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Assam Higher Secondary Geology study materials
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Assam Higher Secondary Geology practice questions

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Question 1Geology

Which rock type forms directly from the cooling and solidification of molten material?

  1. Bioclastic limestone
  2. Sedimentary rock
  3. Igneous rock Correct answer.
  4. Metamorphic rock

Explanation

Igneous rocks crystallise from magma or lava as it cools, so they are the primary product of melt solidification. Sedimentary rocks form by deposition and lithification of grains or chemical precipitates, and metamorphic rocks form by recrystallisation of pre-existing rocks under heat and pressure. Bioclastic limestone is a sedimentary rock built from shell fragments.

Question 2Geology

In the geological time scale, the Phanerozoic eon is subdivided, from oldest to youngest, into which three eras?

  1. Mesozoic, Palaeozoic, Cenozoic
  2. Cenozoic, Mesozoic, Palaeozoic
  3. Archaean, Proterozoic, Palaeozoic
  4. Palaeozoic, Mesozoic, Cenozoic Correct answer.

Explanation

The Phanerozoic eon comprises the Palaeozoic, Mesozoic and Cenozoic eras in ascending order of time. The Archaean and Proterozoic belong to the Precambrian, which precedes the Phanerozoic. Reversing the order would place the age of mammals before the age of invertebrates.

Question 3Geology

The property of a mineral describing the colour of its powder when rubbed on unglazed porcelain is called:

  1. Hardness
  2. Cleavage
  3. Streak Correct answer.
  4. Lustre

Explanation

Streak is the colour of the fine powder left when a mineral is rubbed across a porcelain streak plate, and it is often more diagnostic than the colour of the bulk specimen. Lustre describes how a surface reflects light, cleavage describes planar breaking, and hardness measures resistance to scratching.

Question 4Geology

A fault in which the hanging wall has moved down relative to the footwall is best described as:

  1. A normal fault Correct answer.
  2. A strike-slip fault
  3. A reverse fault
  4. A thrust fault

Explanation

In a normal fault the hanging wall block moves down relative to the footwall, which is consistent with crustal extension. Reverse and thrust faults show the opposite sense, with the hanging wall pushed up, and strike-slip faults involve dominantly horizontal displacement along the fault plane.

Question 5Geology

Which of the following is the most abundant gas in the Earth's present-day atmosphere?

  1. Nitrogen Correct answer.
  2. Argon
  3. Carbon dioxide
  4. Oxygen

Explanation

Nitrogen makes up roughly 78 percent of the dry atmosphere by volume, making it the dominant constituent. Oxygen is second at about 21 percent, while argon is near 0.9 percent and carbon dioxide is a minor trace component.

Question 6Geology

Which seismic wave type is compressional, travels fastest, and can pass through both solids and fluids?

  1. S wave
  2. P wave Correct answer.
  3. Rayleigh wave
  4. Love wave

Explanation

P waves are longitudinal body waves in which particle motion is parallel to propagation, and they are the fastest seismic arrivals. Because fluids resist compression but not shear, P waves travel through liquids while S waves cannot, and Love and Rayleigh waves are slower surface waves.

Question 7Geology

The Mohorovicic discontinuity marks the boundary between which two layers?

  1. Crust and mantle Correct answer.
  2. Lithosphere and asthenosphere
  3. Outer core and inner core
  4. Mantle and outer core

Explanation

The Mohorovicic discontinuity, or Moho, is the seismic boundary where P-wave velocities jump sharply at the base of the crust, separating crustal rocks from mantle peridotite. The mantle-core boundary is the Gutenberg discontinuity, and the inner-outer core boundary lies deeper still.

Question 8Geology

A rock composed mainly of alternating light and dark mineral bands formed under high-grade metamorphism is termed:

  1. Slate
  2. Quartzite
  3. Gneiss Correct answer.
  4. Marble

Explanation

Gneiss is a high-grade metamorphic rock characterised by compositional banding of light felsic and dark mafic minerals. Slate is a low-grade foliated rock with slaty cleavage, marble is recrystallised limestone, and quartzite is recrystallised sandstone, none of which show such banding.

