sentences: most canyons are formed by flowing rivers through erosion over long periods of time
docs: river is flowing_body_of_water. group_of_canyons is group_of_valleys. canyon is valley. not_all_rivers has waterfalls canyon is flow_terrain. rivers has flowing_water member_of_undergrounds_canyons is canyon. river is water_moving_downhill. canyon is ravine. river is long_flowing_body_of_water.