THE IMPACTS OF GEOLOGY ON LANDFORM DEVELOPMENT IN THE BLUE MOUNTAINS

1. The present distribution of the rocks in the Blue Mountains is influenced by:

   a. The initial nature of deposition – sediments were laid down more or less horizontally but were compress into a sinking basin which angled their orientation. Additionally as later sedimentation concentrated more in the centre of the basin, initial deposits covered a greater area than subsequent.

   b. Earth movements associated with the rifting processes that formed the Tasman Sea uplifted blocks of land in some areas and downwrapped others. Geological strata were displaced. It accentuated further the situation whereby older rocks are found at higher elevations than newer.

   c. Subsequent weathering and erosion have carved out great river valleys exposing older strata and completely removed or reduced the thickness of some strata.

2. Valley formation depends on a number of factors including the age of the stream, size of the catchment and geological strata involved.
   
   Headwater streams in the upper Blue Mountains generally flow at a low gradient then enter the larger valleys via waterfalls. These deep wide valleys eventually narrow to exit the Blue Mountains via the bottle neck valleys of the eastern escarpments.
   
   Valleys are wider in the upper Blue Mountains because streams have eroded down to weaker rock layers (eg clay formation and Illawarra coal measurements) which are worn away quickly, leaving the sandstone unsupported. It eventually falls along joint planes to form the sheer cliff faces and to widen out in the valleys. This process is known as sapping. In the Lower Blue Mountains the Hawkesbury sandstone tends to lack the weaker layers which allow the valleys to widen, so they often remain quite narrow and V shaped.
   
   
3. Waterfalls represent a break in the long profile of the stream that is created by escarpments, rock layers that are very resistant to erosion and so on. The length and number of leaps of the falls depends on the degree of escarpment development and the nature of the rock strata in the Blue Mountains.
   
   Many of the waterfalls in the Blue Mountains have a double leap like Katoomba Falls and Wentworth Falls.
   
   
4. The rifting process that commenced approximately ninety two million years ago contributed to patterns of joints which play an important role in landform development in the Blue Mountains. Stream direction and patterns, canyon formation, the sculpting of individual rock formations like the Three Sisters, Orphan Rock and The Pagodas are related to both vertical and horizontal joints or cracks.
   
   a. Canyons in the Blue Mountains are typically narrow, step sided and deep. It is considered that the main cause of these valleys is the upstream migration of stream knickpoint developed on soft claystone layers and following a vertical joint. Swirl holes and rock mills indicate rapid stream flow with rotary abrasion are common.
   
   b. Pagodas are also determined in their initiation by vertical and horizontal joints, with their shapes then being related to the presence of relatively hard bands or iron oxide which has been dissolved from overlaying rocks and travelling downwards to accumulate long bedding plains and joints. Ironstone ledges are probably best preserved where destructive fires are relatively absent.
   
   
5. Other factors of landscape sculpting in the Blue Mountains include:
   
   a. Fire: increases rate of erosion, directly through the breakdown of sandstone surfaces, and indirectly through the loss of vegetation and soil in post fire rain.
   
   b. Biological agents: influence weathering, erosion, soil formation and it’s downslope movement. For example, a study of lyrebirds have shown that they move more than sixty tonnes of material a year.
   
   c. People: Aboriginal fire would have contributed to the natural effect of bushfires. Mining is a major contributor to cliff face collapse. Development and road works also affect erosion rates and downstream sedimentation.