Question 9Geology

A mineral has a specific gravity of about 2.7 and shows perfect rhombohedral cleavage with a hardness of 3. It is most likely:

  1. Calcite Correct answer.
  2. Quartz
  3. Galena
  4. Gypsum

Explanation

Calcite has a specific gravity near 2.7, a hardness of 3 on Mohs scale, and perfect rhombohedral cleavage in three directions. Quartz has conchoidal fracture and hardness 7, gypsum has hardness 2, and galena is far denser at about 7.5.

Question 10Geology

A stream flows from a steep mountain course onto a flat plain. Which change in its behaviour is expected?

  1. Gradient decreases, so erosion intensifies and the channel straightens
  2. Sediment load becomes coarser because transport energy rises
  3. Gradient decreases, so deposition replaces erosion and channel sinuosity increases Correct answer.
  4. Discharge falls to zero because the plain is flat

Explanation

As gradient and velocity fall on reaching a plain, the stream loses competence and deposits much of its load, building alluvial features. Lower velocity also favours meandering, so sinuosity generally increases downstream. Erosion and coarser load are associated with the steeper upstream reach.

Question 11Geology

Which sequence correctly shows increasing coal rank?

  1. Lignite, peat, anthracite, bituminous coal
  2. Bituminous coal, peat, lignite, anthracite
  3. Peat, lignite, bituminous coal, anthracite Correct answer.
  4. Anthracite, bituminous coal, lignite, peat

Explanation

Coal rank increases with progressive loss of moisture and volatiles and rising carbon content. The standard order is peat, lignite (brown coal), bituminous coal, and finally anthracite, the hardest and most carbon-rich variety. Any other ordering misplaces a lower-rank member after a higher-rank one.

Question 12Geology

An aquifer confined between two impermeable layers and in which water rises above the top of the aquifer in a borehole is called:

  1. A perched aquifer
  2. A confined aquifer under artesian conditions Correct answer.
  3. A vadose zone
  4. An unconfined aquifer with a water table

Explanation

A confined aquifer is bounded above and below by aquicludes, so the water is under pressure and rises in a well to a potentiometric surface. When that level lies above the top of the aquifer, artesian conditions exist. Unconfined aquifers have a free water table open to the atmosphere.

Question 13Geology

A rock sample from the ocean floor shows symmetrical magnetic stripes about a ridge axis. This pattern is best explained by:

  1. Sea-floor spreading with geomagnetic field reversals recorded during cooling Correct answer.
  2. A single magnetic reversal affecting the whole ocean basin at once
  3. Hydrothermal alteration that magnetised the crust in one direction only
  4. Sedimentary deposition of magnetite-rich layers on the abyssal plain

Explanation

New basaltic crust acquires a remanent magnetisation as it cools at the ridge, recording the field polarity at that time. As spreading continues, successive reversals produce symmetrical stripes mirrored on either side of the axis. This symmetry is a key line of evidence for plate tectonics.

Question 14Geology

Which metamorphic facies indicates the highest temperature conditions among the following?

  1. Blueschist facies
  2. Granulite facies Correct answer.
  3. Greenschist facies
  4. Zeolite facies

Explanation

Metamorphic facies reflect increasing temperature and pressure; granulite facies represents high-temperature, medium- to high-pressure conditions typical of the deep crust. Greenschist and zeolite facies are low-grade, and blueschist indicates high pressure with low temperature at a subduction zone.

Question 15Geology

A fold in which the two limbs dip away from a common central axis, with the oldest rocks in the core, is called:

  1. An anticline Correct answer.
  2. A recumbent fold
  3. A monocline
  4. A syncline

Explanation

An anticline is an upward-arching fold with limbs dipping away from the crest and stratigraphically older rocks exposed in the core. A syncline is the complementary downward fold with younger rocks in the core. A monocline is a single step-like flexure, and a recumbent fold has a near-horizontal axial plane.

Question 16Geology

In the context of the Assam-Arakan basin, which structural feature is classically associated with hydrocarbon accumulation?

  1. Meteorite impact craters
  2. Large-scale glacial moraines
  3. Salt domes formed by evaporite diapirs
  4. Anticlinal structures and fault-bounded traps Correct answer.

Explanation

Hydrocarbons in the Assam-Arakan basin are trapped in anticlinal folds and fault-bounded structural traps developed during Himalayan collision and basin inversion. Traps concentrate buoyant oil and gas that migrate upward through permeable carrier beds. Glacial and impact features are not the trapping mechanism here.

Question 17Geology

Which of the following best describes the difference between weathering and erosion?

