FM 5-410 CHAPTER 2 Structural Geology Structural geology describes the form, pat- secondary structural features. These secon- tern, origin, and internal structure of rock dary features include folds, faults, joints, and and soil masses. Tectonics, a closely related schistosity. These features can be identified field, deals with structural features on a and m appeal in the field through site inves- larger regional, continental, or global scale. tigation and from remote imagery. Figure 2-1, page 2-2, shows the major plates of the earth’s crust. These plates continually Section I. Structural Features undergo movement as shown by the arrows. in Sedimentary Rocks Figure 2-2, page 2-3, is a more detailed repre- sentation of plate tectonic theory. Molten material rises to the earth’s surface at BEDDING PLANES midoceanic ridges, forcing the oceanic plates Structural features are most readily recog- to diverge. These plates, in turn, collide with nized in the sedimentary rocks. They are adjacent plates, which may or may not be of normally deposited in more or less regular similar density. If the two colliding plates are horizontal layers that accumulate on top of of approximately equal density, the plates each other in an orderly sequence. Individual will crumple, forming mountain range along deposits within the sequence are separated the convergent zone. If, on the other hand, by planar contact surfaces called bedding one of the plates is more dense than the other, planes (see Figure 1-7, page 1-9). Bedding it will be subducted, or forced below, the planes are of great importance to military en- lighter plate, creating an oceanic trench along gineers. They are planes of structural the convergent zone. Active volcanism and weakness in sedimentary rocks, and masses seismic activity can be expected in the vicinity of rock can move along them causing rock of plate boundaries. In addition, military en- slides. Since over 75 percent of the earth’s gineers must also deal with geologic features surface is made up of sedimentary rocks, that exist on a smaller scale than that of plate military engineers can expect to frequently tectonics but which are directly related to the encounter these rocks during construction. reformational processes resulting from the force and movements of plate tectonics. Undisturbed sedimentary rocks may be relatively uniform, continuous, and predict- The determination of geologic structure is able across a site. These types of rocks offer often made by careful study of the stratig- certain advantages to military engineers in raphy and sedimentation characteristics of completing horizontal and vertical construc- layered rocks. The primary structure or tion missions. They are relatively stable rock original form and arrangement of rock bod- bodies that allow for ease of rock excavation, ies in the earth’s crust is often altered by as they will normally support steep rock Structural Geology 2-1 FM 5-410 Structural Geology 2-2 FM 5-410 faces. Sedimentary rocks are frequently military engineers do not determine the sub- oriented at angles to the earth’s “horizontal” surface conditions before committing surface; therefore, movements in the earth’s resources to construction projects. Therefore, crust may tilt, fold, or break sedimentary where outcrops are scarce, deliberate excava- layers. Structurally deformed rocks add com- tions may be required to determine the type plexity to the site geology and may adversely and structure of subsurface materials. To affect military construction projects by con- determine the type of rock at an outcrop, the tributing to rock excavation and slope procedures discussed in Chapter 1 must be stability problems. followed. To interpret the structure of the bedrock, the military engineer must measure Vegetation and overlying soil conceal most and define the trend of the rock on the earth’s rock bodies and their structural features. surface. Outcrops are the part of a rock formation ex- posed at the earth’s surface. Such exposures, FOLDS or outcrops, commonly occur along hilltops, Rock strata react to vertical and horizontal steep slopes, streams, and existing road cuts forces by bending and crumpling. Folds are where ground cover has been excavated or undulating expressions of these forces. They eroded away (see Figure 2-3). Expensive are the most common type of deformation. delays and/or failures may result when Folds are most noticeable in layered rocks but Structural Geology 2-3 FM 5-410 rarely occur on a scale small enough to be ob- dips up to 90 degrees. The elevation of the served in a single exposure. Their size varies beds on opposite sides of the fold may differ by considerably. Some folds are miles across, hundreds or thousands of feet. Anticlines are while others may be less than an inch. Folds upfolds, and synclines are downfolds (see Fig- are of