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The parent material includes both, hard, resistant rocks such as granite, marble as well as slate, apart from less resistant rocks such as recent volcanic lavas and ashes, and the metamorphic (schist, gneiss) and sedimentary rocks (sandstone, clay, silt, and limestone). The term ‘rock’ is strictly applied not only to granite, sandstone and the like, but also to gravel, clay, and unconsolidated sand, loess, and alluvium which are less compact and less resistant soils.