  1. Weathering breaks rock in place, whereas erosion removes and transports the debris Correct answer.
  2. Weathering occurs only in cold climates, whereas erosion occurs only in deserts
  3. Weathering is always chemical, whereas erosion is always physical
  4. Weathering moves material downslope, whereas erosion only dissolves minerals

Explanation

Weathering is the in-situ breakdown of rock by physical, chemical or biological processes without significant movement. Erosion involves the detachment and transport of the resulting material by agents such as water, wind or ice. The two are sequential processes rather than interchangeable terms.

Question 18Geology

A sandstone contains well-rounded, well-sorted quartz grains with a clay matrix. Which inference about its history is most reasonable?

  1. Deposition very close to the source with minimal transport
  2. Formation by direct crystallisation from a silica-rich melt
  3. Formation by regional metamorphism of a limestone precursor
  4. Long transport distance with significant reworking before deposition Correct answer.

Explanation

Well-rounded and well-sorted grains imply prolonged transport and winnowing, which removes unstable minerals and leaves resistant quartz. A clay matrix records fine material deposited with the sand. Short transport would instead give angular, poorly sorted grains.

Question 19Geology

Which of the following is a primary structure useful for determining the tops of sedimentary beds in the field?

  1. Joint set orientation
  2. Mineral cleavage
  3. Graded bedding Correct answer.
  4. Fossil species diversity

Explanation

Graded bedding shows a systematic upward decrease in grain size within a single bed and therefore indicates the original top direction. This makes it a reliable way-up criterion in deformed sequences. Joints, cleavage and diversity counts do not directly record younging direction.

Question 20Geology

A rock contains 25 percent of a radioactive parent isotope and 75 percent of its stable daughter. If the half-life is 100 million years, the age of the rock is approximately:

  1. 300 million years
  2. 200 million years Correct answer.
  3. 100 million years
  4. 400 million years

Explanation

After one half-life, 50 percent parent remains; after two half-lives, 25 percent parent remains, with 75 percent converted to daughter. Since 25 percent parent is observed, two half-lives have elapsed. With a 100-million-year half-life, the age is about 200 million years.

Question 21Geology

Which plate boundary type is most directly associated with the generation of new oceanic lithosphere?

  1. Passive continental margin
  2. Transform boundary along a fracture zone
  3. Divergent boundary at a mid-ocean ridge Correct answer.
  4. Convergent boundary at an oceanic trench

Explanation

At mid-ocean ridges, mantle upwelling and decompression melting produce basaltic magma that solidifies to form new oceanic crust as plates diverge. Trenches mark subduction and destruction of lithosphere, transform faults only slide plates past one another, and passive margins are not plate boundaries.

Question 22Geology

Barchan dunes are crescent-shaped sand bodies. Their geometry is best explained by:

  1. Tidal currents acting on submarine sand sheets
  2. Glacial abrasion of bedrock surfaces
  3. Unidirectional wind with horns pointing downwind and a gentle stoss slope Correct answer.
  4. Bidirectional winds producing symmetrical ridges with horns pointing upwind

Explanation

Barchans form where sand supply is limited and wind blows consistently from one direction. The gentle windward (stoss) slope passes to a steeper lee slip face, and the horns extend downwind around the crescent. They are therefore useful palaeowind indicators.

Question 23Geology

In a sequence of folded strata, a bed is found to young consistently towards the east on both limbs of a large fold. The fold axis most probably:

  1. Is a recumbent fold with a horizontal axial plane
  2. Plunges gently towards the east Correct answer.
  3. Has been refolded into a dome
  4. Is vertical with a horizontal axis

Explanation

If younging directions on opposite limbs both point the same way, the fold is not a simple upright structure but a plunging one whose axis trends in that direction. The consistent younging reflects the closure geometry as the fold axis plunges, so the axis trends gently eastward. An upright fold would give opposite younging senses on its two limbs.

Question 24Geology

A sandstone bed shows a sharp erosional base overlain by coarse grains that fine upward into shale. This motif is best interpreted as:

  1. A single turbidite deposited by a waning current Correct answer.
  2. A beach prograding seaward over offshore muds
  3. A glacial till deposited directly by melting ice
  4. An evaporite sequence formed in a sabkha

Explanation

A sharp erosional base followed by upward fining reflects deposition from a decelerating sediment gravity flow, the classic Bouma-type turbidite signature. As flow energy declines, progressively finer material settles. Beach progradation and glacial till produce different vertical arrangements and sorting characteristics.

