New England was covered with rounded stones when the first settlers got here. In some places there was hardly enough room to put feet down between them. I have seen areas that were never cultivated because there were just too many of them. I have a mental video of the ice slowly moving forward like a colossal bulldozer, rolling the jagged stones over as it passed over them, then the ice retreating perhaps a quarter of a mile in summer, then advancing a half mile in winter, rolling the stones over again and again, until the sharp edges were gone. I sure would like to see a diagram of how the geologists picture that action.