significant importance to military en- ure 2-5c and d, respectively). They are the gineers due to the change in attitude, or most common of all fold types and are typi- position, of bedding planes within the rock cally found together in a series of fold undula- bodies (see Figure 2-4). These can lead to rock tions. Differential weathering of the rocks excavation problems and slope instability. composing synclines and anticlines tends to Folds are common in sedimentary rocks in mountainous areas where their occurrence produce linear valleys and ridges. Folds that may be inferred from ridges of durable rock dip back into the ground at one or both ends strata that are tilted at opposite angles in are said to be plunging (see Figure 2-6). nearby rock outcrops. They may also be Plunging anticline and plunging syncline recognized by topographic and geologic map folds are common. Upfolds that plunge in all patterns and from aerial photographs. The directions are called domes. Folds that are presence of tilted rock layers within a region bowed toward their centers are called is usually evidence of folding. basins. Domes and basins normally exhibit roughly circular outcrop patterns on geologic Types maps. There are several basic types of folds. They are— Symmetry Homocline. Folds are further classified by their sym- Monocline. metry. Examples are- Anticline. Asymmetrical (inclined). Syncline. Symmetrical (vertical). Plunging. Overturned (greatly inclined). Dome. Recumbent (horizontal). Basin. The axial plane of a fold is the plane that A rock body that dips uniformly in one direc- bisects the fold as symmetrically as possible. tion (at least locally) is called a homocline (see The sides of the fold as divided by the axial Figure 2-5a). A rock body that exhibits local plane are called the limbs. In some folds, the steplike slopes in otherwise flat or gently in- plane is vertical or near vertical, and the fold clined rock layers is called a monocline (see is said to be symmetrical. In others, the axial Figure 2-5b). Monoclines are common in plane is inclined, indicating an asym- plateau areas where beds may locally assume metrical fold. If the axial plane is greatly Structural Geology 2-4 FM 5-410 Structural Geology 2-5 FM 5-410 inclined so that the opposite limbs dip in the mapped to help determine the structure of a same direction, the fold is overturned. A rock mass. Their attitudes can complicate recumbent fold has an axial plane that has rock excavation and, if unfavorable, lead to been inclined to the point that it is horizontal. slope stability problems. Figure 2-7 shows the components of an ideal- ized fold. An axial line or fold axis is the FAULTS intersection of the axial plane and a par- Faults are fractures along which there is ticular bed. The crest of a fold is the axis line displacement of the rock parallel to the frac- along the highest point on an anticline. The ture plane; once-continuous rock bodies have trough denotes the line along the lowest part of been displaced by movement in the earth’s the fold. It is a term associated with synclines. crust (see Figure 2-8). The magnitude of the displacement may be inches, feet, or even CLEAVAGE AND SCHISTOSITY miles along the fault plane. Overall fault dis- Foliation is the general term describing the placement often occurs along a series of small tendency of rocks to break along parallel sur- faults. A zone of crushed and broken rock faces. Cleavage and schistosity are foliation may be produced as the walls are dragged terms applied to metamorphic rocks. past each other. This zone is called a “fault Metamorphic rocks have been altered by heat zone” (see Figure 2-9). It often contains and/or pressure due to mountain building or crushed and altered rock, or “gouge,” and an- other crustal movements. They may have a gular fragments of broken rock called pronounced cleavage, such as the metamor- “breccia.” Fault zones may consist of phic rock slate that was at one time the materials that have been altered (reduced in sedimentary rock shale. Certain igneous strength) by both fault movement and ac- rocks may be deformed into schists or igneis- celerated weathering by water introduced ses with alignment of minerals to produce along the fault surface. Alteration of fault schistosity or gneissic foliation. The attitudes gouge to clay lowers the resistance of the of planes of cleavages and schistosity can be faulted rock mass to sliding. Recognition of Structural Geology 2-6 FM 5-410 faults is extremely important to military en- rock beds often indicates faulting, it may also gineers, as they represent potential weakness be caused by igneous intrusions and uncon- in the rock mass. Faults that cut very young formities in deposition. Faults that are not sediments may be active and create seismic visibly identifiable can be inferred by sudden (earthquake) damage.