Vol. 65.] KARROO SYSTEM IN NORTHERN RHODESIA. 423 



pebbles subside into the ground and form a pavement that partly 

 protects it from tbe scouring effect of the rains. Some such pro- 

 tected areas form hills as high as 300 feet above the surrounding 

 country, while there is a sheet of shingle of some hundreds of 

 square miles in extent on the eastern side of the Kasafa Mountains. 



The pebbles are also a notable feature in the Luangwa Valley, 1 

 and they are to be seen in the Lukasashi ' trench ' — thus ranging 

 over a. vast area, and claiming consideration as important material 

 in the Karroo strata. It was noticed that each stone, however 

 well rounded, was slightly dimpled with one or more faint con- 

 cavities, and a careful examination showed this to be a distinctive 

 feature possessed alike by those of walnut-size, boulders measuring 

 12 inches in diameter, and subangular blocks of 40 inches across — 

 in the last-named increasing to more exaggerated cup or trough- 

 like shapes. (See PI. XIX.) These outlines could only have been 

 gained by the submission of fragments to a process of crush-move- 

 ment when the mass was in a semi-consolidated state, and before 

 its complete and final settlement. 



The search for the original rock resulted in coming upon the 

 conglomerate, first as blocks in storm-channels, then in situ in the 

 kloofs, and, lastly, up the plateau-slopes of Eakanunga Creek, on 

 the north side of the Muchinda Valley, and some 350 feet above 

 the centre of the plain, lying unconformably, with a dip of 10° to 

 1 2° towards the valley, upon the nearly vertical edges of gneiss. 



Before describing this conglomerate and its distribution, it will 

 be perhaps better to finish my remarks on the superficial shingle- 

 mounds. By the transition of one to the other up the hillside, it is 

 easy to see that they are derived from the breaking-up of beds, due 

 to the rapid disintegration of the matrix. It will be shown that 

 they still form the domes of anticlines in the flats, and probably 

 bridged over parts of the promontories, rising to a height of many 

 hundreds of feet above the present plain-level. The Archaean arches 

 of these anticlines are now exposed ; but the removal of the 

 mass of conglomerate from the domes, and also from the limbs 

 of the synclines, must have yielded immense supplies of pebbles. 

 When the slopes were steep, the shingle would be carried down 

 quickly ; and thus, immediately below the line of the present 

 exposure on each side of Eakanunga Creek, the foothills are formed 

 of liberated boulders, pebbles, and the granular matrix or cement. 

 (See PI. XIX.) From the foot of these slopes the pebbles ' creep ' 

 towards the lower valley-plains, aided by the shifting nature of 

 the clayey soil, and the effect of rains. 



In this onward movement insects play an important part. Under 

 each pebble is a maze of passages formed by all manner of burrow- 

 ing insects (mostly ants), in the formation of which the excavated 

 soil is dumped on the surface. This either becomes desiccated and 



1 L. A. Wallace, Geogr. Journ. vol. xsix (1907) p. 394. 



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