(Mishnah): A canal that is wide enough that one is not able to stand on one side of it and harvest produce on the other side - R. Yehuda says that it divides. (The Tanna of the previous Mishnah (daf 17-1(a)) simply lists a canal as something that divides.)

(Yeshayahu 7:25), "And all the mountains that will be dug with a spade" - even if a mountain can only be dug up and plowed with metal tools because the cattle cannot pass (to be able to plow), it is all considered to be one field for Peah.

(Gemara): The students wanted to suggest that there is no dispute and R. Yehuda is explaining and qualifying the teaching of the first Tana. However, they discovered a Baraisa that teaches that according to the first Tana, it is specifically a permanent canal that divides (and R. Yehuda disagrees and says that if it is wide enough that one is not able to stand on one side of it and harvest produce on the other side, even if it is not permanent, it divides.)

The students wanted to suggest that R. Yehuda was referring to when he cannot harvest both sides of the canal even if he would be standing in the middle of the canal (due to the great width of it), but if he could stand in the middle and harvest both sides, it does not divide.

However, they discovered a Baraisa that taught that unless he can stand specifically on one side and harvest the other, it divides.

(Chiya bar Ada citing R. Shimon ben Lakish): If there is a rock across the entire width of the field, if he must lift the plow over it in order to continue plowing, it divides.

Question (Baraisa): 'If a field was on multiple elevations, with one level ten tefachim, or more, higher than the next, he gives Peah for each level separately; less than 10 tefachim, he gives one Peah for all of them.' But even less than 10 would still require lifting the plow over?

Answer: True, but the number 10 was actually taught for the latter clause of the Baraisa - that if the ends of the levels were mixed (as they sloped to become within 10 tefachim of each other), he gives one Peah.