Odd Marking on rocks

[ddub1642]

These are two rocks I found in Sanilac County Michigan. The first is obviously a natural occurrence however I wondered if anyone knew what type of rocks would have these types of marks. The second two pics are of a sandstone type rock I found in a old field ditch. It is about 3 inches across and two inch top to bottom. It is a circle as you can see, however, I could not get it to show that there is actually a very faint round dot in the middle of the circle. Thought is was odd and picked it up. Any opinions would be appreciated. Thanks!

 

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[Archeodeb]

Could have been a rock placed under the bottom of a pipe that wore away a ring over time. As to why someone would place a rock under a pipe -- who knows? Could have been under a gate or part of a fence locking mechanism (some old farmers would use a pipe and wire on one end of a loose fence section as a kind of makeshift gate) but this is all just speculation. That looks like limestone -- which is soft and easily carved -- so pretty much anything round could have been rotated against it to dig in and leave an impression. Might have been for a reason, or might merely have been for something to do -- like doodling.