Spell Study Series: 5th Level Wall Spells

I'm probably not going to do full write-ups of the 5th and 6th level spells, since most of the spells have long descriptions and are very idiosyncratic; the purpose of the detailed re-reading was to find more usable patterns to simplify the spell descriptions, but spells like Teleport and Contact Other Plane won't help much. (I'd be happy to open a debate on a skipped spell, if someone's interested in talking about it, though.) The Wall spells, though, definitely shed some light on a previous post.

Wall of Stone: The creation of a stone wall two feet thick with a maximum length and height equalling 10 square inches. The wall will last until dispelled, broken down or battered through as a usual stone wall. Range: 6".

Wall of Iron: Like a Wall of Stone, but the thickness of the wall is three inches and its maximum area 5 square inches. Duration: 12 turns. Range: 6".

Brendan and I discussed whether Wall of Ice should represent a conjured material that persists until it melts naturally. He contrasts it with Wall of Fire, which burns without a fuel source, so it only lasts a short while. However, the durations of Wall of Stone and Wall of Iron call this into question. The Wall of Stone can last indefinitely, but it is definitely magical and can be dispelled; the Wall of Iron only lasts a short time. It seems to me that Wall of Ice should exist somewhere between those two.

The differences between the Stone and Iron durations probably derive from the more natural state of the stone versus the artificial nature of the iron wall.