Fold (geology)

In structural geology, folds occur when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales.

Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature gradient, as evidenced by their presence in soft sediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones. Folds are commonly formed by shortening of existing layers, but may also be formed as a result of displacement on a non-planar fault (fault bend fold), at the tip of a propagating fault (fault propagation fold), by differential compaction or due to the effects of a high-level igneous intrusion e.g. above a laccolith.

Spaghetti Rock
Folds in Paleoproterozoic marble in Nunavut, Canada (with hammer for scale).
Agiospavlos DM 2004 IMG002 Felsenformation
Folds in alternating layers of limestone and chert in Crete, Greece.
Fold98
An asymmetric angular fold in Ukrainian Carpathians in Dora (near Jaremcze, Ivano-Frankivsk region, West Ukraine

Describing folds

Flank & hinge
Fold terminology. For more general fold shapes, a hinge curve replaces the hinge line, and a non-planar axial surface replaces the axial plane.
Cylindrical fold
Cylindrical fold with axial surface not a plane.[1]

Folds are classified by their size, fold shape, tightness, and dip of the axial plane.[2]

Fold terminology in two dimensions

A fold surface seen in profile can be divided into hinge and limb portions. The limbs are the flanks of the fold and the hinge is where the flanks join together. The hinge point is the point of minimum radius of curvature (maximum curvature) for a fold. The crest of the fold is the highest point of the fold surface, and the trough is the lowest point. The inflection point of a fold is the point on a limb at which the concavity reverses; on regular folds, this is the midpoint of the limb.

Fold terminology in three dimensions

The hinge points along an entire folded surface form a hinge line, which can be either a crest line or a trough line. The trend and plunge of a linear hinge line gives you information about the orientation of the fold. To more completely describe the orientation of a fold, one must describe the axial surface. The axial surface is the surface defined by connecting all the hinge lines of stacked folding surfaces. If the axial surface is a planar surface then it is called the axial plane and can be described by the strike and dip of the plane. An axial trace is the line of intersection of the axial surface with any other surface (ground, side of mountain, geological cross-section).

Finally, folds can have, but don't necessarily have a fold axis. A fold axis, “is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold.” (Davis and Reynolds, 1996 after Donath and Parker, 1964; Ramsay 1967). A fold that can be generated by a fold axis is called a cylindrical fold. This term has been broadened to include near-cylindrical folds. Often, the fold axis is the same as the hinge line.[3][4]

Fold shape

A fold can be shaped as a chevron, with planar limbs meeting at an angular axis, as cuspate with curved limbs, as circular with a curved axis, or as elliptical with unequal wavelength.

Fold tightness

Fold tightness is defined by the size of the angle between the fold's limbs (as measured tangential to the folded surface at the inflection line of each limb), called the interlimb angle. Gentle folds have an interlimb angle of between 180° and 120°, open folds range from 120° to 70°, close folds from 70° to 30°, and tight folds from 30° to 0°.[5] Isoclines, or isoclinal folds, have an interlimb angle of between 10° and zero, with essentially parallel limbs.

Fold symmetry

Not all folds are equal on both sides of the axis of the fold. Those with limbs of relatively equal length are termed symmetrical, and those with highly unequal limbs are asymmetrical. Asymmetrical folds generally have an axis at an angle to the original unfolded surface they formed on.

Deformation style classes

Folds that maintain uniform layer thickness are classed as concentric folds. Those that do not are called similar folds. Similar folds tend to display thinning of the limbs and thickening of the hinge zone. Concentric folds are caused by warping from active buckling of the layers, whereas similar folds usually form by some form of shear flow where the layers are not mechanically active. Ramsay has proposed a classification scheme for folds that often is used to describe folds in profile based upon curvature of the inner and outer lines of a fold, and the behavior of dip isogons. that is, lines connecting points of equal dip on adjacent folded surfaces:[6]

Ramsay Classification
Ramsay classification of folds by convergence of dip isogons (red lines).[7]
Ramsay classification scheme for folds
Class Curvature C Comment
 1 Cinner > Couter Dip isogons converge
    1A Orthogonal thickness at hinge narrower than at limbs
    1B Parallel folds
    1C Orthogonal thickness at limbs narrower than at hinge
 2 Cinner = Couter Dip isogons are parallel: similar folds
 3 Cinner < Couter Dip isogons diverge

