A number of rockery plants would work. Look at Arabis (rockcress), Aubretia, creeping phlox, basket of gold (Aurinia saxatilis), snow in summer (Cerastium tomentosum). Various hardy geraniums and lambs ear (Stachys byzantina) could work also. Depending on where you are located in Missouri, I'd also look into Cotoneaster dammeri and sunrose, Helianthemum nummularia. If your wall is long enough, a careful mix could offer a riot of color and a long period of bloom.

These are all creeping type plants that will cascade over the side of a wall and especially like the full sun, well drained situation this offers. How much coverage they offer will depend on the height of the wall. In my zone most are evergreen as well.

thanks for the info. this looks like alot of good info.