Crevice gardens are a variant of the rock garden. I don't know a lot about them but I do know they look beautiful.
Apparently cracks and crevices in cliffs and mountain peaks give protection to many plant species. Alpine plants grow happily in sun warmed natural crevice gardens in a summer that may last only a few weeks. The solar heat absorbed by the stone adds a few extra days to the growing season as well.
A crevice garden can be a great alternative solution to a typical stone retaining wall for accommodating the change in grades of a hilly garden while also providing an environment for some more unusual species of hardy plants. Just remember to plant things that don't need much water, grow slowly and stay small. You don't want to hide the stonework.
After all, a crevice garden is basically a dry stone wall laying on its side, buried flush to the ground.