Structural Control of Landforms

1
Structural Control of Landforms
Parts of Chapters 3 and 12
Plus a review of folds and faults
Photo from Drury Two distinct units.  One
dendritic drainage pattern is sparsely
vegetated.  Parallel contours suggest it is
horizontal.  Other formation banded, with
straight wooded ridges, controlled by steep
dips. The boundary truncates the ridges.
Horizontal unit lies unconformably on the steeply
dipping strata (angular unconformity).  The wide
spacing of drainage in the younger unit suggests
that it is a massive, coarse clastic rock.  The
older unit comprises shales and limestones. From
Steve Drury, Image Interpretation in Geology,
adopted for this course
Some photos in this PowerPoint made available
online, courtesy of Steve Dutch, click here
Slickensided Rock, Pencil, Rubber band Gum, Foam
sediments, Cardboard fault models, 2 Plastic
boxes, Food Coloring ,Paper, wood, Ice
From our lab workbook Image Interpretation in
Geology by Steve Drury
2
Lithology

• Relative Erodability
• Layered rocks wide range
• Sedimentary
• Volcanic
• Massive rocks narrow range
• Metamorphic
• Intrusive igneous
• Erodability is not absolute but
• typically shale gt limestone gt sandstone gneiss

Canadian Shield. Pale granite and darker
metavolcanic rocks, the granite having resisted
glaciation best. Drury IIG
3
Erodability

• " shale, limestone, marble and some types of
  schist are less resistant "valley-makers" in
  humid climates"
• "whereas sandstone, quartzite, conglomerate and
  various igneous rocks are resistant
  "ridge-makers" .
• Easterbrook (1969) Principles of Geomorphology

4
Lithology/Climate
Erodability shale gt limestone gt sandstone
gneiss
In humid areas, weathering and erosion are
faster, slopes are more eroded, gentler after
the same duration of exposure to weathering
5
Horizontally layered rocks outcrops parallel
topographic contours. 
In arid terrains (a) the intermittent violent
erosion develops steep-sided gullies and valleys.
Note differential erosion
In humid climate the topography is more muted.
Monadnocks resistant rock ridges Colorado
6
Undisturbed Sediments showing differential
erodability
Dry Climate, intermittent strong storms
7
Review Stream Vees
8
Tablelands
In horizontal beds, rocks outcrops would follow
contours
9
Tablelands note horizontal layers, differential
erosion
In horizontal beds, rocks outcrops would follow
contours
Butte chimney
mesa
Inselberg
Pediment (gentle slope lt 5, erosional concave up
surface w thin veneer of gravel etc.)
Dry Climate, intermittent strong storms

• Plateaugtmesagtbuttegtchimney
• Ratio surface area of top to height

10
Desert Landforms near Mountains
Alluvial Fan
(often exposed bare rock with gravel veneer)
Rain-shadow desert in the lee of mountains
Mountains eventually erode away to Inselbergs
11
Compression, Tension, and Shearing Stress
Convergent Divergent Transform
12
Convergent Plate Boundaries and Folding
Continent-Continent collision forms Fold and
Thrust Mountains Alps, Himalayans, Appalachians
Subduction causes Arc Under Ocean Lithosphere
Japan, Aleutians, Cent. Am. under continent
Andes, Cascades
13
Strike and Dip
Map Symbols Strike shown as long line, dip as
short line. Note the angle of dip shown 45o
Strike intersection w horizontal, dip
perpendicular, angle from horizontal down toward
surface
14
Tilted Strata

• Monoclinal folds, or one side (limb) of a fold
• Name f(dip angle)
• Cuesta (gentle)
• Hogback (steep)
• Flatiron remnant of dissected Hogback w
  triangular face

