The author under a natural bridge into the "oscuros" of Barranco Pallas. "oscuros" is a very dark part of a narrow canyon in this area. This is the best conglomerate canyon of the left bank of the Rio Usuala. The last rappel of the Barranco Pallas exit into the Estrecho de Balces, a seven kilometers narrow canyon of the Rio Usuala with several wading and long swimming part. Sierra de Guara Regional Park, Spanish Pyrenees.