CHAPTER 3 Surficial Geology · the vicinity of Philipp, Mississippi. Surficial Geology 3-5. FM 5-410 Stream Deposits Coarse-grained (gravels and sands) and fine-g-rained (silts and

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CHAPTER 3

S u r f i c i a l G e o l o g y

An integral part of the military engineer’smission is the location and processing ofmaterials for construction use. Most con-struction materials are derived from rocksand soils that occur naturally on or near thesurface of the earth. These materials may beobtained by developing a quarry or a borrowpit.

Quarries are sites where open excavationsare made into rock masses by drilling, cut-ting, or blasting for the purpose of producingconstruction aggregate. These operations re-quire extensive time, manpower, andmachinery. Borrow pits are sites where un-consolidated material has been deposited andcan be removed easily by common earth-moving machinery, generally withoutblasting.

This chapter covers the processes that formsurficial features which are suitable forpotential borrow pit operations and the typesof construction materials found in these fea-tures.

FLUVIAL PROCESSThe main process responsible for the

erosion and subsequent deposition ofweathered material suitable for the develop-ment of borrow pits is that of moving water.When water moves very quickly, as over asteep gradient, it picks up weatheredmaterial and carries it away. When thestream slows down (for example, when thegradient is reduced), the capacity of the

stream to carry the weathered materialdecreases; then it deposits the material in avariety of possible surficial features.

Stream deposits are characteristicallystratified (layered) and composed of particleswithin a limited size range. Fluvial depositsare sorted by size based on the velocity of thewater. When the velocity of the stream fallsbelow the minimum necessary to carry theload, deposition occurs beginning with theheaviest material. In this way, rivers buildgravel and sandbars on the inside of meanderloops and dump fine silts and muds outsidetheir levees during floods. This createsdeposits of reasonably well-sorted, naturalconstruction materials.

Drainage PatternsWithout the benefit of geologic maps, it is

difficult to determine the type and structureof the underlying rocks. However, by study-ing the drainage patterns as they appear on atopographic map, both the rock structure andcomposition may be inferred.

Many drainage patterns exist; however,the more common patterns are—

Rectangular.Parallel.Dendritic.Trellis.Radial.Annular.Braided.

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Rectangular. This pattern is characterizedby abrupt, nearly 90-degree changes instream directions. It is caused by faulting orjointing of the underlying bedrock. Rectan-gular drainage patterns are generallyassociated with massive igneous andmetamorphic rocks, although they may befound in any rock type. Rectangular drainageis a specific type of angular drainage and isusually a minor pattern associated with amajor type, such as dendritic (see Figure3-1a). Angular drainage is characterized bydistinct angles of stream juncture.

Parallel. This drainage is characterized bymajor streams trending in the same direction,Parallel streams are indicative of gently dip-ping beds or uniformly sloping topography.Extensive, uniformly sloping basalt flows andyoung coastal plains exhibit this type ofdrainage pattern. On a smaller scale, theslopes of linear ridges may also reflect thispattern (see Figure 3-1b).

Dendritic. This is a treelike pattern, com-posed of branching tributaries to a mainstream. It is characteristic of essentially flat-lying and/or relatively homogeneous rocks(see Figure 3-1c).

Trellis. This is a modified version oft h edendritic pattern. Tributaries generally flowperpendicular to the main streams and jointhem at right angles. This pattern is found inareas where sedimentary or metamorphicrocks have been folded and the main streamsnow follow the strike of the rock (see Figure3-1d).

Radial. This pattern, in which streams flowoutward from a high central area, is found ondomes, volcanic cones, or round hills (see Fig-ure 3-1e).

Annular. This pattern is usually associatedwith radial drainage where sedimentaryrocks are upturned by a dome structure. Inthis case, streams circle around a high centralarea (see Figure 3-1f).

