GHARDAIA. 37 



At Ghardaia are several well-defined plant habitats, which may or may 

 not be distinct topographical areas, and which differ from one another in 

 exposure, soil conditions, and water relations. These are the plain of the 

 Chebka (hamada), the low and flat-topped mountains resting on the plain 

 of the Chebka, the walls of the M'Zab Valley, and the valley floor with its 

 gardens, cemeteries, and waste lands. 



The soil conditions of the areas mentioned are very diverse. On the 

 valley walls and the mountains there are bold rock outcrops with soil in 

 the interstices only, and here the most intensely arid conditions prevail. 

 The soil on the hamada also is exceedingly meager. Rocks of various sizes 

 strew the surface. It is only between them, as well as in the washes of 

 gentle gradient, that the best soil conditions of the plain are to be found. 

 Here a cursory examination shows a large admixture of small stones to the 

 fine clay, the prevailing soil type, and that at a depth less than 50 cm. A 

 white hardpan, similar in appearance to the caliche of the southwestern 

 United States, may usually be encountered. In the drainage depressions 

 the soil is relatively more coarse than on the more level portions of the 

 hamada. There is also great variation in the character of the soils of the 

 valley. Above the upper palm gardens, which are about 2 kilometers 

 above the town of Ghardaia, will be found much sand and fairly large sta- 

 tionary dunes, while smaller dunes, shifted by the winds, are to be found 

 at various places in the valley. About 10 kilometers down the valley, 

 toward the east, the sand is blown against the valley sides, and in certain 

 places where the walls are low it has been sifted in a thin layer over the 

 plain itself. At the sister city of El Ateuf the drifting sand is a continuous 

 menace to the gardens. 



Between Ghardaia and the upper palm gardens, and also between this 

 city and Beni Isguen and Melika, are bare areas, free from sand or clay, 

 where the soil is so hard as to be used for threshing floors and where the 

 small amount of grain grown in the valley is threshed and winnowed. The 

 hardpan is similar in appearance to the caliche of the southwestern 

 United States and may be essentially the same. It is of wide extent in 

 the valley and probably underlies the largest portion of it. Near the thresh- 

 ing floors the upper portion of the hardpan stratum is from 2 to 3 meters 

 above the bed of the oued M'Zab; the stratum is about 30 centimeters in 

 thickness and is of fairly uniform structure throughout. Beneath this is 

 another stratum, less well defined perhaps, of approximately the same 

 thickness, and with nearly the same character, but carrying a noticeably 

 large percentage of sand. The lower stratum is less hard than the upper 

 one. Underneath the second stratum is soil, largely sand, containing rocks 

 of various sizes. Where erosion of the oued banks has occurred the soft 

 lower hardpan stratum and the yet more soft underlying soil are both 

 removed, leaving the upper stratum projecting as a shelf, sometimes as large 

 as 2 by 4 meters in extent. When the shelving banks break they remain prac- 

 tically intact, partly buried by the sandy floor of the oued. (See fig. 17.) 



