I had not seen the top of the rock because the grass was high and I was in a “no danger” zone. So what happened?
Of course nature does its own thing and it all makes sense. We know that in the summertime the streets of Manhattan seem extremely hot. And they are, because the stone buildings bank the heat during the day and keep it much longer than- for example, Central Park. The rock on- or near the surface of my lawn is a better conductor than the soil around it. It conducts heat away from the cooler soil beneath it, saturating the soil under the rock. In the winter time that saturated soil under the rock freezes before other dirt at the same depth. We all know that when water freezes it expands. This why frozen water pipes burst in the Winter. The soil under the rock freezes, it expands and pushes the rock up a little.
When spring is here and everything thaws out, the space under the rock that froze will be filled with sediment and my rock is up a little higher. This process will be repeated every year and over many years the magic happens. The rock sees daylight and the blades of my mower.
The same thing happens in my vegetable garden. My “stone harvest” every spring is amazing and never ending.
Soils that have high clay content will hold more water and therefore expand (heave) more. Therefore, when constructing a basement, attention to water drainage around the foundation is very important. Heaving soil can exert enough force to collapse a concrete block wall! This is also the reason that your local building codes mandate how deep the foundation footings for your house must be. Heaving soils can lift an entire house unevenly, cracking the foundations and endangering the whole structure, if they aren’t below the local frost line. For us, the frost line is 3- 4 feet deep.
I have come to the conclusion that my lawn does not have any “no danger zones” anymore. We have too many rocks that are striving to see daylight!