Question 25Geology

A magma body assimilates a large volume of carbonate country rock during ascent. Which change is most likely in the resulting melt?

  1. Decreased silica activity with no change in volatile content
  2. Increased calcium and carbon dioxide content with possible explosive degassing Correct answer.
  3. Formation of a pure granitic melt identical to the parent magma
  4. Complete solidification without any compositional change

Explanation

Assimilation of carbonate wall rock adds calcium and carbonate components to the melt and releases carbon dioxide, which raises volatile pressure and can promote explosive volcanism. The process also shifts bulk composition away from the original magma. The melt does not remain identical to the parent.

Question 26Geology

In the geological time scale, which era is divided into the Paleogene, Neogene, and Quaternary periods?

  1. Mesozoic
  2. Cenozoic Correct answer.
  3. Precambrian
  4. Paleozoic

Explanation

The Cenozoic Era is subdivided into the Paleogene, Neogene, and Quaternary periods, covering roughly the last 66 million years. The Mesozoic is divided into Triassic, Jurassic, and Cretaceous, while the Paleozoic contains the Cambrian through Permian periods.

Question 27Geology

Which mineral property is defined as the resistance of a mineral surface to scratching and is commonly tested using Mohs' scale?

  1. Hardness Correct answer.
  2. Luster
  3. Cleavage
  4. Streak

Explanation

Hardness measures resistance to scratching and is ranked on Mohs' scale from talc (1) to diamond (10). Cleavage, luster, and streak describe different properties, namely breakage planes, surface appearance, and powder colour.

Question 28Geology

Which type of unconformity is represented by sedimentary strata resting on an eroded surface of igneous or metamorphic rock?

  1. Disconformity
  2. Paraconformity
  3. Nonconformity Correct answer.
  4. Angular unconformity

Explanation

A nonconformity occurs where sedimentary layers overlie eroded crystalline igneous or metamorphic basement. An angular unconformity involves tilted older strata, while a disconformity separates parallel sedimentary beds with an erosional break.

Question 29Geology

Which rock type forms when lava cools rapidly at or near the Earth's surface and therefore has a very fine-grained or glassy texture?

  1. Chemical sedimentary rock
  2. Intrusive igneous rock
  3. Extrusive igneous rock Correct answer.
  4. Regional metamorphic rock

Explanation

Extrusive igneous rocks solidify quickly at the surface, so crystals have little time to grow, producing fine-grained or glassy textures. Intrusive rocks cool slowly at depth and generally develop coarse crystals.

Question 30Geology

Which term describes the total volume of pore spaces in a rock expressed as a percentage of the rock's total volume?

  1. Porosity Correct answer.
  2. Permeability
  3. Specific yield
  4. Hydraulic conductivity

Explanation

Porosity is the ratio of void volume to total rock volume, expressed as a percentage. Permeability and hydraulic conductivity measure the ease of fluid flow, not the amount of void space.

Question 31Geology

Which of the following is the most common mineral group in the Earth's crust and includes minerals such as quartz and feldspar?

  1. Sulfides
  2. Carbonates
  3. Silicates Correct answer.
  4. Oxides

Explanation

Silicate minerals, built around the silicon-oxygen tetrahedron, dominate the crust and include quartz, feldspars, micas, and olivine. Carbonates, sulfides, and oxides are important but far less abundant in crustal rocks.

Question 32Geology

Which type of volcano is typically built of gently sloping layers of fluid basaltic lava and has a broad, shield-like profile?

  1. Stratovolcano
  2. Shield volcano Correct answer.
  3. Caldera
  4. Cinder cone

Explanation

Shield volcanoes form from low-viscosity basaltic lava that flows far from the vent, producing broad, gently sloping profiles. Stratovolcanoes and cinder cones are built from more viscous lava and pyroclastic debris with steeper slopes.

Question 33Geology

Which natural process removes weathered material from its place of origin and transports it downslope?

  1. Erosion Correct answer.
  2. Lithification
  3. Diagenesis
  4. Metasomatism

Explanation

Erosion is the removal and transport of weathered debris by agents such as water, wind, and ice. Diagenesis and lithification are post-depositional processes that convert sediment into rock.