Fold types

NJ Route 23 anticline
An anticline in New Jersey
Monocline
A monocline at Colorado National Monument
Caledonian orogeny fold in King Oscar Fjord
Recumbent fold, King Oscar Fjord
  • Anticline: linear, strata normally dip away from axial center, oldest strata in center irrespective of orientation.
  • Syncline: linear, strata normally dip toward axial center, youngest strata in center irrespective of orientation.
  • Antiform: linear, strata dip away from axial center, age unknown, or inverted.
  • Synform: linear, strata dip toward axial center, age unknown, or inverted.
  • Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.
  • Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.
  • Monocline: linear, strata dip in one direction between horizontal layers on each side.
  • Chevron: angular fold with straight limbs and small hinges
  • Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold.
  • Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.
  • Ptygmatic: Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.
  • Parasitic: short wavelength folds formed within a larger wavelength fold structure - normally associated with differences in bed thickness[8]
  • Disharmonic: Folds in adjacent layers with different wavelengths and shapes[8]

(A homocline involves strata dipping in the same direction, though not necessarily any folding.)

Causes of folding

Folds appear on all scales, in all rock types, at all levels in the crust. They arise from a variety of causes.

Layer-parallel shortening

When a sequence of layered rocks is shortened parallel to its layering, this deformation may be accommodated in a number of ways, homogeneous shortening, reverse faulting or folding. The response depends on the thickness of the mechanical layering and the contrast in properties between the layers. If the layering does begin to fold, the fold style is also dependent on these properties. Isolated thick competent layers in a less competent matrix control the folding and typically generate classic rounded buckle folds accommodated by deformation in the matrix. In the case of regular alternations of layers of contrasting properties, such as sandstone-shale sequences, kink-bands, box-folds and chevron folds are normally produced.[9]

Rollover
Rollover anticline
Faultbendfold
Ramp anticline
Thrust with fault propagation fold
Fault-propagation fold

Fault-related folding

Many folds are directly related to faults, associated with their propagation, displacement and the accommodation of strains between neighbouring faults.

Fault bend folding

Fault-bend folds are caused by displacement along a non-planar fault. In non-vertical faults, the hanging-wall deforms to accommodate the mismatch across the fault as displacement progresses. Fault bend folds occur in both extensional and thrust faulting. In extension, listric faults form rollover anticlines in their hanging walls.[10] In thrusting, ramp anticlines form whenever a thrust fault cuts up section from one detachment level to another. Displacement over this higher-angle ramp generates the folding.[11]

Fault propagation folding

Fault propagation folds or tip-line folds are caused when displacement occurs on an existing fault without further propagation. In both reverse and normal faults this leads to folding of the overlying sequence, often in the form of a monocline.[12]

Detachment folding

When a thrust fault continues to displace above a planar detachment without further fault propagation, detachment folds may form, typically of box-fold style. These generally occur above a good detachment such as in the Jura Mountains, where the detachment occurs on middle Triassic evaporites.[13]

Folding in shear zones

Dextral shear folds
Dextral sense shear folds in mylonites within a shear zone, Cap de Creus

Shear zones that approximate to simple shear typically contain minor asymmetric folds, with the direction of overturning consistent with the overall shear sense. Some of these folds have highly curved hinge-lines and are referred to as sheath folds. Folds in shear zones can be inherited, formed due to the orientation of pre-shearing layering or formed due to instability within the shear flow.[14]

Folding in sediments

Recently-deposited sediments are normally mechanically weak and prone to remobilisation before they become lithified, leading to folding. To distinguish them from folds of tectonic origin, such structures are called synsedimentary (formed during sedimentation).

Slump folding: When slumps form in poorly consolidated sediments, they commonly undergo folding, particularly at their leading edges, during their emplacement. The asymmetry of the slump folds can be used to determine paleoslope directions in sequences of sedimentary rocks.[15]

Dewatering: Rapid dewatering of sandy sediments, possibly triggered by seismic activity, can cause convolute bedding.[16]

Compaction: Folds can be generated in a younger sequence by differential compaction over older structures such as fault blocks and reefs.[17]

Igneous intrusion

The emplacement of igneous intrusions tends to deform the surrounding country rock. In the case of high-level intrusions, near the Earth's surface, this deformation is concentrated above the intrusion and often takes the form of folding, as with the upper surface of a laccolith.[18]