Braided. A braided stream pattern com-monly forms in arid areas during flash


flooding or from the meltwater of a glacier.The stream attempts to carry more materialthan it is capable of handling. Much of thegravels and sands are deposited as bars andislands in the stream bed (see Figure 3-1g andFigure 3-2, page 3-4), Figure 3-2 shows thevicinity of Valdez, Alaska. Both the Copperand Tonsina Rivers are braided streams.

DensityThe nature and density of the drainage pat-

tern in an area provides a strong indicator asto the particle size of the soils that havedeveloped. Sands and gravels are usuallyboth porous and permeable. This means thatduring periods of precipitation, water perco-lates down through the sediment. Thedensity of the drainage and the surface runoffare minimal due to this good internaldrainage.

Clays and silts are normally porous but notpermeable. Most precipitated water is forcedto run off, creating a fine network of streamerosion.

Sandstone and shale may exhibit the sametype of drainage pattern. Sandstone, due toits porosity and permeability, has good inter-nal drainage while shale dots not. Therefore,the texture or density of the drainage pattern—which develops on the sandstone is coarsewhile that on shale is fine.

Stream EvolutionThe likelihood of finding construction

materials in a particular stream valley can becharacterized by the evolution of that valley.The evolutionary stages are described as—

Youth.Maturity.Old age.

Youth. Youthful stream valleys, which arelocated in highland areas, are typified bysteep gradients, high water velocities withrapids and waterfalls present, downcutting instream bottoms resulting in the creation ofV-shaped valleys, and the filling of the entire

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valley floor by the stream (see Figure 3-3a).Although there is considerable erosion takingplace, there is very little deposition.

Maturity. A mature system has a developedfloodplain and, while the stream no longerfills the entire valley floor, it meanders toboth edges of the valley. The stream gradientis medium to low, deposition of materials canbe found, and (when compared with theyouthful stream) there is less downcuttingand more lateral erosion that contributes towidening the valley (see Figure 3-3b).

Old Age. In an old-age system, the streamgradient is very gentle, and the water velocityis low. The river exhibits little downcutting,and lateral meandering produces an exten-sive floodplain. Because of the low watervelocity, there is a great amount of deposition.

The river only occupies a small portion of thefloodplain (see Figure 3-3c).

Recognition of the stream evolution stage ofa particular river system is required todevelop sources of construction aggregate.Rivers in maturity or old age provide thegreatest quantities of aggregate. In youthfulrivers, sources of aggregate are often scarce orunobtainable due to the steep gradients andhigh velocity. Table 3-1, page 3-6, sum-marizes the characteristics of each stage ofstream evolution, Figure 3-4, page 3-7, showsan example of the topographic expression of ayouthful stream valley in the vicinity ofPortage, Montana. Figure 3-5, page 3-8,shows a mature stream valley in the vicinityof Fort Leavenworth, Kansas. Figure 3-6,page 3-9, shows an old age stream valley inthe vicinity of Philipp, Mississippi.


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Stream DepositsCoarse-grained (gravels and sands) and

fine-g-rained (silts and clays) deposits can befound by map reconnaissance. Certain surfi-cial features are comprised of coarse-grainedmaterials, others are made up of medium-sized particles, and still others of fine-g-rainedsediments. However, if the source area for astream is composed only of fine- grainedmaterials, then the resulting depositionalfeatures will also contain fine-grainedsediments, regardless of their usual composi-tion.

The following surficial features can be iden-tified by their topographic expressions onmilitary maps and are likely sources of con-struction materials.


Point bars.Channel bars.Oxbow lakes.Natural levees.BackSwamps/floodplains.Alluvial terraces.Deltas.Alluvial fans.

Point Bars. Meandering is the process bywhich a stream is gradually deflected from astraight-line course by slight irregularities.Most streams that flow in wide, flat-flooredvalleys tend to meander (bend). Thesestreams are alternately cutting and fillingtheir channels, and as the deflection progres-ses, the force of the flowing waterconcentrates against the channel wall on the

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outside of the curve. This causes erosion onthat wall (a body in motion tends to remain inmotion in the same direction and with thesame velocity until acted on by an externalforce) (see Figure 3-7). Consequently, there isa decrease in velocity and carrying power ofthe water on the inside of the curve, and thegravels and sands are deposited, formingpoint bar deposits (see Figure 3-8). Point bardeposits on many maps will not be apparentbut can be inferred to be at the inside of eachmeander loop.

