The county's western and southern boundaries lie deep within gorges carved by the Colorado and San Juan Rivers.  Tributary canyons, cutting through rock layers of the surrounding deserts, have carved the land up with chasms, cliffs and plateaus.  In the center of the county are Cedar Mesa, Comb Wash, Natural Bridges and Hovenweep National Monuments.  Canyonlands National Park is primarily within the county borders.  The Eastern side of Glen Canyon National Recreation Area / Lake Powell in also in San Juan County.  Rising above all, the Blue  Mountains reach to nearly 12,000 feet  and the La Sal Mountains rise to 13,000 feet .  Both ranges are covered with lush forests vividly contrasting with the scenery below.  The elevation change within the county is from near 13,000 feet  in the La Sal Mountains to 3,000 feet  at Lake Powell, an elevation change of 10,000 feet .  The county is cut by deep and spectacular canyons, red rock and mountain meadows, desert, and evergreen forest.  The towns run primarily on a north/south axis along U.S. Route 191 and U.S. Route 163  from La Sal in the north to Monument Valley in the south.. What do Tributary canyons NOT form, cliffs, gorges, or chasms?