At the very back of the spectacular Big Bend NP cirque canyon that is lower Burro Mesa, the canyon dead-ends in a slender cleft of rock, a fissure in deep blue shadow. You scramble over boulders only to literally hit a wall. But beneath those walls lie tiny surprises, little mysteries. A twig, a submerged rock. A ripple of sunlight. The soft sound of a spray of falling water still trickling down off the pour-off.