Shevi'is Chapter 3, Mishnah 6. This mishnah also deals with the idea of removing stones from a field during the Shemittah year, and with the measures and conditions established for doing so.
"Gader sheyesh bo eser avanim shel masoy shenayim shenayim":
The case is a stone fence containing at least ten stones, each one so heavy that two men are needed to carry it. In such a case the mishnah rules: "Harei elu yinatlu" - and not only those ten stones, but one may remove all the stones of the fence, even down to the ground itself. There is no need to be concerned that it will look as though he is preparing the field for plowing and planting, since when a large quantity of large stones is involved, it is obvious that the stones are being taken for building purposes or in order to clear away the fence, and not for the sake of planting.
The measures that permit removal:
A fence that meets the conditions: the fence stands ten tefachim high and contains ten stones that require two men to carry - in that case one may remove everything down to the ground.
Less than that: if the fence is lower than ten tefachim, or it does not contain ten stones of that weight - "Mechtzah v'gomro ad pachos mitefach". That is, he may clear it away, but he must leave on the ground a layer of stone less than a tefach high. Leaving this amount makes clear that he does not intend to use the spot for plowing and planting, and therefore it is permitted.
Two conditions - "Bameh devarim amurim":
"Mitoch shelo": these conditions were stated specifically where a person is clearing the stones from his own field. But "mitoch shel chaveiro" - when he clears them from his fellow's field - "mah shehu rotzeh yitol", and he may take as much as he wishes. The reason is clear: since the field is not his, he has no interest in preparing it for planting, and his only interest is in the stones themselves.
"Bizman shelo hischil bo me'erev shevi'is": the conditions were stated specifically where he did not begin removing the stones before Shemittah. But if "hischil bo me'erev shevi'is" - "mah shehu rotzeh notel", and he may take as much as he wishes, and he is not bound by any of these conditions.
These two clauses of "bameh devarim amurim" - that the restrictions apply specifically to his own field and specifically where he did not begin before Shemittah - we explained as referring to the opening of the mishnah, to the law of a fence containing ten stones. There are those, however, who say that they apply to the previous mishnah as well: that one may not open a quarry during Shemittah until it contains a certain quantity of stones and the like. That permission too applies only in his own field, and only where he did not begin before Shemittah.
In summary: in this mishnah we learned the measures that permit clearing away the stones of a fence during Shevi'is - a fence ten tefachim high containing ten stones that require two men to carry may be taken down to the ground, and if it does not meet that measure it may be cleared until less than a tefach remains. We also examined the two limiting conditions - that it be his own field and that the work began during Shevi'is - and the opinion that these conditions apply as well to the law of opening a quarry taught in the previous mishnah.