These formations are called trove or popular "living stones". They say it would be over 6 million years old and would have occurred due to local cementing of sand, possibly due to a seismic activities. Most trove of Costesti ellipsoidal in shape.
Locals insisted that after a long rain of small rises trove wet sand and stones actually grow larger.
This theory is confirmed.
It is a natural phenomenon that deserves to be seen. Here is the only museum in the world of trovantilor.