the.com/natural bridge

means A rock arch or span formed when erosion — usually water cutting through stone — removes everything except a connecting overhead bridge of rock.

from Simply descriptive English: 'natural' (from Latin naturalis, 'by birth/nature,' marking it as nature's work rather than human engineering) plus 'bridge' (Old English brycg, a span for crossing). The phrase shows up as a literal label wherever geology does the work masons usually get credit for — famous examples like Virginia's Natural Bridge gave the generic term extra staying power once it became a proper name too.