Page 162 - Fundamentals of Geomorphology

SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 145

10 km

San
N
Elizabeth Antelope
Lake Andreas Valley

Rift
Bouquet
Reservoir
Palmdale

Lake Zone
Palmdale

Santa Clara River Creek Little

Rock

Figure 5.27 Offset drainage along the San Andreas Fault, California, USA.

formed before a particular landmass began to drift would physical properties of the rock mass. Outcrops of resis-
need rotating back to their original positions. In conse- tant rocks such as granite may be reduced to plains,
quence, a worldwide set of lineaments with common given time, because fractures allow water and therefore
alignments must be fortuitous. That is not to say that weathering to eat into the rock. If the granite has a high
there is not a worldwide system of stress and strain that density of fractures, the many avenues of water penetra-
could produce global patterns of lineaments, but on a tion promote rapid rock decay that, if rivers are able to
planet with a mobile surface its recognition is formidable. cut down and remove the weathering products, may pro-
duce a plain of low relief. This has happened on many
old continental shields, as in the northern Eyre Peninsula,
Australia. Even granite with a moderate density of frac-
Joints
tures, spaced about 1 to 3 m apart, may completely decay
All rocks are fractured to some extent. A broad range of given sufficient time, owing to water penetrating along
fractures exists, many of which split rock into cubic or the fractures and then into the rock blocks between the
quadrangular blocks. All joints are avenues of weath- fractures through openings created by the weathering
ering and potential seats of erosion. The geomorphic of mica and feldspar. The weathering of granite with
significance of a set of joints depends upon many factors, moderately spaced joints produces distinctive landforms
including their openness, pattern and spacing, and other (Figure 5.28). Weathering of the joint-defined blocks