Flow folding

Advancing ramp in incompetent layers
Flow folding: depiction of the effect of an advancing ramp of rigid rock into compliant layers. Top: low drag by ramp: layers are not altered in thickness; Bottom: high drag: lowest layers tend to crumple.[19]

The compliance of rock layers is referred to as competence: a competent layer or bed of rock can withstand an applied load without collapsing and is relatively strong, while an incompetent layer is relatively weak. When rock behaves as a fluid, as in the case of very weak rock such as rock salt, or any rock that is buried deeply enough, it typically shows flow folding (also called passive folding, because little resistance is offered): the strata appear shifted undistorted, assuming any shape impressed upon them by surrounding more rigid rocks. The strata simply serve as markers of the folding.[20] Such folding is also a feature of many igneous intrusions and glacier ice.[21]

Folding mechanisms

NAT F2foldsF1
Example of a large-scale crenulation, an example of chevron-type flexural-slip folds in the Glengarry Basin, W.A.

Folding of rocks must balance the deformation of layers with the conservation of volume in a rock mass. This occurs by several mechanisms.

Flexural slip

Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata, which, altogether, result in deformation. A good analogy is bending a phone book, where volume preservation is accommodated by slip between the pages of the book.

The fold formed by the compression of competent rock beds is called "flexure fold".

Buckling

Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by layer parallel shortening the volume, which grows in thickness. Folding under this mechanism is typically of the similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically.

Mass displacement

If the folding deformation cannot be accommodated by flexural slip or volume-change shortening (buckling), the rocks are generally removed from the path of the stress. This is achieved by pressure dissolution, a form of metamorphic process, in which rocks shorten by dissolving constituents in areas of high strain and redepositing them in areas of lower strain. Folds created in this way include examples in migmatites, and areas with a strong axial planar cleavage.

Mechanics of folding

Folds in rock are formed in relation to the stress field in which the rocks are located and the rheology, or method of response to stress, of the rock at the time at which the stress is applied.

The rheology of the layers being folded determines characteristic features of the folds that are measured in the field. Rocks that deform more easily form many short-wavelength, high-amplitude folds. Rocks that do not deform as easily form long-wavelength, low-amplitude folds.

