Most natural soil deposits represent one of the followingprincipal types: river-channel deposits, flood-plain deposits,delta deposits, shore deposits, glacial deposits, windlaiddeposits, deposits formed by sedimentation in standingwater, and residual soils formed in place by weathering.The only ones likely to have a fairly regular structureare the flood-plain and wind-laid deposits and thoseformed in large bodies of standing water at a considerabledistance from the shore. All the others are likely to bedistinguished by large and erratic variations, at least inconsistency or relative density, and usually in grain sizeas well.In the upper reaches of river systems, river-channeldeposits generally occupy the bottoms of valleys carvedout of rock. In the lower reaches they may be laid downin winding and interlaced channels eroded out of thebroad sheet of fine-grained sediments that have previouslybeen deposited by the river under different conditionsof sedimentation. The average grain size decreases withincreasing distance from the source, and at any one pointit is likely to increase in a general way with increasingdepth below the surface. However, the details of stratificationare always erratic, and both grain size and relativedensity vary in an unpredictable manner. Still more abruptand conspicuous are the variations in glacial outwashdeposited by the melt waters along the rim of continentalice sheets.Flood-plain deposits are laid down during the highwaterseason on both sides of the lower courses of rivers.They commonly consist of continuous layers of silt orclay of fairly uniform thickness, separated from eachother by equally persistent layers of coarser sediments.However, at any point or line the continuity of these stratacan be broken by bodies of other sediments occupyingtroughs or abandoned river channels (Kolb and Shockley1959). If such a body is located between two drill holes,its presence may escape attention. Several well-knownfoundation accidents have been ascribed to this cause.Delta deposits are formed where water courses enterbodies of standing water. The main features of deltasare simple, but the details of their structure can be verycomplex, as shown in Fig. 12.1, because the currents thattransport the sediments shift continually.Shore deposits are composed of sediments that wereeroded by waves or carried into a body of standing waterby rivers and transported and deposited by shore currents.They generally consist of sand and gravel. However, asa result of important fluctuations in the lake or sea levelcombined with the shifting of the water courses that crossthe coastal belt, the sand and gravel deposits may alternatein an intricate manner with layers or pockets of silt, clay,or peat. Deposits of this kind are referred to as compositeshore deposits. Figure 12.7 and the upper half of Fig.10.2 illustrate the structure of deposits of this type.The constituents of glacial deposits were picked upand transported by ice and laid down when the ice melted.The wasting away of ice sheets always alternates withperiods of temporary growth and advancement. Theadvancing ice plows up or deforms previously depositedlayers of glacial material. Furthermore, at the ice rim

random sorting and shifting are carried on by the streamsof water that emerge from beneath the ice. Hence, glacialdeposits are among the most erratic with which the engineerhas to deal. Irregular pockets and lenses of fine- andcoarse-grained materials intermingled with boulders mayfollow each other in a chaotic manner (Fig. 12.1).In contrast to glacial deposits, wind-laid sediments areremarkably uniform. However, their boundaries may bevery irregular, because the wind may drop its burden inirregular heaps on very uneven surfaces. Furthermore,the fine-grained varieties known as loess (Article 2) maycompletely lose their original homogeneity on accountof local leaching or weathering. Many faulty foundationson loess have resulted from the failure of the designersto recognize the existence of these partial alterations.