Moed Katan 6A

ודלמא טומאה מגואי ואילנות מבראי?

The Talmud asks: But perhaps the ritual impurity was on the inside and the trees were on the outside, and only the area between the trees was plowed, while the inner portion of the field with the grave was not plowed?

במסובכין. ואיבעית אימא, הא אמרן: אין מרחיקין ציון ממקום טומאה שלא להפסיד את ארץ ישראל.

The Talmud answers: The case is where the trees are scattered throughout the entire field, so that it is likely that the entire field was plowed. And if you wish, say instead: This is not a concern, as we said earlier that one does not distance the marker too far from the actual site of ritual impurity, so as not to cause a loss of Eretz Yisrael. As the marker is located near the trees, presumably the trees are close to the actual site of the grave, and the site of the grave was plowed.

רבי יהודה אומר: עד שיהא שם זקן או תלמיד, לפי שאין הכל בקיאין בדבר. אמר אביי: שמע מינה צורבא מרבנן דאיכא במתא ⁠— כל מילי דמתא עליה רמיא.

It is taught in the baraita that R' Yehuda says: One relies on these signs only when there is an Elder or a rabbinic scholar who can testify about the matter, as not all are well versed in this matter, and perhaps the field was not plowed at all. Abaye said: Learn from this statement of R' Yehuda that when there is a Torah scholar in the city, all affairs of the city are thrust upon him, i.e., are his responsibility. Consequently, he is expected to know what has happened in the city.

אמר רב יהודה: מצא אבן מצוינת ⁠— תחתיה טמא. שתים, אם יש סיד ביניהן ⁠— ביניהן טמא, ואם אין סיד ביניהן ⁠— ביניהן טהור.

§ The Talmud continues its discussion of marking graves. Rav Yehuda said: If one found a single marked stone, this indicates that the ground underneath it is ritually impure. If he found two marked stones, the following distinction applies: If there is lime on the ground between them, this indicates that the area between them is ritually impure and the two stones mark the boundaries of the impure area; and if there is no lime on the ground between them, this indicates that the area between them is ritually pure and each stone marks a separate area of ritual impurity.