Channel Bars. When a stream passesthrough a meander loop, its speed increaseson the outer bank due to the greater volume ofwater that is forced to flow on the outside ofthe loop. When the stream leaves themeander and the channel straightens out, theforces that caused the stream to move fasterare no longer in control and the stream slowsdown and deposits materials. Thesematerials are coarse-grained (gravels andsands) and are on the opposite bank anddownstream of the point bar. If there is aseries of meander loops, these deposits mayormay not be present between point bars,depending on the spacing of the meanders.However, a channel bar can be expected afterthe last meander loop. Figure 3-9, page, 3-12,shows channel bar deposits, oxbow lakes, andbackswamp/floodplain deposits in the vicinityof Fort Leavenworth, Kansas. A prominentchannel bar is located north of Stigers Island.

Mud Lake, Burns Lake, and HorseshoeLake are oxbow lakes. Backswamps on thefloodplain are represented by swampy groundsymbols.

Oxbow Lakes. During high-water stages, astream that normally flows through ameander loop may cut through the neck of apoint, thus separating the loop. When thishappens, the stream has taken the path ofleast resistance and has isolated the bend.The cutoff meander bend is eventually sealedfrom the main stream by fine deposits. Thebend itself then forms an oxbow lake (see Fig-ure 3-10, page 3-13). These deserted loopsmay become stagnant lakes or bogs, or thewater may evaporate completely leaving a U-shaped depression in the ground.Fine-grained deposits (silts and clays) arenormally located in oxbow lakes. An old pointbar deposit can be found on the inside of the U(see Figure 3-11, page 3-13). In Figure 3-9,page 3-12. Horseshoe Lake is an example ofthe topographic expression of an oxbow lake,

Natural Levees. Stream velocity increasesduring flooding as the stream swells withinthe confines of its bank to move a greatervolume of water. As the stream moves faster,it has the ability to carry more material. Ifthe volume of water becomes so great that thewater cannot stay in the channel, the streamspills over its banks onto the surrounding


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floodplain, which is a flat expanse of land ad-jacent to a stream or river. Once the streamspills over its banks, the water velocitydecreases as the water spreads out to occupya larger area. As the velocity decreases, sedi-ment carried by the floodwater is deposited.The size of this material depends primarily onthe character of the material in the sourcearea upstream and the velocity of the water inthe stream channel. Generally, gravels andsands can be found in a natural levee, withthe larger material deposited near the streambank and a gradual gradation to smaller sandparticles away from the stream.

Backswamps/Floodplains. After a floodends and the stream regresses into its chan-nel, much of the water that spilled over thebanks onto the floodplain is trapped on theoutside of the natural levees. The finematerials (silts and clays) that are suspendedin this water settle onto the floodplain. Con-sequently, these areas are often used foragricultural production. In the lower-lyingareas of the floodplain, a large amount of finesmay accumulate, inhibit drainage, and formswamplike conditions called a backswamp(see Figure 3-9, page 3-12).

Alluvial Terraces. A depositing streamtends to fill its valley with a fair amount of

granular alluvial material. If a change in thegeological situation results in the uplift of alarge area or rejuvenation of the stream, anincrease in the stream velocity by othermeans, or a change in the sedimentation anderosion process, the stream may begin toerode away the material it had deposited pre-viously. As the eroding stream meandersabout in its new valley, it may leave benchlikeremnants of the preexisting valley fillmaterial perched against the valley walls asterraces. This action of renewed downcuttingmay occur several times, leaving several ter-race levels (see Figure 3-12). These are easilyrecognized on a topographic map becausethey show up as flat areas with no contourlines, alternating with steeply sloping regionswith many contour lines. Alluvial terracesusually occur on one side of the stream butcan be found on both sides. They are a normalfeature of the history of any fluvial valley.They are usually a good source of sandsand gravels. Figure 3-13 shows alluvialterraces in the vicinity of Souris River, NorthDakota.

