Geology Reference
In-Depth Information
10 km
N
Elizabeth
Lake
Bouquet
Reservoir
Palmdale
Lake
Palmdale
Figure 5.27
Offset drainage along the San Andreas Fault, California, USA.
formed before a particular landmass began to drift would
need rotating back to their original positions. In conse-
quence, a worldwide set of lineaments with common
alignments must be fortuitous. That is not to say that
there is not a worldwide system of stress and strain that
could produce global patterns of lineaments, but on a
planet with a mobile surface its recognition is formidable.
physical properties of the rock mass. Outcrops of resis-
tant rocks such as granite may be reduced to plains,
given time, because fractures allow water and therefore
weathering to eat into the rock. If the granite has a high
density of fractures, the many avenues of water penetra-
tion promote rapid rock decay that, if rivers are able to
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-
tures, spaced about 1 to 3 m apart, may completely decay
given sufficient time, owing to water penetrating along
the fractures and then into the rock blocks between the
fractures through openings created by the weathering
of mica and feldspar. The weathering of granite with
moderately spaced joints produces distinctive landforms
(Figure 5.28). Weathering of the joint-defined blocks
Joints
All rocks are fractured to some extent. A broad range of
fractures exists, many of which split rock into cubic or
quadrangular blocks. All
joints
are avenues of weath-
ering and potential seats of erosion. The geomorphic
significance of a set of joints depends upon many factors,
including their openness, pattern and spacing, and other
