Explanation: [The cavities and rocks are not measured with it] according to the calculation written in the Parshah (50 Shekalim for a Beis Kor for all 49 years between Yovels).

Suggestion: Perhaps they are not measured with it because they do not get Kedushah at all!

Rejection #1: A Mishnah above (71a) says that one who was Makdish his field was Makdish all of them, even a pit and winepress, and all the more so rocks and cavities, like the Rashbam explained!

Rejection #2: If so, what was the question after this 'a Beraisa teaches "Sadeh..." (to teach that a field of any size is redeemed according to the calculation)'? There surely it gets Kedushah, for he was Makdish it explicitly. Here that he was not Makdish it explicitly, it does not get Kedushah at all!

Explanation #1 (Rashbam): He should be like one who is Makdish two fields. He can redeem one without the other.

Rebuttal (Ri): If so, even in one field, he can redeem half according to the calculation of [50 Shekalim for] a field in which one sows a Kor of barley;

Regarding Sedei Achuzah (an inherited field), it says in Kidushin (20b) that all agree that one may borrow and redeem, and one may redeem half. There, R. Shimon and Chachamim argue only about Batei Ir Chomah;

This is unlike Rashi explained in Erchin (31a) that they argue about Sedei Achuzah.

Explanation #2 (Ri): The Gemara means as follows. They should get Kedushah by themselves! I.e. how can you say that because they are considered by themselves, they are not measured with it? They should get Kedushah, even though they are considered by themselves!

Question: The words "if so" are awkward. They connote that had we not established that the cavities are full of water, it would be fine;

In any case we can ask from rocks, which cannot be sown!

Also now [that we established that the cavities are full of water], we ask [also] from [rocks], since we needed to answer that they are called mounds of the land.

Answer #1: In several places, the Gemara could have said "also according to you (the Makshan, this is difficult)", and it does not say so.

Answer #2 (Ri): Initially, we did not think that it must be proper to be sown. Rather, even if it is not proper to be sown, it gets Kedushah [to be redeemed] according to the calculation of [50 Shekalim for] a field in which one sows a Kor of barley;

Therefore, it was fine that less than this (10 Tefachim), everything is one field, and when they are 10 tall or 10 deep, they are considered by themselves;

However, now that it says that we require proper to be sown, even less than this [should not be counted with the field]!

Implied question: Here we say, with difficulty, that [our Mishnah means that] when they are less [than 10 Tefachim, they are like rocks]! (Why did we say above "Deika Nami"? The Mishnah of Hekdesh can discuss cavities without water. It equates them to rocks only when they are less than 10!)

Answer: Even so, we properly said above Deika Nami, for [the Mishnah] connotes more that regarding [rocks] 10 tall it taught [cavities] like rocks, for the Reisha (which discusses when they are 10 deep or 10 tall, explicitly) teaches rocks and cavities.

Explanation (R. Tam): "A lone rock" means that it has an accompanying name - "Sela Ploni", even if it is any size. Therefore, even if it is in the middle of the field, it is not measured with it.

Explanation (cont. - R. Tam): The text says "that it has a Beis Rova." It says so in the Yerushalmi. It refers to this that we said above, that Trashin (rocky areas) that were said (that they are not measured with it) are Beis Arba Kabim, provided that they are not contained in less than five Kabim;

Inference: If they are less than four, even if they are not contained in less than five, they are measured with it.

Now [the Beraisa] says that if it was a lone rock, that it is all one rock, even if it is only a Beis Rova, and there are not Arba Kabim, and there are no others to complete the Shi'ur of four Kabim, even so, it is not measured with it.