. Soils, their properties and management . ly a coarse,loose soil, rather unfavorable for plant growth. It isformed, as its name indicates, in regions of precipitoustopography,, and is made up of fragments of rocks de-tached from the heights above and carried down theslopes by gravity. Talus slopes, cliff debris, and otherheterogeneous rock detritus are examples of colluvialsoil. Avalanches are made up largely of such the physical forces of weathering are most acti\e inthe formation of these soils, the amount of solution andoxidation is small. The upper part of the accumulation 1 K
. Soils, their properties and management . ly a coarse,loose soil, rather unfavorable for plant growth. It isformed, as its name indicates, in regions of precipitoustopography,, and is made up of fragments of rocks de-tached from the heights above and carried down theslopes by gravity. Talus slopes, cliff debris, and otherheterogeneous rock detritus are examples of colluvialsoil. Avalanches are made up largely of such the physical forces of weathering are most acti\e inthe formation of these soils, the amount of solution andoxidation is small. The upper part of the accumulation 1 Kerr, W. Geology of North Carolina, Vol. I, p. 195. GEOLOGICAL CLASSIFICATION OF SOILS 39 exhibits this isolated physical action to the greatest extent,the particles being angular, coarse, and comparativelyfresh; farther down the slope the material may mergeby degrees into ordinary soil. Such soils are usuallyshallow and stony, and approach the original rock incolor unless large amounts of organic matter have ac-cumulated (Fig. 5).. Fig. 5. — Diagram showing the formation of a colluvial soil, (a), bedrock ; (6), dismantled cliff ; (d), coarse unproductive talus; (p), soilcapable of bearing plants. 29. Alluvial soils. — In considering the importance ofwater as a weathering agent, it was found that it hadboth cutting and transporting powers. The alluvial 40 SOILS: PROPERTIES AND MANAGEMENT soil is a direct result of both these activities. The earn-ing power of water varies directly as the sixth power ofits velocity; so that a doubling of the velocity increasesthe transportive ability sixty-four times. It is estimated ^that water flowing at the rate of three inches a secondwill carry onl} fine clay, but if this rate is increased totwenty-four inches a second, pebbles the size of an eggwill be moved along the stream bed. Any checking ofthe velocity of a stream will cause it to deposit the ma-terial carried in suspension, the larger particles first andthe finest when the
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