You wouldn't mark field ends with stones, in an arable landscape, for fear of the plough dinging on them as it turned. You'd mark the field ends with sticks or a hedge, maybe, and make heaps of rocks or chuck them onto a river terrace or put them around the base of trees. Pete's right about this being the best time of the year to find lost stones, but. Another couple of weeks or so and the greenery will be growing again.