Rock Spiraea
Petrophyton caepitosum  (Nutt.) Rydb.

Prostrate or cushion-forming shrubs growing on rocks. A flat little evergreen perennial that is actually a shrub. It's not very woody. Woody plants that seem to be molded over rocks, forming dense mats. The flowers are on a spike a few inches tall towering over these flat plants. The flowers are creamy white. Located in the wild on exposed rock surfaces, rooting in cracks and crevices, usually on limestone cliffs and outcrops from 1000 to 3000 mountain altitude. Found in western Montana, through southern Idaho to eastern Oregon. Nuttall noted the plant on outcrops of the Rocky Mountains.  Original material from Nuttall's specimen is at the British Museum.