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 threshing 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 practically intact, partly buried by the sandy floor of the oued. (See fig. 17.) Along the sides of the valley, and at a distance more remote from the oued, there are occasional washed-out areas, really box cañons, where the banks show a slightly different condition of the hardpan from that just described. Here there may be three strata of hardpan. The uppermost is of the same stratum as the top stratum by the oued, and the second stratum also resembles the lower one just described. There is also a third hardpan stratum of a much different character, in that it has a very large admixture of sand and gravel, and large as well as small stones, making it more easily eroded than either of the upper strata. The soil in which the roots studied were found varied from a fine sand, with waterworn pebbles, near the oued, to a clay mixed with sand nearer the sides of the valley. In places the sand is cemented so as to resemble one of the hardpan strata above described, but it is less hard and apparently is penetrated by water without great difficulty.