See also

Notes

  1. ^ DD Pollard; RC Fletcher (2005). "Figure 3.14: Geometric attributes of folded geological surfaces". Fundamentals of Structural Geology. Cambridge University Press. p. 92. ISBN 0-521-83927-0.
  2. ^ For a discussion of fold nomenclature, see for example, Robert J. Twiss; Eldridge M. Moores (1992). Structural geology (2nd ed.). Macmillan. pp. 220–221. ISBN 0-7167-2252-6.
  3. ^ Sudipta Sengupta; Subir Kumar Ghosh; Kshitindramohan Naha (1997). Evolution of geological structures in micro- to macro-scales. Springer. p. 222. ISBN 0-412-75030-9.
  4. ^ RG Park (2004). "Fold axis and axial plane". Foundations of structural geology (3rd ed.). Routledge. p. 26. ISBN 0-7487-5802-X.
  5. ^ Lisle, Richard J (2004). "Folding". Geological Structures and Maps: 3rd Edition. Elsevier. p. 33. ISBN 0-7506-5780-4.
  6. ^ See, for example, R. G. Park (2004). "Figure 3.12: Fold classification based upon dip diagrams". Foundations of structural geology (3rd ed.). Routledge. p. 31 ff. ISBN 0-7487-5802-X.
  7. ^ Neville J. Price; John W. Cosgrove (1990). "Figure 10.14: Classification of fold profiles using dip isogon patterns". Analysis of geological structures. Cambridge University Press. p. 246. ISBN 0-521-31958-7.
  8. ^ a b Park, R.G. (2004). Foundation of Structural Geology (3 ed.). Routledge. p. 33. ISBN 978-0-7487-5802-9.
  9. ^ Ramsay, J.G.; Huber M.I. (1987). The techniques of modern structural geology. 2 (3 ed.). Academic Press. p. 392. ISBN 978-0-12-576922-8. Retrieved 2009-11-01.
  10. ^ Withjack, M.O.; Schlische (2006). "Geometric and experimental models of extensional fault-bend folds". In Buiter S.J.H. & Schreurs G. (ed.). Analogue and numerical modelling of crustal-scale processes. Special Publications 253. R.W. Geological Society, London. pp. 285–305. ISBN 978-1-86239-191-8. Retrieved 2009-10-31.
  11. ^ Rowland, S.M.; Duebendorfer E.M.; Schieflebein I.M. (2007). Structural analysis and synthesis: a laboratory course in structural geology (3 ed.). Wiley-Blackwell. p. 301. ISBN 978-1-4051-1652-7. Retrieved 2009-11-01.
  12. ^ Jackson, C.A.L.; Gawthorpe R.L.; Sharp I.R. (2006). "Style and sequence of deformation during extensional fault-propagation" (PDF). Journal of Structural Geology. 28 (3): 519–535. Bibcode:2006JSG....28..519J. doi:10.1016/j.jsg.2005.11.009. Retrieved 2009-11-01.
  13. ^ Reicherter, K., Froitzheim, N., Jarosinki, M., Badura, J., Franzke, H.-J., Hansen, M., Hübscher, C., Müller, R., Poprawa, P., Reinecker, J., Stackebrandt, W, Voigt,T., von Eynatten, H. & Zuchiewicz, W. (2008). "19. Alpine Tectonics north of the Alps". In McCann, T. (ed.). The Geology of Central Europe. Geological Society, London. pp. 1233–1285. ISBN 978-1-86239-264-9. Retrieved 2009-10-31.CS1 maint: uses authors parameter (link)
  14. ^ Carreras, J.; Druguet E.; Griera A. (2005). "Shear zone-related folds". Journal of Structural Geology. 27 (7): 1229–1251. Bibcode:2005JSG....27.1229C. doi:10.1016/j.jsg.2004.08.004. Retrieved 2009-10-31.
  15. ^ Bradley, D.; Hanson L. (1998). "Paleoslope Analysis of Slump Folds in the Devonian Flysch of Maine" (PDF). Journal of Geology. 106: 305–318. Bibcode:1998JG....106..305B. doi:10.1086/516024. Retrieved 2009-10-31.
  16. ^ Nichols, G. (1999). "17. Sediments into rocks: post-depositional processes". Sedimentology and stratigraphy. Wiley-Blackwell. p. 355. ISBN 978-0-632-03578-6. Retrieved 2009-10-31.
  17. ^ Hyne, N.J. (2001). Nontechnical guide to petroleum geology, exploration, drilling, and production. PennWell Books. p. 598. ISBN 978-0-87814-823-3. Retrieved 2009-11-01.
  18. ^ Orchuela, I.; Lara M.E.; Suarez M. (2003). "Productive Large Scale Folding Associated with Igneous Intrusions: El Trapial Field, Neuquen Basin, Argentina" (PDF). AAPG abstracts. Retrieved 2009-10-31.
  19. ^ Arvid M. Johnson; Raymond C. Fletcher (1994). "Figure 2.6". Folding of viscous layers: mechanical analysis and interpretation of structures in deformed rock. Columbia University Press. p. 87. ISBN 0-231-08484-6.
  20. ^ Park, R.G. (1997). Foundations of structural geology (3rd ed.). Routledge. p. 109. ISBN 0-7487-5802-X.; RJ Twiss; EM Moores (1992). "Figure 12.8: Passive shear folding". Structural geology (2nd ed.). Macmillan. pp. 241–242. ISBN 0-7167-2252-6.
  21. ^ Hudleston, P.J. (1977). "Similar folds, recumbent folds and gravity tectonics in ice and rocks". Journal of Geology. 85: 113–122. Bibcode:1977JG.....85..113H. doi:10.1086/628272. JSTOR 30068680.

General references

  • David D. Pollard; Raymond C. Fletcher (2005). Fundamentals of structural geology. Cambridge University Press. ISBN 0-521-83927-0.
  • Davis, George H.; Reynolds, Stephen J. (1996). "Folds". Structural Geology of Rocks and Regions. New York, John Wiley & Sons. pp. 372–424. ISBN 0-471-52621-5.
  • Donath, F.A., and Parker, R.B., 1964, Folds and Folding: Geological Society of America Bulletin, v. 75, p. 45-62
  • McKnight, Tom L; Hess, Darrel (2000). "The Internal Processes: Folding". Physical Geography: A Landscape Appreciation. Upper Saddle River, NJ: Prentice Hall. pp. 409–14. ISBN 0-13-020263-0.
  • Ramsay, J.G., 1967, Folding and fracturing of rocks: McGraw-Hill Book Company, New York, 560p.
  • Lisle, Richard J (2004). "Folding". Geological Structures and Maps: 3rd Edition. Elsevier. p. 33. ISBN 0-7506-5780-4.
Anticlinal

Anticlinal may refer to:

Anticline, in structural geology, an anticline is a fold that is convex up and has its oldest beds at its core.