Deltas. When streams carrying sedimentsin suspension flow into a body of standingwater, the velocity of the stream is immedi-ately and drastically reduced. As a result, thesedimentary load begins to settle out of


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suspension, with the heavier particles set-tling first. If the conditions in the body ofwater (sea or lake) are such that these par-ticles are not spread out over a large area bywave action, or if they are not carried away bycurrents, they continue to accumulate at themouth of the stream. Large deposits of thesesediments gradually build up to just abovethe water level to form deltas (see Figure3-14). These assume three general forms,

depending mainly on the relative influence ofwaves, fluvial processes, and tides. Theseforms are—

Arcuate (see Figure 3-15a and b).Bird’s-foot (see Figure 3-15c,),Elongate (see Figure 3-15d).

Arcuate deltas are arc- or fan-shaped andare formed when waves are the primary forceacting on the deposited material. Arcuate


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deltas usually result from deposition bystreams carrying relatively coarse material(sands and gravels) with some occasional finematerial. Arcuate deltas consisting primar-ily of coarse material have very good internaldrainage; therefore, they have few minorchannels. On the other hand, an arcuatedelta having a considerable amount of finematerial (silts and clays) mixed with thecoarse material does not have good internaldrainage. In this case, a larger number ofminor channels develop. Generally, arcuatedeltas are considered good sources of sandsand gravels. An example of an arcuate deltais the Nile Delta in Egypt.

Bird’s-foot deltas are formed in situationswhere fluvial processes have a major in-fluence on deposited sediments. Bird’s-footdeltas resemble a bird’s foot from the air,hence the name. They are generally com-posed of fine-g-rained material and have verypoor internal drainage. These deltas are flatwith vegetation, have many small outlets,and are a good source of fine materials. TheMississippi Delta is a classic example of thisdelta type.

Elongate deltas form where tidal currentshave a major impact on sediment deposition.

They contain only a few distributaries, butthe distributaries that occur are large.

Alluvial Fans. These are the dry landcounterpart of deltas. They are formed bystreams flowing from rough terrain, such asmountains or steep faults, onto a flat plain.This type of deposit is found in regions thathave an arid to semiarid type of climate, suchas the western interior, the Basin and RangeProvince of the United States, and the desertmountain areas worldwide. The valleys inthese areas are normally dry much of theyear, with streams resulting only after tor-rential rainstorms or following the springsnow melt. The mountains themselves aredevoid of vegetation, and erosion by thestreams is not impeded. These streams rushdown a steep gradient, and when they meetthe valley floor, there is a sudden reduction invelocity. The sediment load is deposited atthe foot of the rough terrain. This deposit isin the form of a broad “semitone” with theapex pointing upstream. Coalescing alluvialfans consist of a series of fans that have joinedto form one large feature. This is typical inarid areas. Figure 3-16 depicts alluvial andcoalescing alluvial fans. Alluvial fans maybe readily identified by their topographic


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expressions of concentric half-circular con-tour lines. Figure 3-17 is a topographic mapshowing the Cedar Creek alluvial fan in thevicinity of Ennis Lake, Montana. This al-luvial fan is approximately four miles inradius. Figure 3-18, page 3-20, shows coalesc-ing alluvial fans in the vicinity of Las Vegas,Nevada.

The types of materials found in alluvialfans are gravels, sands, and fines based on a1/3 rule. The first 1/3, the area adjacent tothe highland, is primarily composed ofgravels; the middle 1/3 is composed of sands;and the final 1/3, the area farthest from thehighland, is composed of fines.

Fluvial features are found throughout theworld and are the primary source of borrowpit materials for military engineers. Table3-2, page 3-21, and Figure 3-19, page 3-21,present a summary of fluvial features. Fig-ure 3-20, page 3-22, shows a generalizeddistribution of fluvial surficial featuresthroughout the world.

