Hmm.  I don't think I agree with Bill's advice here.  Inside the
canyon, you will often be faced with choices.  Remember, it's dark in
there.  I had many 2 minute+ exposure times using velvia recently.  If
you can compose a shot you like while leaving the bright incoming light
out of the frame, then I agree with Bill.  In many cases, however, some
of the small bright light sources will be in the frame and therefore
you might choose to blow them out in order to get some detail in the
shadows.  Even the classic shot of the light beam at the entrance might
include portions of highlights that are blown out.

Enjoy,
Tom K