OPINIONS: The Gemara discusses a case in which one side of a Mavoy ends at the ocean, and the other side ends at a garbage heap. What exactly is the case?

(a) RASHI and the RITVA apparently do not have the word "ends" ("Kalah") in their texts of the Gemara. They explain that the sea and the garbage heap comprise the sides of the Mavoy, but are not at the ends of the Mavoy. The reason why they explain that only the sides of the Mavoy are the sea and the garbage heap, and not that the Mavoy opens into those areas at each end, is because the Gemara says that we are concerned that the water might recede and the area will turn into a sand bank, effectively taking away the Mavoy's wall on that side. If the Gemara is discussing a Mavoy that ends at the sea, though, what is the Halachah if the sea recedes and leaves only a sand bank? Since the other end is still a garbage heap, the Mavoy would be open on one side like any other Mavoy Sasum, and a Lechi or Korah would suffice to permit carrying there! Only if both the sea and the garbage heap are taken away does the Mavoy become invalid (and it needs more than a Lechi or Korah to permit carrying there).

(b) The RITVA cites others (see RABEINU CHANANEL, RAMBAM in Hilchos Shabbos 17:5, and ME'IRI) who explain that the garbage heap and the sea are at the two opposite ends of the Mavoy. Apparently, they understand that the Mavoy does not have a Lechi. Therefore, even if one of the end-walls is taken away (such as if the sea turns into a sand bank) one would become prohibited from carrying in the Mavoy until a Lechi or Korah is erected.