GLACIAL PROCESSBetween ten and twenty-five thousand

years ago, much of North America, Europe,and Northern Asia was covered by glaciers.Significant ice sheets still cover Greenlandand Antarctica, and lesser ice sheets can befound at high elevations and latitudes (seeFigure 3-21, page 3-23).

Glaciation produces great changes in theexisting topography by reshaping the landsurface and depositing new surficial featuresthat may serve as a source of construction ag-gregate for military engineers.

Types of GlaciationThe glaciation process may be described as

either continental or alpine glaciation.

Continental. Continental glaciation occurson a large, regional scale affecting vast areas.It may be characterized by the occurrence ofmore depositional features than erosionalfeatures. Continental glaciers can be oftremendous thickness and extent. They

move slowly in a plastic state with the icechurning the soil and rocks beneath it as wellas crushing and plucking rocks from theground and incorporating large amounts ofmaterial within the glacier itself. The overallrange of particle size of these materials isfrom clays through cobbles and boulders (seeFigure 3-22, page 3-24).

Alpine. Alpine or mountain glaciation takesplace in mountainous areas and generallyresults in the creation of mainly erosionalforms. Alpine glacial features are very dis-tinctive and easy to recognize. In the past,glaciers scooped out and widened the valleysthrough which they moved, producing valleyswith a U-shaped profile in contrast to theV-shaped profile produced by fluvial erosion(see Figure 3-23, page 3-25).

Glacial DepositsMaterials deposited by glaciers are fre-

quently differentiated into two types. Theyare—

Stratified.Unstratified.

Stratified. The features composed ofstratified deposits are actually the result ofdeposition of sediment by glacial streams(glaciofluvial) and not by the movement of theice itself. These features are—

Outwash plains.Eskers.Kames.Kame terraces.Glacial lake deposits.

They result when the material in the glacierhas been carried and deposited by meltwaterfrom the glacier. The water selectivelydeposits the coarsest materials, carryingthe fines away from the area. The endresult is essentially deposits of sands andgravels.

Outwash plains result when melting ice atthe edge of the glacier creates a great volumeof water that flows through the end moraineas a number of streams rather than as a con-tinuous sheet of water. Each of the streams


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builds an alluvial fan and each of the fansjoins together and forms a plain that slopesgently away from the end moraine area. Thecoarsest materiaI is deposited nearest the endmoraine, and the fines are deposited atgreater distances. Much of the prairie land inthe United States consists of outwash plains.Drainage and trafficability in the outwashplains are much better than in a groundmoraine; however, kettles can be formed in

outwash plains due to large masses of ice leftduring the recession of the ice front. If thekettles are numerous, the outwash area iscalled a pitted plain (see Figure 3-22, page3-24).

Eskers are winding ridges of irregularlystratified sands and gravels that are foundwithin the area of the ground moraine. Theridges are usually several miles long but are


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rarelv more than 45 to 60 feet wide or morethan 150 feet high. They are formed by waterthat flowed in tunnels or ice-walled gorges inor beneath the ice. They branch and wind likestream valleys but are not like ordinary val-leys in that they may cross normal drainagepatterns at an angle, and they may also passover hills (see Figures 3-22, page 3-24, andFigure 3-24, page 3-26). Figure 3-24, page3-26, shows kettle lakes, swamps, and es-kers.

A similar feature that resembles an esker,but is rarely more than a mile in length, is aridge known as a crevasse filling. A crevasseis a large, deep crevice or fissure on the sur-face of a glacier. Unsorted debris washes intothe crevasse, and when the surrounding icemelts, a ridge containing a considerableamount of fines is left standing.

Kames are conical hills of sands andgravels deposited by heavily laden glacialstreams that flowed on top of or off of theglacier. They are usually isolated hills thatare associated with the end or recessional

moraine; kettle lakes are commonly found inthe same area. The formation of kames nor-mally occurred when meltwater streamsdeposited relatively coarse materials in theform of a glacioalluvial fan at the edge of theice; the fine particles were washed away.This material accumulated along the side ofthe ice, and when the ice receded, thematerial slumped back on the side formerly incontact with the glacier.

