11.   To the west, the seemingly impenetrable wall of the Sierra Nevada crumbles one grain at a time.

12.   Without such prompts, Ms. Donovan said, Shakespeare can often appear to students as an impenetrable wall.

13.   Word from the southern frontier is that there is now an almost impenetrable wall of federal agents on both sides of the border guarding against terrorists.

14.   As the elevator doors opened, they were hit by an impenetrable wall of black smoke, according to a reconstruction of events.

15.   The violence and seemingly impenetrable wall of mistrust has also confounded US peace efforts.