Source
ISumerOP•

First picture is from beach 3 at Awenda Provincial Park, Ontario, Canada. I forget where I took the second one, but most likely at Killbear Provincial Park, Ontario, Canada. Not sure what's going on with the cool patterns on them.

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UnseenDegree•

The Killbear one is almost certainly because of erosion. Cool to see some differential erosion of the crosscut.

The ones from Awenda are likely not native to that areas, probably brought there from the shield rock. Can’t imagine what they’ve been though over the years lol

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Busterwasmycat•

felsic dikes crosscutting another rock type. only makes a ring in the first pic because of the way the rock has eroded (circular shape is what you get when you intersect a sphere with a plane). That gneissic-looking stuff down below in the first pic looks interesting.

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ISumerOP•

Thank you, that is so cool! So you're saying that the dike formed first, and then then erosion happened in a spherical form around the dike, resulting in the circle we see now?

Also, is there anything you have to say regarding the second picture? All of this seems so fascinating to me.

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Busterwasmycat•

pretty much what I said, but the erosion didn't happen around the dike in particular, it happened to the rock which happened to contain a dike. The plane of the dike doesn't separate the mass into two equal halves, which can happen but generally doesn't. Striped cobbles looking like giant pool balls (13 ball with a nice orange stripe), are not so common.

The only thing I can add about the second pic is that the foliation, the orientation of the minerals in the host rock (sort of left to right in the pic), is at a high angle to the orientation of the dike. Dike apparently came after the host rock got its flattening (or most of it) because the dike isn't deformed in the same way.

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