Delta kames are another type of kame thatmay be formed when the meltwater flows intoa marginal lake and forms a delta. After thelake and the ice disappear, deltas are left asflat-topped, steep-sided hills of well-sortedsands and gravels (see Figure 3-22, page3-24).

Kame terraces are features associated withalpine glaciation. When the ice moves down avalley, it is in contact with the sides of the val-ley. As the glacier melts away from the valleywall, glacial water flows into the spacecreated between the side of the glacier and thevalley wall. The void is filled with gravels and


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sands, while the fines are carried away by thestream water. A terrace is left where the icewas in contact with the valley; gravels andsands can be found at the base of the terrace(see Figure 3-25).

Glacial lake deposits occur during the melt-ing of the glacier when many lakes and pondsare created by the meltwater in the outwashareas. The streams that fed these waterswere laden with glacial material. Most of thegravels and sands that were not depositedbefore reaching the lake accumulated as adelta (later to be called a delta kame) aftermelting of the ice. The fines that remainedsuspended in the water were, on the otherhand, deposited throughout the lake. Duringthe summer, a band consisting of light-colored, coarse silt was deposited, whereas athinner band of darker, finer-grainedmaterial was deposited in the winter. Thetwo bands together represent a time span ofone year and are referred to as a varve.

Unstratified. Unstratified glacial deposits(sediments deposited by the ice itself) are themost common of the of glacial deposits. Theycomprise the following surfical features:

Ground moraines.End moraines.

Recessional moraines.Drumlins.

Unstratified deposits make up landformsthat may be readily identified in the field, onaerial photographs, and from topographicand other maps. Unstratified deposits arecomposed of a heterogeneous mixture of par-ticle types and sizes ranging from clays toboulders. Till is the name given to this mix-ture of materials. It is the most widespread ofall the forms of glacial debris. In general, fea-tures comprised of till are undesirable assources of military construction aggregatesince the material must be washed andscreened to provide proper gradation.

Ground moraines, sometimes called tillplains, are deposits that are laid down aS theglacier recedes. Melting ice drops materialthat blankets the area over which the glaciertraveled. A deposit of this kind forms gentlyrolling plains. The deposit itself may be athin veneer of material lying on the bedrock,or it may be hundreds of feet thick. Morainesoil composition is complex and often indeter-minate. This variation in sediment makeupis due to the large variety of rocks and soilpicked up by the moving glacier (see Figure3-22, page 3-24).


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Morainic areas have a highly irregulardrainage pattern because of the haphazardarrangement of ridges and hills, althougholder till plains tend to develop dendritic pat-terns. Frequent features associated withgrouncl moraines are kettle holes andswamps. Kettles are usually formed by themelting of ice that had been surrounded by orembedded in the moraine material. Largeamounts of fines in the till prevent water frompercolating down through the soil. This mayallow for the accumulation of water in the ket-tle holes forming kettle lakes or, in low-lyingareas, swamps. Figure 3-22, page 3-24, andFigure 3-24, page 3-26, show ground morainewith an esker.

End moraines, sometimes called terminalmoraines, are ridges of till material that werepushed to their locations at the limit of theglacier’s advance by the forceful action of theice sheet. Generally, there is no one linearelement, such as a continuous ridge, evidentin either the field or on aerial photos. Nor-mally. this deposit appears as adiscontinuous chain of elongated to oval hills.These hills vary in height from tens tohundreds of feet. The till material is. attimes. quite clayey. Kettle lakes are some-times associated with terminal morainedeposits also (see Figure 3-22, page 3-24).

Recessional moraines. which are similar toend moraines. are produced when a recedingglacier halts its retreat for a considerableperiod of time. The stationary action allowsfor the accumulation of till material along theglacier’s edge. A series of these moraines may

result during the retreat of a glacier (see Fig-ure 3-22, page 3-24).

