Illusions can mimic any terrain.

    Effect: Creatures that realize that an object is an illusion ignore its effects, while those that do not realize the truth behind the illusion react to it as appropriate.
    Usage: Use characters’ passive Insight checks to determine if they notice something “not right,” but don’t allow them to make active checks without good reason. Once a character has reason to be suspicious, he can make an Insight check as a minor action to attempt to disbelieve an illusion.
    Illusory Wall: An illusory wall blocks line of sight. Creatures can walk through it without penalty, though obviously creatures that believe the illusion aren’t likely to try doing so. Some illusory walls are similar to one-way mirrors in that they are transparent from one side (allowing a viewer to see creatures on the other side) while from the other side they appear to be normal walls (blocking line of sight, and looking like normal wall terrain).

Published in Dungeon Master's Kit, page(s) 210, Dungeon Master's Guide, page(s) 68.