I have a similar wall and I planted sedums. They look beautiful once they have established. They will grow with little soil -any kind and will latch on to the rocks and cover it up.

They'll grow out of character if it's too shady.

Hey,
Seattle li,
I believe the other two feed backs you've received say it all. The ground covering type sedum don't need much. I've seen them growing beneath massive spruce trees in full shade surviving for years but leggy and not all that attractive. But I've got some in a south facing rock wall where they get all the water draining from above and they are full and thick. I suppose you might not want the soil too lean since the water is draining quickly anyway but it shouldn't matter much. Sedum will grow in clay especially in your situation. It's the big and tall sedums that need deeper soil.
Tom

I grow Sedum in a north-facing rock wall. They all do very well. Just be sure your planting pockets between the rocks are large enough to support the ever-increasing size of your perennial Sedums.

Another trailing plant that does well for me is Lamium maculatum (Spotted Dead Nettle - not really nettle, but a member of the Mint Family). 'Beacon Silver' and 'White Nancy' are especially attractive. 'Beedham's White' has yellow foliage. 'Golden Anniversary' has tri-colored foliage. The flowers are usually a shade of pink or white.

Here is a link that might be useful: Mountain View Experimental Gardens

seattle, what you have sounds like my neighbor's

{{gwi:369801}}

she has phlox, sweet woodruff, lupines, creeping jenny...

You could try checking out the Stepables web site. This line of plants are sold at area garden centers and nurseries. This company sells plants that can be used as ground covers or between rocks in a path. You can do a search by plant name (enter the name at the top of the page) or by criteria (light conditions, soil type etc.). They will give you some ideas of plants suitable for your conditions. These plants are usually a good price in our area not too much more than the cost of the small quart sized perennials. Look for them at your local garden center or check the web site to see if there is one selling in your area.

Here is a link that might be useful: Stepables Plant Search