THE PLANT ECOLOGY OF Di; AKKXSBERG RANGE. 551 



8. VEGETATION OF THE MOUNTAIN TOP. 



At liig"li altitudes we have a type of soil and plant forma- 

 tion which differs from the tiissoek veld found on the tops of 

 the lower foothills. The top of the main Drakensberg* 

 escarpment is^ on the whole, fiat, but there are fairl}^ large 

 depressions and bare portions which are higher. The detritus 

 occurring on the mountain top recalls that which is formed 

 on tlie mountains of Scotland, described by Crampton ^ in his- 

 memoir on the vegetation of Caithness. It is formed in situ,. 

 and in Scotland, according to Crampton, is due chiefly to the 

 action of frost and wind. On the Drakensberg these factors 

 are in action also, and, in addition, probably the intense insola- 

 tion, which, as has been ali-eady pointed out, leads, through 

 the rarity of the air, to great heating of the surface, and the 

 sudden cooling by thunderstorms must be taken into con- 

 sideration as an important factor, the heated rock-surface 

 being sliattered by the sudden cooling. The alternate freezing 

 and thawing during the colder season and tlie percolation of 

 water continue the process of disintegration. 



'i'he debris consists of rather small, angular fragments of 

 the underlying rock (aniygdaloidal lavas), and forms a 

 spongj', porous layer from about 4 in. to a foot or two in 

 thickness in the depressions. On the higher, bare portions- 

 it continues to be formed. After n, rainstorm it appears as a 

 wet, oozing mass with innumerable rivulets; but the water 

 I'uns off, and it dries very quickly. Some of the deeper 

 hollows, however, remain permanently wet, and in winter the 

 water becomes frozen into a mass of ice. 



The vegetation is partly of an extremely xerophytic- 

 chai'acter, the growth - forms being designed to give the 

 greatest possible protection. The rhizomes and shoots inter- 

 twine to form low mats, and in places a certain amount of 

 black humous soil is formed among the heathers. On the 

 bare rocks we get lithophytes and chomophytes, as described 



* Crampton, C. B., '• Vegetation of Caithness considered in relationi 

 to the Geology," 'Brit. Veg. Comni..' 1911. 