Question 34Geology

A geologist measures a bedding plane that dips 30 degrees towards the southeast. Which structural quantity does the 30 degrees represent?

  1. Strike direction
  2. Plunge of a fold axis
  3. Dip amount Correct answer.
  4. Vertical exaggeration

Explanation

The dip amount is the angle between the bedding plane and the horizontal, measured perpendicular to strike. Strike is the compass direction of the line formed by the intersection of the plane with the horizontal.

Question 35Geology

A rock sample is composed chiefly of the mineral calcite and fizzes readily when dilute hydrochloric acid is applied. Which rock type is most likely?

  1. Limestone Correct answer.
  2. Shale
  3. Sandstone
  4. Granite

Explanation

Calcite-rich rock that reacts vigorously with dilute acid is diagnostic of limestone, a carbonate sedimentary rock. Sandstone and shale are clastic rocks dominated by quartz and clay minerals, which do not effervesce.

Question 36Geology

Two rock samples are collected: one has coarse interlocking crystals and the other has fine crystals with some vesicles. Which interpretation of their cooling histories is most reasonable?

  1. The coarse sample cooled slowly at depth, while the fine vesicular sample cooled quickly near the surface Correct answer.
  2. The coarse sample cooled quickly at the surface, while the fine vesicular sample cooled slowly at depth
  3. The fine vesicular sample is metamorphic and the coarse sample is sedimentary
  4. Both samples cooled at identical rates but differed only in chemical composition

Explanation

Coarse interlocking crystals indicate slow cooling, which allows crystals to grow, typical of intrusive settings. Fine crystals and vesicles indicate rapid near-surface cooling with trapped gas, typical of extrusive rocks.

Question 37Geology

A coastal area experiences strong longshore drift. What is the most likely consequence for beach sediment?

  1. Sediment is transported parallel to the shoreline and may accumulate at a spit or harbour wall Correct answer.
  2. Sediment is sorted by density so that heavy minerals collect at the dune crest
  3. Sediment is deposited only at the high-tide mark and never moves again
  4. Sediment is carried directly offshore into deep water and permanently lost

Explanation

Longshore drift moves sediment along the shore in the direction of the dominant wave approach, commonly building spits or accumulating against obstructions. It does not simply remove sediment permanently offshore.

Question 38Geology

A confined aquifer has a potentiometric surface above the ground level at a particular site. What is the expected behaviour when a well is drilled there?

  1. Water will remain below the water table and require pumping
  2. Water will rise above ground level without pumping Correct answer.
  3. The well will encounter only unsaturated rock
  4. The aquifer will yield no water because it is confined

Explanation

When the potentiometric surface of a confined aquifer lies above ground level, hydrostatic pressure drives water up the well without pumping, producing a flowing artesian well. Confinement does not prevent yield.

Question 39Geology

A mineral specimen shows two directions of cleavage at about 90 degrees, is dark coloured, and contains iron. Which mineral is most consistent with these properties?

  1. Muscovite
  2. Quartz
  3. Pyroxene Correct answer.
  4. Calcite

Explanation

Pyroxene typically shows two cleavages near 90 degrees and is a dark, iron-bearing ferromagnesian silicate. Muscovite shows one perfect cleavage, calcite shows rhombohedral cleavage, and quartz lacks cleavage.

Question 40Geology

A sequence of sedimentary beds shows a gradual change from conglomerate at the base through sandstone to shale at the top. What does this vertical change most likely indicate?

  1. A reversal of the Earth's magnetic field
  2. A decrease in depositional energy over time Correct answer.
  3. An increase in depositional energy over time
  4. A period of metamorphism after deposition

Explanation

Coarse conglomerate reflects high-energy transport, whereas shale reflects low-energy settling of fine particles. The upward fining therefore records decreasing energy, as in a transgressive or waning-flow setting.

Question 41Geology

A fault plane has a dip of 60 degrees and the hanging wall has moved down relative to the footwall. Which fault type is this?

  1. Reverse fault
  2. Normal fault Correct answer.
  3. Strike-slip fault
  4. Thrust fault

Explanation

In a normal fault the hanging wall moves down relative to the footwall, typically under extension. Reverse and thrust faults show hanging-wall upward movement due to compression.

Question 42Geology

A groundwater sample has a high concentration of dissolved bicarbonate and calcium and deposits scale in pipes. Which interpretation is most consistent?

