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
