Someone who was searching and reached a river, an irrigation canal, or a public highway, stops [his search]. In a field in which people were killed, [if] all the bones are collected the field is then considered tahor. [If] someone is removing a grave from his field, he gathers up every bone and the whole [field] is considered tahor. [Regarding] a pit into which stillborn fetuses or slain people were thrown, the bones are gathered, and the whole [pit] is considered tahor. R’ Shimon says: If [the pit] was initially intended to be a grave, the surrounding earth must be removed.