This is the front step of the Josiah Dennis Manse. It is a millstone or a stone that was used for grinding grain in a windmill or water wheel. Many old Cape Cod style houses have millstones for front steps. This looks to me like it was a bedstone or a bottom stone. The bedstone was the bottom stone mounted in a fixed position while the runner stone was mounted on the mill spindle and turned or ground on top of the bedstone or bottom stone. The grooves were to move the grain to the edges of the stone after it was ground.