Weathering, erosion, transportation, and deposition
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| 6A. Minnamurra Falls |
The geomorphic and hydrologic processes such as earth movements, weathering, erosion,
transport and deposition are all very important in Minnamurra Rainforest
because of its position on the steep slopes of the Illawarra Escarpment.
Remember that this rainforest has survived until today, unlike much
of the rainforest in the Illawarra, because of this steep position.
Looking at the geological
cross-section you can see that there are three main types of
parent rock on the steep slopes. There are sandstones at the top
and bottom of the escarpment, with those at the top representing
the Triassic Period (225 million years old) and those at the bottom
representing the Permian Period (280 million years old), with basalt
(tertiary between 65 million and 10 million years old) and latite
(volcanic) rocks (Permian) making up the other two broad types.
The ages of the sandstones roughly gives the period (280 million
years ago to 225 million years ago) during which this area was
uplifted by mass earth movements. Ever since then, the plateau,
of which the Illawarra Escarpment is the edge, has been weathering
to create the steep horseshoe-shaped area that ensures the perfect
conditions for the Minnamurra Rainforest. The appearance of an
area of basalt (10-65 million years old) in the much older rock
is probably the result of a volcanic intrusion such as a sill,
where volcanic lava has run in between layers of older rock, taking
advantage of a weakness in the layers.
Water, over the millions of years since this area was uplifted,
has weathered away the rock to produce the horseshoe shape which
can be seen in the air
photo. This is a result of the rock being weathered so that
the plateau grows smaller, making the coastal plain bigger. As
the water has run down the cliffs of the escarpment, it has established
drainage lines which today we call the Minnamurra River. These
drainage lines have become prominent as the water has weathered
away the rock to form a permanent path. The weathering of the
rock depends on how hard the rock is and how fast the water is
moving. Materials such as shale and sandstone tend to be softer
than materials such as basalt or harder quartz-based sandstone.
This gives rise to different structures caused by weathering of
different rocks.
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| 6B. Pool on a rock platform |
As the water comes over the escarpment from the plateau above, it hits an area
of harder rocks made up of quartz-rich sandstone, which has formed
a jutting-out piece of rock called a bench, around which all the
other rock has weathered around. This harder rock bench gives rise
to the 25-metre-high upper falls at Minnamurra, as shown on the
topographic map. Below
this are the 50-metre falls, known as Minnamurra Falls, which are
formed in a similar fashion in the harder rock of the Illawarra
coal measures. The path that the water has weathered out in the
rock then cuts through a section of the Minnamurra latite and has
been able to form a narrow gorge curving around harder rocks of
volcanic origin. Further down, where the rock is soft shale, the
river is able to flow straighter because it travels through rock
which has been easily weathered by the water.
When the weathering to create this dramatic drainage pattern
which is the Minnamurra River occurred, the processes of erosion,
transportation and deposition came into play, with the result
that the river has transported rocks and sediment from the upper
reaches on the plateau, down the escarpment. If you look in the
foreground of photo 6A, you can see a number of rocks which have
been deposited there by the river in times of high rainfall. If
you were to examine the rocks closely to work out what type of
rock they are, you could even work out where along the river's
path the rocks originated. How far they have been transported
(carried) depends on the size of the rock and the speed and volume
of the water. If water is running fast it can carry heavy rocks;
if it is running slowly it can carry smaller sediment, such as
individual bits of sand which have been eroded or weathered. Where
big rocks have been moved great distances it tells us that, at
some point in the past, the river has experienced an extreme event
like a flood, which allowed the big rocks to be transported that
far. If you look at photo
5A you can see large numbers of rocks that have been deposited
at the bottom of a steeper area of slope.
This means that over time; rock and sediment from the higher
areas of the escarpment, which have the richer volcanic rock and
soils, have been transported down the river's catchment, leaving
large rocks and a type of soil we call alluvium next to the river.
This alluvium is quite fertile and is responsible (along with
the extra warmth and sunlight) for the growth of riparian rainforest
next to the river.
So you can see that the processes of weathering, erosion, transportation
and deposition have all contributed to creating the environment
of the Minnamurra Rainforest. These processes then feed into soil
formation processes as shown in the lithosphere
cycle.
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