The various habitats, therefore, not including the oases, are naturally closely associated with soil differences. These are few and, in nearly each case, to name is sufficient definition. There are dunes (areg), hamada or stony desert, reg or alluvial desert, the daya, and the flood-plain of the oueds (reg?). Modification of the hamada, reg, and oued flood-plain occurs whenever white hard-pan (caliche) is present. The soil of the reg, often that of the oued flood-plain and that of the daya, is fine alluvial and is relatively or actually deep. The hamada has the poorest soil condition, being underlaid by rock, and often or always by hard-pan as well. Large stones and boulders are embedded in the soil or lie on the surface. A modification of the hamada occurs whenever sand is strewn over its surface, even if the sand is only a few centimeters in thickness. This acts as an effectual mulch, increasing the retentive capacity of the soil, and very strikingly changes the character of the vegetation. The final habitat to be mentioned is the salt spot, or chott, where gypsum constitutes an important salt. As regards their relative importance the habitats in southern Algeria can probably be grouped in the following sequence, a relation which very possibly holds good for the Sahara taken as a whole: Hamada, dune, oued, flood-plain, reg, daya.[36] The habitats are unlike as regards the relation to the rainfall and its effects. This is in part due to the differences in soils, or their depth, and in part to topographical differences. By the latter the low-lying areas receive a relatively large amount of water; and since their soil is relatively deep the water is retained longer than on the hamada, for instance, where the soil is shallow.