15
Dip Slope vs. Scarp slope
Cuesta
Hogback
16
Ridges

• Dip of Cuesta lt Hogback

17
Folds are typical of convergenceFolded Rock
Before Erosion
18
Folded Rocks, Hwy 23 Newfoundland, New Jersey
Note highest point
Source Breck P. Kent
Adjacent Anticline and Syncline
19
Folded Rock After Erosion
Eroded Anticline, older rocks in center. Syncline
is opposite.
20
Topography may be opposite of Structure Anticline
Before/After Erosion
Notice center rock oldest
21
Topography may be opposite of Structure Syncline
Before/After Erosion
Notice center rock youngest
22
Various Folds
23
Various Folds (cont'd)
24
Various Folds (cont'd)
25
Various Folds (cont'd)
Axis
Axial plane near axis should be close to
horizontal
26
Plunging Folds and Nose Rules
Demo Plastic box, water, paper folds
Up End
Down End
Nose of anticline points direction of plunge,
syncline nose in opposite direction
27
Plunging Folds
Nose
Nose
Nose
28
Joints Fractures with no movement
vs. Faults with relative movement
Sandstone, note no streams here, too many cracks
Source Martin G. Miller/Visuals Unlimited
29
Dip-Slip Faults
Demo Cardboard Models
30
Continental Rift into Ocean Basin - Tension gt
Divergence
Rift Valleys and Oceans are the same thing
31
Normal Faults at Divergent Margins - Iceland
A new graben, down dropped hanging wall block -
Normal Fault divergent zone MOR
Overhanging Block
Footwall
32
Fault Line scarp (High-angle Normal Fault)
33
Shallow Reverse Fault Thrust Fault
34
Lewis Thrust Fault (cont'd)
Same layer
35
Lewis Thrust Fault (cont'd)
Source Breck P. Kent
PreCambrian Limestone over Cretaceous
Shales
36
Geologists are frequently called upon to find the
ore body
This guy is rich
Younger
What phase of magma fractionation would result in
the placement of this ore body? Which formed
first, the ore body or the fault? What common
mineral is mostly likely in the ore body?
Reverse
Miners pay geologists to find their lost
orebody One friend earned enough to buy a house
Normal
This poor guy is out of luck
37
Horizontal Movement Along Strike-Slip Fault
38
Landscape Shifting, Wallace Creek
San Andreas Fault
39
Normal Fault Quake - Nevada
Reverse Fault Quake - Japan
Divergent
HW Down
Convergent
HW Up
Transform
Strike Slip Fault Quake - California
40
Fracture Zones and Slickensides
http//pangea.stanford.edu/laurent/english/resear
ch/Slickensides.gif
41
Fluvial Landforms of Streams

• Consequent streams follow slope of the land over
  which they originally formed.
• Subsequent streams are streams whose course has
  been determined by erosion along weak strata.
• Resequent streams are streams whose course
  follows the original relief, but at a lower level
  than the original slope
• Obsequent streams are streams flowing in the
  opposite direction of the consequent drainage.

Memorize These
42
Insequent Streams Initial Consequent

• Almost random drainage often forming dendritic
  patterns.
• Typically tributaries - developed by headward
  erosion on a horizontally stratified or
  homogeneous rocks.
• NOT controlled by the original slope of the
  surface, its structure or the type of rock.

Headward Erosion
43
subsequent (s along weak)
consequent (c follow slope)
Insequent (random dendritic)
obsequent (o opposite main slope)
resequent streams (original slope but lower level)
44
(No Transcript)
45
Drainage Patterns with and without structural
control
None Joints
fold limbs
Volcano, exposed pluton, diapir
46
Dendritic Patterns

• Underlying bedrock has no structural control over
  where the water flows.
• Characteristic pattern of acute angles
• No repeating pattern.

47
Trellis Patterns

• Form where underlying bedrock has repeating
  weaker and stronger types of rock.
• Streams cut down deeper into the weaker bedrock
• Nearly parallel streams
• Branch at higher angles.

48
Rectangular patterns

• Branching of tributaries at nearly right angles
• Form in jointed igneous rocks or horizontal
  sedimentary beds with well-developed jointing or
  intersecting faults.

49
Parallel Erosion

• Form on unidirectional regional slope or parallel
  landform features. Small areas.

Near Capitol Reef National Park, Utah
50
Radial Erosion

• Flow of water outward from a central point
• Down a newly formed cinder volcano cone
• or an intrusive dome.

51
Annular patterns

• form on domes of alternating weak and hard
  bedrocks.
• The pattern formed is similar to that of a
  bull's-eye when viewed from above
• weaker bedrocks are eroded and the harder are
  left in place.

52
Centripetal patterns

• Form where water flows into a central location,
  such as in a karst limestone terrain where the
  water flows down into a sinkhole and then
  underground.

53
Structural Control of Drainage

• Contorted
• Folded Rocks

54
Stream Capture vs. Structural Control
Headward Erosion
55
Stream Capture vs. Structural Control
56
Dry Valley
Elbow of Capture
Brodhead Creek
Godfrey Ridge
Stream Capture
57
Terraces 1
1. Old river meanders across floodplain 2. Base
level drops (how?), or region uplifts. Area now
much higher above sea level than before.
Potential energy increases, water flows faster,
better erosion, stream straightens and cut down
to base level, less floodplain width and cut
lower. 3.Terrace forms from previous floodplain.
Further incision cuts another terrace
Next time Terraces 2 and 3 Isostatic Rebound and
high water shorelines as glaciers melt
Potential rgh to Kinetic Energy 1/2mV2
58
A flight of river terraces
59

• Antecedent Streams and Superimposed Streams
• Meanders in steep, narrow valleys
• Caused by a drop in base level or uplift of region

Incised (entrenched) meanders
Delaware Water Gap

• River is older than uplift

60
"In this panorama in southwestern Colorado, a
stream flows from the right across an uplift
(anticline) in the rocks. As soon as the stream
enters the uplift, its canyon becomes deep. Note
the entrenched meanders, a couple of which were
cut through and abandoned when the canyon was
about half its present depth. As soon as the
river exits the uplift, the canyon once again
becomes shallow. Clearly, the river was there
first and the rocks arched upward across its
course." Steve Dutch
Some photos in this PowerPoint made available
online, courtesy of Steve Dutch, click here