I am having trouble picturing what this is. Is this a retaining wall? Is it smooth like a granite slab? Is it vertical or a gentle slope with granite boulders? If it is a slope is there exposed soil throughout the hillside? Can you walk down it? Does water pool at the bottom? Is your house at the top or the bottom of the slope/retaining wall. Where will the view be? What kind of sun exposure does it have? Will this be an area that you can get a hose to for watering?

I agree with Flowerfan, the picture would help greatly. Can you post a picture of this sloped area?

Whatever the situation, most of the clematis that I grow (not all, but most) tend to languish if they don't have something on which to twine their leaves/stems from a very early age. I can't imagine most clematis would "drape" down a wall or a slope unless they had first got going significantly by twining upward first.

I'm ready for all the more knowledgeable people on here to correct me...go ahead.

Robert

Actually clematis will do fine draping down if planted at the top. The only way they will scramble up the rocks is if the slope is very mild and they are just groundcovers, or if you covered the rocks with flexible netting so that they could grasp onto something.

Pictures would help determine size and degree of slope.

There are also quite a few non-vining species and cultivars that do very well sprawling in addition to the vine types. Is this a sunny location?
Is the soil moist enough and are you going to water? Clematis do not like dry soil.

From everything I've read, "Praecox" makes a great ground cover. It gets about 8 feet wide when allowed to scramble. It is non-clinging. Flowers mid summer to early fall.

There is a photo of "Praecox", being shown as a ground cover, in Mary Toomey's book "Timber Press Pocket Guide to Clematis. Check your local book stores to see if they carry the book. Look on page 12 and see if this is the effect you are hoping to achieve.

Another nice larger non-clinging variety is Arabella - it gets about 6 feet when allowed to scramble, and will bloom most of the summer. Pamiat Serdtsa is nice too, and there are other larger non-clinging varieties.

As far as going crazy or conservative? How do you decorate your home? If you lean more to conservative, then I think the crazy in the landscape might drive you crazy. KWIM?

If you like the rock, you won't have to fill up all the space. However, you could plant the non-clingers in drifts, such as 3 "Praecox" 2 at the top, 1 at the bottom, and 3 of another non-clinger in another drift, and so on. Then plant a couple of viticellas to grow between the drifts. Stake very strong fishing line in the direction you want them to grown, and in theory, you should have a spectacular slope. I say "in theory" because I've never tried this, but it sounds nice. lol