1.   We followed their tracks down into the swamp where a recent clearcut had left impenetrable thickets of young fir.

2.   But they remain an impenetrable thicket that, like the labels, send prospective customers away shaking their heads.

3.   History on television is not an intellectual wasteland any more than academia is an impenetrable thicket that protects specialists interested in speaking only among themselves.

4.   Recently Ms. Vierny searched for the route, only to be confronted by an impenetrable thicket.