Anticlinal, in stereochemistry, a torsion angle between 90° to 150°, and –90° to –150°; see Alkane_stereochemistry

Foliation (geology)

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others). The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation (orogenic belts).

More technically, foliation is any penetrative planar fabric present in metamorphic rocks. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and gneiss. The slatey cleavage typical of slate is due to the preferred orientation of microscopic phyllosilicate crystals. In gneiss, the foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as S-tectonite in sheared rock masses.

Examples include the bands in gneiss (gneissic banding), a preferred orientation of planar large mica flakes in schist (Schistocity), the preferred orientation of small mica flakes in phyllite (with its planes having a silky sheen, called phylitic luster – the Greek word, phyllon, also means "leaf"), the extremely fine grained preferred orientation of clay flakes in slate (called "slaty cleavage"), and the layers of flattened, smeared, pancake-like clasts in metaconglomerate.

Hidaka Mountains

Hidaka Mountains (日高山脈, Hidaka-sanmyaku) is a mountain range in southeastern Hokkaido, Japan. It runs 150 km from Mount Sahoro or Karikachi Pass in central Hokkaidō south, running into the sea at Cape Erimo. It consists of folded mountains that range from 1,500 to 2,000 metres in height. Mount Poroshiri is the highest at 2,053 m. The Hidaka Mountains separate the subprefectures of Hidaka and Tokachi. Most of the range lies in the Hidaka-sanmyaku Erimo Quasi-National Park (日高山脈襟裳国定公園, Hidaka-sanmyaku Erimo Kokutei-kōen). Since the mountain range lies so far north, the alpine climate zone lies at a lower altitude.

Lineation (geology)

Lineations in structural geology are linear structural features within rocks. There are several types of lineations, intersection lineations, crenulation lineations, mineral lineations and stretching lineations being the most common. Lineation field measurements are recorded as map lines with a plunge angle and azimuth.

Mount Karifuri

Mount Karifuri (狩振岳, Karifuri-dake) is a mountain in the Hidaka Mountains of Hokkaidō, Japan. The mountain sits on the border between Minamifurano and Shimukappu. It is 1,323.2 metres (4,341.2 ft) high. It is the source of the Mu River (Hokkaidō).Mount Karifuri is split between two different rock types. The western side consists of plutonic rock formed 40–32 million years ago. The eastern side is made of metamorphic rock formed under low-to-mid pressure 50–20 million years ago.

Mount Memuro

Mount Memuro (芽室岳, Memuro-dake) is located in the Hidaka Mountains, Hokkaidō, Japan. The western summit of Mount Memuro (1746 m) is named Mount Pankenūshi.

Mount Odasshu

Mount Odasshu (オダッシュ山, Odasshu-yama) is located in the Hidaka Mountains, Hokkaidō, Japan. The Yasuda River route leads to the peak.

Mount Pankenūshi

Mount Pankenūshi (パンケヌーシ岳, Pankenūshi-dake) or Mount Memuro Western Peak (芽室岳西峰, Memuro-dake Sei-hō) is located in the Hidaka Mountains, Hokkaidō, Japan. This mountain is the western summit of Mount Memuro.

Mount Pekerebetsu

Mount Pekerebetsu (ペケレベ ツ岳, Pekerebetsu-dake) is located in the Hidaka Mountains, Hokkaidō, Japan. The Nisshō Pass route leads to the peak.

Mount Penkenūshi

Mount Penkenūshi (ペンケヌーシ岳, Penkenūshi-dake) is located in the Hidaka Mountains, Hokkaidō, Japan. It is the source of the Penkenūshi River (ペンケヌーシ川, Penkenūshi-gawa).

Mount Sahoro

Mount Sahoro (佐幌岳, Sahorodake) is located in the Hidaka Mountains, Hokkaidō, Japan. It is the site of the Sahoro Ski Resort.

There are two routes up the mountain:

Karikachi Pass route

Sahoro Ski Resort route

Mount Tomamu

Mount Tomamu (トマム山, Tomamu-san) is located in the Hidaka Mountains, Hokkaidō, Japan. It is the site of the Alpha Resort Tomamu, a ski resort.

