ringwall n.

The second and in certain respects most puzzling type are the numerous ringwalls or craters which range all the way from gigantic ‘walled plains’—of which Clavius is a fine example—to ‘normal’ Moon craters like Copernicus, and small ‘craterlets’ to tiny ‘beads’.

But then the younger crater should be considerably higher than the old eroded ringwall and the floor, again, should be considerably above mean Moon level. And that a secondary crater should break through the old ringwall, just at the spot where a lot of additional weight is piled on, is a harder strain on the imagination that one could reasonably be expected to stand.

He stepped down from the jumper—a rocket motor with a railed platform and some equipment on it, nothing more—and planted his boots on the solid rock of the ringwall’s crest. With a twist of his shoulders to settle the weight of the pressure-suit’s bulky backpack, he shambled over to the packet of seismic instruments and florescent [sic] marker that the priest had left there.

The train climbed Tycho’s ringwall, swooped down again, whizzed across the crater floor, and plunged underground near Skyview Tower.

This had an effect like a slow-motion fusion blast, as the powerful thermal updraft sucked huge quantities of dust up into the firestorm, where the dust ignited in turn, becoming an expanding ringwall of flame that swept across Mindor’s shattered landscape toward the battle that raged around the volcanic dome.