Why does this rock look like a girls?, well see the pic...?
http://img71.imageshack.us/img71/9323/im...
http://img71.imageshack.us/img71/9323/im...
I believe this is mostly granite and pushed up by glaciers. I live in Northeast US. Towards the left side of the picture, there is a really rectangular "cut out" but there is no evidence of cutting, dynamite, or other. It seems natural, so I wonder what could have caused it? My yard is full of rocks like this and some are granite but some flake apart or have ridges where they seem weak or would flake if smaller. The "cutout" I speak of is 2 feet wide and 1 1/2 foot deep. There are many large such rocks in my yard that have a lot of quartz and mica mixed in. I'm wondering also if a weaker stone formed there and washed away, how it formed so square? I'm just guessing and would love to hear other theories.
PS. Said rock is about 4 ft high at the highest and about 12ft lonn
Answer:
I would bet, if anything natural, it is created by flowing water. Glaciers often end up having left a lot of debris behind at the scene., and the square crack is very smooth. This most likely indicates flowing water rather then glaciers unevenly pushing up and against,
It looks like a schistose metamorphic outcrop.
Tectonic rebound might have a little to do with its formation (bouncing back after a glacier) though it is more likely that a glacier removed a major chunk of the formation it was a part of. It is likely your outcrop was on the side of a syncline (dome shape and somewhere ... perhaps a 100 feet, perhaps a 100 miles ... it will match up with another outcrop. A glacier ripped out the rock in between the two.
It was originally laid down as sedimentary rock (the layers are bedding layers), buried, heated and compressed until it formed a crytalline structure, then folded like waves on the ocean ... which may have been a plastic or brittle process depending on the depth at which it occurred. If the deformation was under less pressure and temperature it would more likely undergo brittle fracture which would have created weeknesses in the rock (tiny cracks) that would be more susceptible to erosion. That might account for the square chunk that is missing ... erosion due to water freezing in a crack in the rock. It is more likely that glaciers did the majority of the erosion work but not "pushed up" the formation or caused the square chunk to fall out.
The uneven erosion of the outcrop (right side of pic) is likely due to chemical as well as physical erosion of the outcrop ... try a little bit of vinegar on it see if it bubbles up, that would be a sign of calcium carbonate in those layers (seashells). A few million years ago your backyard was probably a beach.
Of course all this is pure speculation based on a photo of an outcrop Ive never seen in a geological area I may not ever have visited, so take it with a grain of salt. It's just an educated guess.
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