The plants have spreading branches, often drooping from the cliff face. The stems are flaccid, shorter and pendulous or sub-pendulous. The softer, less woody and pendulous nature of the stems (compared to the woody stems of subsp. rupestris in non-cliff habitats) can be viewed as an adaptation to the cliff environment. The leaves are spreading, closely packed, forming a cylindrical body. Leaf pairs are fused by half or more their length forming a disc, and the crowded leaf arrangement is an adaptation to the xeric conditions of the cliff face. The leaf surface is glaucous, an adaptation resulting from the extreme run-off in the sheer habitat. Plants are long-lived perennials. The leaves are also persistent and long-lived, suggesting adaptation to the xeric cliff conditions. The soft-leafed plant bodies are without conspicuous armament nor camouflage.

Fig. 7. Crassula rupestris subsp. marnierana growing on a quarzitic sandstone cliff ledge on the upper quarzitic sandstone cliffs along the Huisrivier pass, Klein Karoo, Western Cape, the plants have become reddish due to drought stress.

The inflorescence is a large round-topped thyrse, conspicuous, in autumn and winter (April–June) bearing pinkish white flowers. The fruits (follicles) are dehiscent, the seed spontaneously released and dispersed by wind,  in time for autumn rains, maximising establishment.

Plants are slow growing, and long-lived perennials. This slow conservative growth can also be viewed as an adaptation to the mineral-poor quarzitic sandstone soil. The succulent nature of leafy branches stores sufficient moisture enabling the plant so survive periods of drought. The closely packed nature forming an oblong, rectangular body (minimising moisture loss), becoming distinctly purplish reddish during dry periods. This columnar feature is typical of many cliff-dwelling succulents and can be viewed as an adaptation to the extreme xeric conditions of the cliff face.

The leafy stems are often encountered with aerial roots, and stems will root when coming in close contact with crevices, an efficient vegetative backup strategy for surviving the harsh conditions on the cliff face. A detached branch will also root when landing on a crevice.

Fig. 8. A densely branched cluster of Crassula rupestris subsp. marnierana growing on a quarzitic sandstone cliff ledge on the upper quarzitic sandstone cliffs along the Huisrivier pass, Klein Karoo, Western Cape.