  1. The water is acidic and originated from sulfide oxidation
  2. The water is saline and originated from seawater intrusion
  3. The water is organic-rich and originated from a peat bog
  4. The water is hard and originated from contact with carbonate rocks Correct answer.

Explanation

High calcium and bicarbonate concentrations produce hard water that precipitates calcium carbonate scale. This composition commonly reflects dissolution of limestone and dolomite along the flow path.

Question 43Geology

A granite is intruded by a basalt dyke. Which principle allows the relative age of the dyke to be determined?

  1. Faunal succession
  2. Original horizontality
  3. Superposition
  4. Cross-cutting relationships Correct answer.

Explanation

The cross-cutting principle states that a feature cutting another must be younger than the material it cuts. The dyke therefore postdates the granite it intrudes.

Question 44Geology

A metamorphic rock contains garnet and staurolite and shows a strong foliation. Which metamorphic grade is most consistent with this assemblage?

  1. Medium to high grade Correct answer.
  2. Very low grade
  3. Contact grade adjacent to a lava flow
  4. Diagenetic grade

Explanation

Garnet and staurolite are index minerals characteristic of medium- to high-grade regional metamorphism. Very low grade assemblages would contain chlorite or clay minerals instead.

Question 45Geology

A river valley shows a series of terraces above the present floodplain. What is the most likely explanation for their formation?

  1. Repeated downcutting as the river adjusted to a fall in base level Correct answer.
  2. Volcanic eruption filling the valley with lava
  3. Glacial scouring removing all valley sediment
  4. Deposition of a single flood layer across the valley

Explanation

River terraces form when a stream incises its former floodplain after a fall in base level or an increase in discharge, leaving abandoned surfaces at higher elevations. They record episodic downcutting.

Question 46Geology

A sedimentary basin in a convergent margin shows rapid subsidence, thick turbidites, and abundant volcaniclastic debris. Which basin type is most consistent?

  1. Forearc basin Correct answer.
  2. Intracratonic sag basin
  3. Rift basin
  4. Passive-margin basin

Explanation

Forearc basins develop between a magmatic arc and a trench in a convergent setting, receiving volcaniclastic and turbidite fill during rapid subsidence. Passive-margin and intracratonic basins lack this arc-related setting.

Question 47Geology

A rock has a radiometric age of 500 million years and contains a mineral that closes at 300 degrees Celsius. Which interpretation is most defensible?

  1. The age is meaningless because minerals never retain parent and daughter isotopes
  2. The age records the age of the surrounding sedimentary layers
  3. The age records the exact moment of magma eruption
  4. The age records cooling below the closure temperature, not necessarily the time of crystallisation Correct answer.

Explanation

Radiometric ages from minerals reflect the time since the mineral cooled below its closure temperature, when daughter isotopes could no longer escape. This cooling age may postdate crystallisation.

Question 48Geology

A soil profile developed on a humid tropical shield area shows intense leaching, kaolinite clay, and iron oxide accumulation. Which soil-forming process is dominant?

  1. Podzolisation
  2. Salinisation
  3. Calcification
  4. Lateritisation Correct answer.

Explanation

Intense chemical weathering and leaching in humid tropical climates remove silica and bases while concentrating iron and aluminium oxides, producing laterite. Salinisation and calcification dominate in arid settings.

Question 49Geology

A sequence of marine strata shows a sudden change from deep-water shale to shallow-water sandstone, with an erosional surface between them. What is the most likely interpretation?

  1. A gradual transgression with continuous deposition
  2. A forced regression with erosion and basinward shift of facies Correct answer.
  3. A period of quiet pelagic sedimentation
  4. A single volcanic ash fall

Explanation

An erosional surface overlain by shallow-water facies above deep-water shale indicates a forced regression, where relative sea-level fall causes erosion and a basinward shift in facies. A gradual transgression would show deepening upward.

Question 50Geology

In an area of active folding, a bed dips 40 degrees towards the north on one limb and 40 degrees towards the south on the other. What fold geometry does this describe?

  1. A recumbent fold with a horizontal axial plane
  2. An upright syncline with a north-south trending axis Correct answer.
  3. A monocline with a single flexure
  4. An upright anticline with an east-west trending axis

Explanation

Opposing dips of equal amount on either side of a common axis indicate a fold whose axis trends east-west and whose limbs dip north and south. The opposing dips towards the axis define a syncline.