ALLUVIAL soil is formed when a soilcarrying 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 suspension. These particles are deposited, finally, at the mouth of the river, where they form DELTAS of fine-grained 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.

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 is often called DRIFT. It consists 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 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

PHYSICAL CHARACTERISTICS OF SOILS

The physical characteristics of soils aid in determining their engineering characteristics and are the basis of the system of soil classification used in the SEABEEs and by the military in general for the identification of soil types. A knowledge of these physical characteristics aids in determining the degree to which local soils can be used in engineering projects to support traffic loads or to serve as a subgrade or foundation material. Those characteristics of concern to the EA are discussed below.