1. An inverted channel with semi-circular cross section seen in cave-passage ceilings, most clearly where the ceiling is an uneroded bedding surface. The half tube originates as part of a phreatic tube guided by the bedding plane, and the lower half is subsequently removed by vadose enlargement. The presence of half tubes provides important evidence of early phreatic-cave development [9]. 2. Trace of a tube remaining in the roof or wall of a cave [10]. See also tube.