Chapter 16
Landforms and Rock Structure
OVERVIEW
This chapter considers how rock properties influence the landforms
and drainage patterns produced by fluvial denudation.
- The earth's crust contains a variety of rock types which differ
in their resistance to denudation. More resistant rock
tends to form uplands and ridges while weaker rock forms lowlands
and valleys.
- Rock layers can be tilted, folded and fractured by tectonic
forces to produce a variety of rock structures. The tilt and orientation
of rock layers and fractures are described by their strike
and dip.
- In areas with horizontal strata and an arid climate, fluvial
denudation produces vertical cliffs of resistant rock separated
by gentler slopes of less resistant rock. These slopes surround
flat-topped plateaus, mesas and buttes capped by resistant rock.
- Different rock types and structures tend to produce different
drainage patterns or stream network characteristics. Drainage
patterns have some interesting, systematic geometric properties.
- Areas of horizontal strata usually have a dendritic drainage
pattern.
- On gently dipping strata along coastal plains, cuestas
form in more resistant rock while lowland valleys develop in the
less resistant rock. The development of consequent streams
across the cuestas and subsequent streams along the lowland
valleys produces a trellis drainage pattern.
- Fluvial denudation of a sedimentary dome produces an
annular drainage pattern and a circular pattern of hogbacks
of more resistant rock separated by lowlands of less resistant
rock.
- Linear fold belts of anticlines and synclines
are eroded into ridge-and-valley landscapes, with the more
resistant strata forming ridges and the less resistant strata
forming valleys. A trellis drainage pattern is typical of these
landscapes.
- A rock face produced by faulting can persist as a fault-line
scarp while a landscape is worn down by denudation. A subsequent
stream often marks the zone of weakness along a fault plane.
- Tightly folded metamorphic rocks tend to erode to ridge-and-valley
landscapes that are less rugged than those developed in folded
sedimentary rock.
- A monadnock is an isolated projection of intrusive
igneous rock surrounded by an eroded plain.
-
Radial drainage patterns develop in the early stage
of erosion of stratovolcanoes. The advanced stage of erosion produces
volcanic necks and radial dikes of resistant igneous
rock.
- The erosion of shield volcanoes results in landscapes of steep
slopes and sharp ridges.
KEY TERMS
strike dendritic sedimentary dome
dip drainage pattern hogback
plateau trellis drainage radial drainage
mesa pattern pattern
butte coastal plain ridge-and-valley
drainage pattern cuesta landscape
bifurcation ratio artesian well fault-line scarp
aquifer monadnock
aquiclude volcanic neck
STUDY QUESTIONS
- What are sequential landforms and how is their shape, size
and arrangement influenced by rock properties?
- Sketch a simple diagram to illustrate the concepts of rock
strike and dip.
- Describe a mesa and discuss the role of rock properties in
the development of its distinctive shape.
- Sketch simple diagrams to illustrate dendritic, trellis and
radial drainage patterns. Give an example of the conditions under
which is expected to be found.
- Plot a graph to show how the number of stream segments normally
varies with stream order.
- Alternating bands of cuestas and lowlands occur on many of
the coastal plains of passive continental margins. Describe these
landforms and discuss the role of rock properties in their development.
- What are artesian wells and why do they occur in the gently
dipping strata of coastal plains?
- What is a hogback? In what type of rock structure do they
usually develop?
- Ridge-and-valley landscapes can develop on two different kinds
of rock structures. What are these rock structures and how do
the characteristics of the ridges and valleys differ between them?
- Subsequent streams are often found at the base of fault-line
scarps. Why?
- What is the relationship between a batholith and a monadnock?
- The denudation of stratovolcanoes and shield volcanoes produce
very different landscapes. How do they differ and why?
Go to this chapter's quiz
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