The statement is ambiguous, so it can still be valid. However the use of "length" twice in the same sentence to describe both the dimensions parallel and perpendicular to the crack is likely to confuse some readers. Some people's brains are not going to free the "length" pointer halfway through the sentence.
but that's not how it's being used. what length of rope does it take to go around a quarter mile track? there's no ambiguity even though the track has length and width. the dimension in question was specified already, the width of the crack, so referring to the measure of that as its length is fine, no ambiguity.
