NASA NTRS1972
Apollo 15 data support the concept that Hadley Rille is a giant collapsed lava tube that originated at its south end. The south half of the rille is sinuous and the bends are not structurally controlled, whereas the north half of the rille follows pre-mare structural troughs. Most of the rille has a V-shaped profile apparently formed by recession of the rims and coalescence of talus from both sides. The rille is scalloped and discontinuous locally, and the deepest parts consistently are the widest. This is a different relationship from that shown by river channels, but can instead be explained by collapse, with most extensive foundering at the deepest points. The upper 60 meters of the walls of the rille at the Apollo 15 site expose several layers of mare basalt. Most are massive units averaging at least 10 m thick; others are layered. Numerous talus blocks derived from the massive outcrops are 10 to 30 m across. The lunar flows are thus both thick and little jointed. The outcrop ledge and blocky talus derived from it are absent where the rille abuts Apennine massifs, showing that the rille cuts through the mare basalt and against massif material.