussion forum, which I took for sigillaria and others have opined is more like a pseudofossil with some interesting geochemical type ridging.  

[FranzBernhard]

22 hours ago, SteveE said:

The latter.... pushing my nerd skills.

I would like to suggest a general sedimentary geology book. Just to get an overview of the possibilities.

I have a related "problem": Up and down in sedimentary rocks! Not always easy, well, mostly impossible, to recognize in specimens from talus etc. 

Franz Bernhard

[SteveE]

31 minutes ago, FranzBernhard said:

I have a related "problem": Up and down in sedimentary rocks! Not always easy, well, mostly impossible, to recognize in specimens from talus etc.

 

Ain't that the truth.... in my *very* jumbled and folded area (Blair County Pennsylvania, USA) I was sure one outcrop had been inverted opposite of the state's surface geology map, based on finding Tonoloway/Keyser limestone in the scree and talus at the wrong end of the site.   I decided that would be my challenge spot, to try to correlate horizons with others in the area, to try to prove it was inverted, and I was looking forward to calling it to area pros' attention.   But in the end, I finally decided the unexpected limestone was really construction fill from above.  Only took me 8 years to figure that out....

 

Edited June 27, 2021 by SteveE