Full record for ringwall n.

Definition a roughly circular ring of cliffs/mountains surrounding an impact crater
OED requirements antedating 1944
Earliest cite C. Bonestell & W. Ley in Astounding Sci. Fiction
Comment Fred Galvin submitted a 1949 cite from "The Conquest of Space"; Mike Christie checked the 1944 first magazine appearance and discovered that the term was used in that version too. Earliest cite in the OED: 1950.
Last modified 6 July, 2008

Citations for ringwall n.

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1944 W. Ley in Astounding Sci. Fiction Dec. 115/2 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’.
1944 W. Ley in Astounding Sci. Fiction Dec. 121/2 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.
1993 P. Anderson Harvest of Stars (1994) 139 The train climbed Tycho's ringwall, swooped down again, whizzed across the crater floor, and plunged underground near Skyview Tower.
1994 B. Bova Craft of Writing Sci. Fict. that Sells iv. 22 He stepped down from the jumper‥and planted his boots on the solid rock of the ringwall's crest.
1994 B. Bova Craft of Writing Sci. Fict. that Sells xiv. 142, I was somewhat surprised to see the barriers rising like concentric ringwalls, each of them centered on the golden glowing ship and the aliens within it.