And expert opinion from a geologist is that those rocks are more likely known as leaverite. Leaverite there or you have a hole in your yard.
I had this bump of a rock that was a wee bit too high to mow over and too pesky to mow around; so I decided to get it out of the ground.
After all; how hard could it be, as it was only the size of an LP record or so.
Yup; it was leaverite!
I now have a boulder about 2 feet in diameter that I have to mow around!
(We have LOTS of illegal alien rocks in the soil here in central Indiana. They came in a mass migration from Canada about 10-12,000 years ago. We grow a GREAT crop every time we plow!)