THE SEDIMENTARY ROCKS OF THE RHODESIAX 



PLATEAU. 



Bv F. P. Mexnell. F.G.S. 



Introductory.— If we confine the term " sedimentary " to 

 the normal deposits whicli have not assumed a metamorphic 

 character, the distribution of such rocks on the Rhodesian plateau 

 is very restricted. Most of the " high veld " is made up of 

 very ancient metamorphic rocks, into which are intruded 

 immense masses of granite, and in travelling about the country, 

 one is apt to get the idea that the sediments are even less im- 

 portant than is really the case, for they weather so readily 

 into loose sand that roads over them are very heavy, and they 

 are conseqiiently avoided as much as jiossible in road-making. 

 Nevertheless, these sedimentary rocks are of considerable 

 interest in themselves, and perhaps even more so for the light 

 they throw on structiu^al and physiographic problems 



Characters, etc. — The sediments with which we propose to 

 deal in these brief notes belong to a single series almost entirely 

 made up of sandstones, but diversified by sheets or flows of 

 basic igneous rock. They have l)een aptly termed " Forest 

 Sandstones " by ]\Ir. A. J. C. Molyneux, who first described 

 them in any detail, and the name of " Zambezi Series " inde- 

 pendently suggested by myself may therefore be dropped. So 

 distinctive are the characters of the sandstone areas, that the 

 natives of Matabeleland give them the special name of " (iusu " 

 (locative, e-Guswini), which is never applied to any other cla^^s 

 of country. Their principal features are the light-coloured 

 sandy soil, and the abundant covering of trees, usually of good 

 size for Rhodesia, and often sufficiently close to merit the name 

 of forest. The underlying rocks are seldom exposed, except on 

 the slopes of basalt-capped hills or in the rare artificial, excava- 

 tions. In fact, even loose stones are entirely absent over large 

 areas where the basalts do not occur. These latter conform to 

 the bedding, or rather what is ]:)resumed to be the bedding, of 

 the sandstones, and as they are very resistant to weathering 

 compared with the sediments, they usually form the cappings 

 of table-topped hills with sharp marginal slopes. Taba-z-Induna. 

 near Bulawayo. is an excellent example on a small scale. The 

 sandstones generally seem to be separable into a white lower 

 and a red upper division, but the exposures are so few that it is 

 difficult to be certain that this is of universal application. 

 It is seldom, by the way, tliat more than one sheet of 

 basalt is seen. The lower white sandstones are entirelv 

 devoid of bedding planes, and often exhibit a kind of 

 spheroidal jointing. They are extremely fine-grained, and the 

 grains are foimd under the microscope to be so well rounded 

 that they must be regarded as wind-worn. The cement is 

 generally chalcedonic silica or even opal (Taba-z-Tndima). The 

 red sandstones are somewhat coarser in grain, and are distinctly 

 bedded, and infleed often flaggy. The cement i*^ chiefly iron 



