Wall of Fire: The spell will create a wall of fire which lasts until the Magic-User no longer concentrates to maintain it. The fire wall is opaque. It prevents creatures with under four hit dice from entering/passing through. Undead will take two dice of damage (2-12) and other creatures one die (1-6) when breaking through the fire. The shape of the wall can be either a plane of up to 6" width and 2" in height, or it can be cast in a circle of 3" diameter and 2" in height. Range: 6".The 4th level Wall spells share some obvious and not-so-obvious features. The shape options and dimensions are identical, although the ranges aren't. Both are opaque. Both cause what I would call "incidental damage" to creatures that break through: a flat 1d6 damage, not scaled to either caster or spell level, but doubled for a class of creatures vulnerable to fire or cold. The wall actually cannot be passed by creatures with fewer than 4 hit dice, which I interpret as a limit imposed by the spell level.
Wall of Ice: A spell to create a wall of ice six inches thick, in dimensions like that of a Wall of Fire. It negates the effects of creatures employing fire and/or fire spells. It may be broken through by creatures with four or more hit dice, with damage equal to one die (1-6) for non-fire employing creatures and double that for fire-users. Range: 12"
The Wall of Ice stops fire attacks from passing through; by analogy, Wall of Fire should stop ice attacks, but that seems to have been overlooked. Similarly, creatures made of ice or adapted to icy environments should take double damage from Wall of Fire, even though that isn't mentioned.
The duration of Wall of Fire is based on concentration, but no duration is listed for Wall of Ice. On the other hand, Wall of Ice has a 120-foot range, which could be changed to the Line of Sight range I've mentioned elsewhere, while Wall of Fire has a much shorter range. Perhaps the difference can be attributed to the fact that passing through Wall of Ice creates a path, perhaps only a temporarily, but a path nonetheless, which a 1 to 3 HD creature could pass through after a more powerful creature clears the way. Wall of Fire is basically insubstantial, so passing through it does not damage it.