There is a neighborhood house which has a short stone wall at the entrance, along the driveway, and they have creeping phlox (phlox subulata) which hangs over the wall and is absolutely spectacular this time of year. I never paid attention to their sun/shade situation, but I think there is a large tree on the other side of the driveway, so it may be part sun/part shade. I know that phlox can grow in lean, rocky soil, so it may be able to live in crevices. I do think it needs more sun, though.

Depending on how high your walls are, I've often seen pictures of a certain kind of thyme hanging over a wall. Darn, I can see the actual picture - it's been used in several catalogs for years now - but I can't think of the kind of thyme, and again, don't know the sun requirements.

Sorry I can't be of more personal-experience-type help. I'm still waiting to be able to build stone walls on my property! Good luck!

:)
Dee

Creeping phlox will work well, but make sure you get the right one - Phlox subulata is a rock garden plant and prefers (needs) full sun, Phlox stolonifera is a native woodland plant and will grow and flower in much shadier conditions. Others to consider are mossy saxifrage and Saxifraga umbrosa (London Pride) or the groundcover forms of Campanula, like portenschlagiana or poscharskyana (Dalmation or Serbian bellflower).

Again, thyme needs full sun to do well. Some stonecrops will also work in a partial shade condition.

in sun saponaria is nice aubrieta also. I mix them in with creeping phlox they flower at slightly different times.

Thank you all! I'm delighted to have these suggestions.

Creeping lysimachia "Copper Penny". You can't beat it for beautiful gold colored round leaves and it grows on virtually anything.

my favorites are the cerastium and vinca minor. My vinca does well in shade and sun. and you have a wide variety of leaves and flower color. and if you have a wall to hand down from it would be lovely. Some people call it invasive but if you have wall or concrete on both sides it is not a problem.

You can try alyssum. I used to have a bed of 'Carpet of Snow' and it was indestructible. I recently saw seeds for 'Mountain Gold' on the web and I'm thinking about getting it. Nice color - in the photo anyway...