Saturday, December 7, 2013

Geo 365: Dec. 7, Day 341: Cracked Pebbles

Looking carefully at this block- the same one as yesterday- you can see a set of joints or very small faults dipping to the right at about 25 degrees. These fractures cut across the pebbles they intersect, which is the defining characteristic of a cracked pebble conglomerate. Now if they were really joints, I'd expect them to be caused by tensional stress, which I think is unlikely in this setting. Furthermore, it seems likely that the fractures would have followed the pebble edges rather than plowing through them. On the other hand, looking carefully at full resolution, I can't make out any offset. So while I suspect these were caused by shearing, it's not observable macroscopically. Below are a couple of crops (run through autolevel routine in to bring out details) highlighting said fractures.
Note also that the white mineral surrounding the clasts- likely calcite- is more abundant to the upper left and lower right of the pebbles. I think this phenomenon is called a "pressure shadow."

Photo unmodified. May 7, 2013. FlashEarth location.


flask said...

hey, hi.

i don't remember where i started reading, or when.

and i'm no geologist. by training and trade i'm a musician but i have been learning a little geology.

catch is that the more i learn, the more i wish i knew. can you recommend some sources?

something with lots of pictures for non-geologists? i keep finding rocks that show something interesting happened, but don't know how to interpret properly.

Lockwood said...

The best way to get into it is to learn what's around you, specifically, IMO, then when/if you want more rigor, pick up a cheap intro text at a used bookstore. So, as for recommendations, where are you? (Doesn't need to be too specific: state and quadrant is plenty, e.g. SE Ohio.) If I don't have anything, I'll ask for ideas from people in your vicinity.

flask said...

i'm in vermont.

i keep wading into the state geological maps, but understand enough to know i don't know enough.

Lockwood said...

According to @geoflier "Roadside Geology of VT and NH isn't bad, really." I don't know the area very well, nor which books are a good starting point, but I'm not seeing much of anything else in a quick skim at Amazon. Might be useful to ask a local librarian. Sorry I can't be of more help.