Mount Tsurugi (Hokkaido)

Mount Tsurugi (剣山, Tsurugi-san) is located in the Hidaka Mountains, Hokkaido, Japan. The Mount Tsurugi Shrine route leads up to the peak.

Pressure solution

In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress within the same rock or their complete removal from the rock within the fluid. It is an example of diffusive mass transfer.The detailed kinetics of the process was reviewed by Rutter, and since then such kinetics has been used in

many applications in earth sciences.

Stylolite

Stylolites or styolite (Greek: stylos, pillar; lithos, stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a process that decreases the total volume of rock. Insoluble minerals, such as clays, pyrite and oxides, as well as insoluble organic matter, remain within the stylolites and make them visible. Sometimes host rocks contain no insoluble minerals, in which case stylolites can be recognized by change in texture of the rock. They occur most commonly in homogeneous rocks, carbonates, cherts, sandstones, but they can be found in certain igneous rocks and ice. Their size vary from microscopic contacts between two grains (microstylolites) to large structures up to 20 m in length and up to 10 m in amplitude in ice. Stylolites usually form parallel to bedding, because of overburden pressure, but they can be oblique or even perpendicular to bedding, as a result of tectonic activity.

Teshio Mountains

Teshio Mountains (天塩山地 Teshio-sanchi) is a mountain range of Hokkaidō, Japan.

Tussey Mountain

Tussey Mountain is a stratigraphic ridge in central Pennsylvania, United States, trending east of the Bald Eagle, Brush, Dunning and Evitts Mountain ridges. Its southern foot just crosses the Mason–Dixon line near Flintstone, Maryland, running north 130 km (80 mi) to the Seven Mountains of central Pennsylvania, near Tusseyville, making it one of the longest named ridges in this section of the Ridge-and-valley Appalachians. The ridge line separates Morrison Cove from the Woodcock Valley and Friends Cove from the Black Valley. Tussey Mountain lies in, and the ridge line forms parts of the borders of, Centre, Blair, Bedford and Huntingdon counties.

The Flintstone Creek runs around the southern end of the mountain in Maryland. North of there, small streams run through deep gorges, the Sweet Root and Rainsburg Gaps, near Martin Hill. At Everett the Pennsylvania Turnpike, U.S. Route 30, and the abandoned Huntingdon and Broad Top Mountain Railroad follow the Raystown Branch Juniata River through a deep water gap known as The Narrows. The Yellow Creek runs through Loysburg Gap at Loysburg, Pennsylvania. Maple Run Road passes through a wind gap near Pulpit Hill and Coot Hill, heading west to Woodbury. Pennsylvania Route 164 runs east out of Martinsburg, and climbs the west slope with a switchback before crossing the crest. The Frankstown Branch Juniata River runs north along the west foot of the ridge before turning east along U.S. Route 22 at Water Street, the river, road, and rail (abandoned PRR Petersburg Branch) crossing the ridge line through a water gap. The Little Juniata River passes through a nearby water gap at Spruce Creek along with the former Pennsylvania Railroad Main Line, which tunnels through a spur of the mountain to cut across a loop in the river. Galbraith Run passes through Galbraith Gap near the north end of the ridge, adjacent to the Tussey Mountain Ski Area in Boalsburg.

The Tussey Mountain Ridge is popular with soaring birds and glider pilots ridge soaring along its slopes. This ridge is part of a chain of ridges that stretch south to Tennessee. Tussey Mountain has been designated a Pennsylvania Important Bird Area (IBA), based primarily on its importance as a spring raptor migration site, but also as a long corridor of intact forest habitat, over 50% of which is publicly owned [1]. It is one of the best sites in the eastern United States for viewing the migration of the golden eagle. Pennsylvania's longest footpath, Mid State Trail, is atop or closely parallels Tussey Mountain for nearly its entire length.

In Blair County, Tussey Mountain is sometimes called Huntingdon Mountain, as one reaches Huntingdon by crossing it going east. Conversely, in some parts of Huntingdon County it is called Williamsburg Mountain as one reaches Williamsburg by crossing it going west.Pennsylvania State Game Lands Number 118 is located along Tussey Mountain in Blair and Huntingdon Counties.

Underlying theory
Measurement conventions
Large-Scale Tectonics
Fracturing
Faulting
Foliation and Lineation
Folding
Boudinage
Kinematic Analysis
Shear zone

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