Drumlins are asymmetrical, streamlinedhills of gravel till deposited at the base of aglacier and oriented in a direction parallel toice flow. The stoss side (the side from whichthe ice flowed) of the drumlin is steeper andblunter than the lee side. The overall ap-pearance of a drumlin resembles an invertedspoon if viewed from above. Drumlins com-monly occur in groups of two or more.Individual drumlins are seldom more than½ mile long, and they can rise to heights of75 to 100 feet (see Figure 3-26 and Figure3-27).

It is important to understand that featuresformed from the glacial process only occur incertain areas of the world. Figure 3-28, page3-30, and Figure 3-29, page 3-31, illustratethe regions of the United States and the worldwhere glacial landforms occur. Table 3-3,page 3-32, is a summary of glacial surficialfeatures.

EOLIAN PROCESSIn arid areas where water is scarce, wind

takes over as the main erosional agent. Whena strong wind passes over a soil, it carriesmany particles of soil with it. The height anddistance the materials are transported is afunction of the particle size and the windvelocity. The subsequent decrease in thewind velocity gives rise to a set of wind-bornedeposits called eolian features.


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Types of Eolian ErosionThere are two types of wind erosion. They

are—Deflation.Abrasion.

Deflation. Deflation occurs when loose par-ticles are lifted and removed by the wind.This results in a lowering of the land surfaceas materials are carried away. Unlike streamerosion, in which downcutting is limited by a“base level” (usually sea level), deflation cancontinue lowering a land surface as long as ithas loose material to carry away. Deflationmay be terminated if the land surface is cutdown to the water table (moist soil is not car-ried away as easily) or if vegetation issufficient to hold the soil in place. In addi-tion, deflation may be halted when the supplyof fine material has been depleted. Thismakes a surface of gravel in the area wheredeflation has taken place. This gravel surfaceis known as desert pavement (see Figure 3-30,page 3-32).

Abrasion. Abrasion occurs when hard par-ticles are blown against a rock face causingthe rocks to break down. As fragments arebroken off, they are carried away by the wind.This process can grind down and polish rocksurfaces. A rock fragment with facets thathave been cut in this way is called a ventifact(see Figure 3-31, page 3-33).

Modes of TransportationSoil particles can be carried by the wind in

the following ways:

Bed Load. Material that is too heavy to becarried by the wind for great distances at atime (mainly sand-sized particles) bouncesalong the ground, rarely higher than two feet.

Suspended Load. These are fines (mostlysilts) that are easily carried by the wind.Suspended loads extend to high altitudes(sometimes thousands of feet) and can betransported for thousands of miles. During aparticularly bad dust storm in the mid-western “dust bowl” on 20 March 1935, the


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suspended load extended to altitudes of over Figure 3-32 illustrates the origin of these12,000 feet. The lowermost mile of the atmos-phere was estimated to contain over 166,000tons of suspended particles per cubic mile.Enough material was transported to bringtemporary twilight to New York and NewEngland (over 2,000 miles away) on 21March.

Eolian FeaturesEolian surficial features may consist of

gravels, sands, or fines. The three main typesof eolian features are-

Lag deposits or desert pavement.Sand dunes.Loess deposits.

deposits.

Lag Deposits or Desert Pavement. As thewind billows across the ground, sands andfines are continually removed. Eventually,gravels and pebbles that are too large to becarried by the wind cover the surface. Theseremnants accumulate into a sheet that ul-timately covers the finer-grained materialbeneath and protects it from further defla-tion. Desert pavement usually developsrapidly on alluvial fan and alluvial terracesurfaces. The exposed surface of the gravelsmay become coated with a black, glittery sub-stance termed desert varnish. In some


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locations, the evaporation of water, broughtto the surface by the capillary action of thesoil, may leave behind a deposit of calciumcarbonate (caliche) or gypsum. It acts as a ce-ment, hardening the pavement into aconglomeratelike slab.

Although desert pavement contains goodgravel material, the layers are normally toothin to supply the quantity required for con-struction. However, it does provide a roughbut very trafficable surface for all types ofvehicles and also provides excellent airfields.

