1. The Gemara discusses what type of mats may be used as Sechach.
2. Rav Huna explains when one may make a Sukah out of a haystack.
3. There is a dispute about whether a wall of a Sukah that does not reach three Tefachim within the ground is valid.
4. The Gemara explains the dispute (#3).
5. Avimi: The Tana Kama would agree that a wall that is physically taller than four Tefachim may be used.
A BIT MORE
1. If the mats are large and made for use as Sechach, they may be used. If they are small and made for use as mattresses, they are considered a vessel which can become Tamei and may not be used as Sechach.
2. One generally may not dig out the middle of a haystack and be left with Sechach on top, since no positive action was done to make the Sechach (and the rule is "Ta'aseh v'Lo Min ha'Asuy"). However, if a height of one Tefach in an area of seven by seven Tefachim had already been dug out, the rest (to complete at least ten Tefachim of height) may be dug out to form a valid Sukah.
3. The Tana Kama: Such a wall is not valid. Rebbi Yosi: Such a wall is valid. Everyone agrees that if a wall is on the ground and is ten Tefachim tall, it is valid.
4. The Gemara explains that the argument between the Tana Kama and Rebbi Yosi is based on whether a hanging wall is considered a valid wall.
5. If a wall is placed less than three Tefachim from the ground and less than three Tefachim from the top of the Sechach in a ten-Tefach-high Sukah, it is a valid wall. This is because the law of Lavud (viewing things that are less than three Tefachim away from each other as connected) applies both to the bottom three and top three Tefachim of the Sukah, creating a ten Tefach wall that is less than three Tefachim from the ground.