. Soils, their properties and management . ated as sedentary and trans-ported/ are subject to considerable subdivision, asfollows: — o 1 ^ \ ResidualSedentary j ^^^^^^^ Gravity — Colluvial [ AlluvialWater s jNIarineTransported ] [ Lacustrine Ice — GlacialWind — iEolian 25. Residual soils. — This group of soils covers wideareas of our arable regions and comes from many kinds ofrocks. Residual soils are old soils, the oldest with whichwe have to deal in agricultural operations. Since a 1 See Trowbridge, A. C. A Classification of Common Sedi-ments. Jour, of, Geol., Vol. 22, No. 4, pp. 420-436. 19
. Soils, their properties and management . ated as sedentary and trans-ported/ are subject to considerable subdivision, asfollows: — o 1 ^ \ ResidualSedentary j ^^^^^^^ Gravity — Colluvial [ AlluvialWater s jNIarineTransported ] [ Lacustrine Ice — GlacialWind — iEolian 25. Residual soils. — This group of soils covers wideareas of our arable regions and comes from many kinds ofrocks. Residual soils are old soils, the oldest with whichwe have to deal in agricultural operations. Since a 1 See Trowbridge, A. C. A Classification of Common Sedi-ments. Jour, of, Geol., Vol. 22, No. 4, pp. 420-436. 1911. 31 32 SOILS: PROPERTIES AND MANAGEMENT residual soil is formed in situ, the rocks that underlie it,if sound, show the character and composition of the rocksfrom which the soil was actually a product. In suchsoils the changes that a rock undergoes in forming aresidual clay are to be studied to the best examination of the various grades of material that arefound overlying the country rock (Fig. 3) in an area where. Fig. 3. The gradual transition of country rock into residual soil byweathering in situ. this residual mantle exists, reveals more or less accuratelythe gradations from rock to soil. Residual soils, besidesbeing old soils, are usually nonstratified and present aheterogeneous mass of material. Since they have beensubjected to leaching over vast periods, a very largeamount of the soluble materials have been washed out,tending to leave high percentages of the persistent ma-terials, such as silica, iron, and aluminium. An analysisof an Arkansas limestone, its residual clay and the cal-culated percentage loss of the various constituents pres-ent in the fresh rock, illustrates this point: — GEOLOGICAL CLASSIFICATION OF SOILS 33 Arkansas Limestone and its Residual Clay i SiOj AI2O3 FeaO, MnO CaO MgO K2O NajO CO2 Fresh Rock .30 .35 .16 Clay .26 .96 .61 .00 PercentageLost .
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