Sand Dunes. Dunes may take severalforms, depending on the supply of sand, thelay of the land, vegetation restrictions, andthe steady direction of the wind. Theirgeneral expressions are as follows:

Transverse.Longitudinal.Barchan.Parabolic.Complex.

Transverse dunes are wavelike ridges thatare separated by troughs; they resemble seawaves during a storm. These dunes, whichare oriented perpendicular to the prevailingwind direction, occur in desert locationswhere a great supply of sand is present overthe entire surface. A collection of transversedunes is known as a sand sea (see Figure3-33a, page 3-34).

Longitudinal dunes have been elongated inthe direction of the prevailing winds. Theyusually occur where strong winds blow acrossareas of meager amounts of sand or where thewinds compete with the stabilizing effect ofgrass or small shrubs (see Figure 3-33b, page3-34).

Barchan dunes are the simplest and mostcommon of the dunes. A barchan is usuallycrescent-shaped, and the windward side has agentle’ slope rising to a broad dome that cutsoff abruptly to the leeward side. Barchansform in open areas where the direction of thewind is fairly constant and the ground is flatand unrestricted by vegetation and topog-raphy (see Figure 3-33c, page 3-34).

Parabolic, or U-shaped, dunes have tipsthat point upwind. They typically form alongcoastlines where the vegetation partiallycovers the sand or behind a gap in anobstructing ridge. Later, a parabolic dunemay detach itself from the site of formationand migrate independently (see Figure 3-33d,page 3-34).


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Complex dunes lack a distinct form anddevelop where wind directions vary, sand isabundant, and vegetation may interfere.These can occur locally when other dunetypes become overcrowded and overlap,thereby losing their characteristic shapes in adisorder of varying slopes (see Figure 3-33e).

Loess Deposits. In a number of regions ofthe world, thick accumulations of yellowish-brown material composed primarily ofwindblown silts make up a substantialamount of surface area. These deposits areknown as loess. The material that makes upthese deposits originated mainly from driedglacial outwash, floodplains, or desert areafines. Loess is composed of’ physically groundrock rather than of chemically weathered

the deposits may range in thickness from afew feet to hundreds of feet. Thickness tendsto decrease with distance from the source. Inthe United States, most of Kansas, Nebraska,Iowa, and Illinois are covered by loess. Aftera loess has been laid down, it is rarely pickedup again. This is due to a very thin layer offines that interlock after wetting. While dryloess is trafficable, it loses all strength with aslight amount of water (see Figure 3-34).

Eolian features occur worldwide and mayconsist of areas of sand dunes and desertpavement or loess; however, their topogra-phic expressions vary. In general, dune areasare specified on maps by special topographicsymbols since they are continually chang-ing unless stabilized by vegetation. Figure3-35, page 3-36, is a topographic expres-sion, using special symbols, of sand

material. The source and deposition point forthe material may be many miles apart, and


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dunes and desert pavement (Summan). Fig- provide construction aggregate to meet mis-ure 3-36 shows the generalized distribution sion requirements. Generally, engineer unitsof eolian landforms throughout the world. attempt to develop borrow pit operations in

fluvial features since they are easy to identifySOURCES OF CONSTRUCTION and are normally accessible. In arid and

AGGREGATE semiarid regions, eolian deposits and alluvialMilitary engineers use their knowledge of fans provide large amounts of aggregate. In

surficial features to develop borrow pits and mountainous regions and continentally


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glaciated regions, fluvial-glacial deposits can materials. Table 3-4 summarizes the types ofprovide large quantities of quality aggregate. aggregates found in common fluvial, glacial,Therefore, their presence should not neces- and eolian surficial features.sarily be discounted in preference to fluvial


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CHAPTER 3 Surficial Geology · the vicinity of Philipp, Mississippi. Surficial Geology 3-5. FM 5-410 Stream Deposits Coarse-grained (gravels and sands) and fine-g-rained (silts and