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

1. What are sequential landforms and how is their shape, size and arrangement influenced by rock properties?
2. Sketch a simple diagram to illustrate the concepts of rock strike and dip.
3. Describe a mesa and discuss the role of rock properties in the development of its distinctive shape.
4. Sketch simple diagrams to illustrate dendritic, trellis and radial drainage patterns. Give an example of the conditions under which is expected to be found.

1. Plot a graph to show how the number of stream segments normally varies with stream order.
2. 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.
3. What are artesian wells and why do they occur in the gently dipping strata of coastal plains?
4. What is a hogback? In what type of rock structure do they usually develop?
5. 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?
6. Subsequent streams are often found at the base of fault-line scarps. Why?
7. What is the relationship between a batholith and a monadnock?
8. The denudation of stratovolcanoes and shield volcanoes produce very different landscapes. How do they differ and why?