Alluvial  Soil

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Alluvial  Soil ALLUVIAL  soil  is  formed  when  a  soil- carrying  stream  gradually  loses  its  carrying capacity   with   decreasing   velocity.   In   slowing down,  a  river  does  not  have  sufficient  power  to keep the large particles of soil suspended; these particles settle to the riverbed. Further decrease in velocity causes smaller particles to settle. As the  river  becomes  slow  and  sluggish  (as  in  the lowlands  where  its  gradient  becomes  small),  it holds only the extremely fine particles in suspen- sion. These particles are deposited, finally, at the mouth of the river, where they form DELTAS of fine-grained  soil. Marine Soil MARINE  soil  is  formed  from  materials carried into the seas by streams and by material eroded from the beaches by the tidal action of the waves.  Part  of  the  material  is  carried  out  and deposited in deep water; part is heaped upon the beaches  along  the  coast. Lacustrine  Soil Freshwater lake deposits are called LACUS- TRINE soils. Generally speaking, they are fine- grained  soils  resulting  from  material  brought into freshwater lakes by streams or rivers. Aeolian Soil Wind-transported grains make up AEOLIAN soils.  Sand  deposits  from  wind  are  called “dunes,”    and   the   finer   particles   (which   are generally  carried  further)  are  deposited  to  form a material called LOESS. Dune deposits seldom contain material larger than sand size. Glacial Soil GLACIAL soil is often called DRIFT. It con- sists  of  material  carried  along  with  or  upon  an advancing ice sheet or of material pushed ahead of   it.   As   glaciers   melt,   deposits   of   various forms   occur,   such   as   MORAINES,   KAME TERRACES,   ESKERS,   and   OUTWASH PLANES.   Moraines   consist   of   mixtures   of unstratified boulders, gravels, sands, and clays. The other forms (kame terraces, eskers, and out- wash   planes)   mentioned   consist   of   somewhat stratified and partly sorted stream gravels, sand, and  fines  transported  outward  from  the  glacier by  streams  during  the  melting  period. Colluvial  Soil COLLUVIAL  soil  consists  of  mixed  deposits of rock fragments and soil materials accumulated at the bases of steep slopes through the influence of  gravity. Table 15-1.-Size Groups as Used in the Unified Soil Classification System Sieve  Size Size  Groups Passing Retained  on Cobbles   ------------ No  maximum  size*  -------------------- 3 in. Gravels  ------------- 3    in.    --------------------------------------- No.  4 Sands  --------------- No.   4-------------------------------------- No.  200 Fines  ---------------- No.   200----------------------------------- No  minimum  size *In  military  engineering,  maximum  size  of  cobbles  is  accepted  as  40  inches,  based  upon maximum  jaw  opening  of  the  crushing  unit. 15-2

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