Landforms Developed on Folded/Tilted Strata

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Landforms Developed on Folded/Tilted
Strata Underlying geologic structure influences
surface landforms in two ways 1. determines
outcrop pattern. 2. The relative elevation and/or
slope of outcrops will vary, because different
rocks erode at different rates (depends on
RELATIVE hardness of rock, cementation strength
(if sedimentary rock) and joint abundance). This
is called DIFFERENTIAL EROSION.
cuesta
Strike valley
Resistant rock layers
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Resistant rock is left standing higher than less
resistant rock, forming various types of cliffs
("scarps or escarpments), ridges or uplands.
The actual feature formed depends on the dip of
the strata. Mesas are wide flat-topped hills
capped by a resistant horizontal rock layer (a
caprock) buttes are similar, narrower hills a
cuesta is the combination of a dipslope and
escarpment (or scarp) a hogback is a fairly
sharp ridge a razorback is a very sharp ridge
(dip of the rock layer determines which will
form). Softer rock layers are eroded lower
forming STRIKE VALLEYS - valleys parallel to
strike direction.
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MARBLE DEFORMATION
Outcrop Patterns and Landforms On Geologic Maps.
Subsurface rock layers are usually exposed by
downcutting streams and rivers. Horizontal
Strata 1. Outcrop pattern parallels valleys 2.
Contacts between rock units parallel contours
because strata are flat.
fracture
flow
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3. Uplifted horizontal strata form plateaus like
the Colorado Plateau. 4. Vigorous downcutting
during/after uplift forms canyons like the Grand
Canyon. 5. Resistant strata form steep cliffs
softer rock erodes to gentler slopes.
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6. Prolonged erosion isolates blocks of rock,
forming mesas, buttes and pinnacles
particularly where a hard caprock protects
underlying softer rock.
Lava flow caprock on a mesa in west Texas
hills
mountains
cliffs
ridges
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Tilted Strata 1. Outcrops are roughly parallel
bands. 2. Law of V's - low dip angle -gt larger
V. 3. Older beds dip towards younger beds. 4.
Differential erosion forms ridges, dipslopes,
escarpments, cuestas, strike valleys.
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4. Cuestas, hogbacks or razorbacks are formed by
resistant beds strike valleys formed by softer
beds.
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CUESTA
Dipslope
Scarp slope
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Domes 1. Outcrops are concentric belts. 2. Oldest
rocks are in the center. 3. Law of V's -gt beds
dip away from center. 4. Resistant beds form
inward-facing scarps.
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5. Core of dome determines relief if resistant
-gt central upland if soft -gt central lowland.
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Basins 1. Outcrops are concentric belts. 2.
Youngest rocks are in the center. 3. Law of V's
-gt beds dip towards center. 4. Resistant beds
form outward-facing scarps.
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Folds 1. Erosion of plunging folds (most plunge)
-gt zigzag pattern. 2. Anticlines V down plunge
oldest beds at center. Law of V's -gt beds dip
away from center resistant rocks -gt
inward-facing scarps. 3. Synclines V up plunge
youngest beds at center Law of V's -gt beds dip
towards center resistant rocks -gt outward-facing
scarps.
ANTICLINE
SYNCLINE
ANTICLINE
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4. Since most folds form at depth, fold outcrop
patterns are typical in eroded ancient mountain
belts such as the Appalachians. The folds have
been exposed by erosion.
compression
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Landforms Developed On Faulted Strata 1. Form
sharp linear boundaries of uplands or valleys
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2. Displacement -gt fault scarps (300 m high in
this example from Utah).
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3. Recognized on geologic maps by abrupt
displacement of structures or rocks.
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4. Characteristic of young mountain belts -gt
fault block mountains
UPLIFT
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5. Basin and Range Province Caused by crustal
stretching due to heat flow from subduction.
Tilting of normal faults resulted in fault-angle
depressions (tilted fault blocks).
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Death Valley
Basin and range in Texas
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Surficial Deposits 1. Unconsolidated (loose or
uncemented) recent sediments e.g. river sands,
glacial deposits, landslide deposits. 2. Usually
form a thin cover on underlying bedrock. 3.
Usually associated with process e.g. river
deposits in present river valleys landslide
debris at base of cliff etc.
bedrock
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Influence of Geologic Structure On Drainage
Patterns 1. Streams that develop on "original
surface" (e.g. emergent coastal plain) follow the
regional slope CONSEQUENT STREAMS 2. Continued
erosion -gt strike valleys occupied by SUBSEQUENT
STREAMS 3. Tributaries to subsequent streams that
follow regional slope RESEQUENT STREAMS
(REnewed conSEQUENT) tributary streams that flow
in the opposite direction to consequent streams
OBSEQUENT STREAMS (Oppsite to conSEQUENT).
Consequent stream, following regional slope
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Consequent stream
Resequent stream
Obsequent stream
Subsequent stream
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YOU WILL NEED A RULER, CALCULATOR, PENCIL, ERASER
FOR LAB 2.