Ohalos, Chapter 17, Mishnah 4. This chapter deals with the beis hapras, a field in which a grave was plowed over so that bone fragments may have been dragged and scattered through the soil, and upon which Chazal decreed tumah. Our Mishnah asks what happens when the earth of such a field does not stay where it is, but is carried by the weather onto a neighboring field that was until now perfectly tahor.
The Picture the Mishnah Draws
The Mishnah's opening words, "sedeh beis hapras," a field with the status of beis hapras, describe it as sitting "al gabei tehorah," over a field that is tahor. Picture, then, a slope, the side of a hill or a mountain, with one field situated above another. The Bartenura, in the first of his explanations, identifies which is which: the higher field is the beis hapras, and the field lying beneath it is the ordinary tahor one.
Now the weather enters the Mishnah. A tremendous downpour comes, "shotfu geshamim mibeis hapras latehorah": the rains washed soil out of the upper field, the beis hapras, and carried it down onto the ordinary tahor field below.
How Do We Know the Soil Really Came Down?
Ordinarily one could wonder: how can I be certain that this earth in the lower field came from above? The Mishnah therefore describes a case in which there is no room for doubt at all, because the two fields had two different colors. "Afilu adumah vehilbinu": the lower, tahor field originally had red soil, while the upper beis hapras had white soil, and now, after the rain, the lower field has turned white. Or the reverse, "levana vehe'edimu": the tahor field below started out white and the field above was red, and now the lower field has taken on a reddish color.
The color of the lower field has become the color of the upper field, and it was not that way before the rain. There is simply no question here: soil from the beis hapras above has come down onto the field below.
The Halachah Nonetheless
And still the Mishnah rules: "ein osin osah beis hapras." We do not declare the lower field a beis hapras. Why not?
Because Chazal never decreed tumah on loose soil that comes washing down a hillside. Their gezeirah of beis hapras was made on the gush, on the earth as a solid clump lying in its place. Here, granted, the topsoil and the loose earth were carried down by the rain, but a gush, a whole clump of packed earth, does not pick itself up and travel down the mountain, not even in a downpour. Since only the loose soil moved, the decree of beis hapras does not follow it to the lower field.
The Rambam explains the ruling along a somewhat different line. He connects it to the halachah taught earlier, that ein beis hapras osah beis hapras, one beis hapras does not generate another beis hapras.
Whichever approach one follows, the conclusion is identical. When Chazal placed the tumah of a beis hapras upon the afar of such a field, they did so only regarding earth that remains an undisturbed clump, resting where the Ribbono Shel Olam set it down. Rain, too, arrives by the will of Hashem; but once it has shifted that earth from its original spot and the clump no longer holds together, the tumah of a beis hapras does not attach to what came down, and the field below retains its taharah.