I’m pretty sure these are man-made in nature and probably has something to do with construction materials used about 100 years ago.
There are several intersting features of these rocks. Take a look at the big rock . It has a rough, sandy texture on the outside, but where it’s broken open there is a smooth, grey substance.
Emerging from the rock are “fins” The fin coming down the bottom of the picture is apparent, but there are other fins going off to the right and left. These fins look like a miniature version of the Great Wall of China.
There are two holes visible in the main rock. These holes are smooth inside, straight, and of a consistent diameter. This picture shows a piece that was broken off, and you can tell how it looks man-made.
Just a WAG, but looks like concrete that was cast into a hole in the ground, thus the sandy texture and irregular shape; it may have had metal rods or wood sticks into it when casted, that have rotten/rusted to nothing over the years, that would explain the holes.
There are a couple of things that argue against the concrete theory. All concrete or mortar mixes that I know of are mixed with some kind of aggregate - sand or gravel. This stuff has a perfectly smooth texture.
The other problem is that a concrete mixture wouldn’t explain the “fins” unless there were some mighty deep cracks in the ground.
