CASE STUDY: The Blue Mountains, (cliffs)

(press for enlargement) Sandstone 'turrets' in jointed sandstone	Eroded plateau remnants, with cliffs and slopes cutting across almost horizontal sedimentary strata The Three Sisters, Echo Point (near Katoomba) Jamieson Valley and Mt Solitary (right)
The view from Echo Point, near Katoomba, reveals a marvellous panorama that includes the famous Three Sisters (above). The strong jointing property of the Narrabeen sandstone is well shown here. The thick sandstone beds developed widely spaced intersecting vertical joint planes sometime after sediment consolidation (perhaps from early shrinkage, during warping at uplift, or as a result of offloading). Once these almost horizontal beds are exposed, the jointing pattern exerts a major influence on subsequent landform evolution, as the jointing promotes faster chemical weathering and the collapse of small and large sandstone blocks at cliff faces. At the location above, erosion has produced a cliff face along a major vertical joint plane. More closely spaced right-angled jointing planes have allowed the formation of vertical rock columns (ie. turrets) referred to as the Three Sisters. Unfortunately, in the future, joint widening between the Sisters will endanger them as they become narrower and narrower. They will eventually tumble into the valley below. Look carefully at the photo enlargement - can you see the remains of former Sisters? (Perhaps there could be possible compensation in the future as weathering creates some new turrets from the existing cliff face here or elsewhere!)
(press for enlargement) Famous Landslide near Dog Face Rock	Plateau surface, landslide sequence (and revegetation), horizontal sedimentary strata View from Narrow Neck Lookout, near Katoomba view page showing slope revegetation
While erosion acts downwards in places, it also 'eats' into the cliffs, forcing their eventual 'retreat'. Undercutting of the cliff base (in softer layers) is partly responsible for this. As the sandstones of the Narrabeen Group of the upper Blue Mountains are traversed by widely spaced vertical joints it is natural for huge chunks to shear from cliff sides parallel to the original cliff. This leaves fresh cream-coloured vertical rock faces and rocky rubble (talus) below the cliff. The large landslide took place in 1931, probably triggered by coal mining operations underneath the cliff some years earlier. Prior to collapse, the joint had opened up, and was a major tourist attraction as people came to examine the feature and hope to see the collapse. As it happened at night, no one saw this massive landslide. Other landslides in this area are older, while other small ones are more recent. Such features are so common that most of the softer layers of older sedimentaries (below the cliff line) are covered by talus (large rock debris) that partially protects them from rapid breakdown. With time vegetation takes root in the weathering materials below the cliff base. (Go to slope revegetation site)
(press for enlargement) Creek at cliff base	Sandstone boulders from the cliff above cover the lower valley slopes Katoomba Creek (below Katoomba Falls)
What happens to the massive sandstone blocks that fall from the cliffs above? The rubble eventually comes to rest on the slopes below the cliff line. If a major landslide occurs, a band of vegetation will be destroyed, and the bare, rocky patch may be clearly visible for decades. The rubble gradually weathers down into sand and soil. Some materials (especially finer fragments) creep down the slope, to eventually be carried away by creeks and rivers (sand will later be deposited along river banks while fine sediment may end up on the sea floor). The sandstone blocks falling from cliff faces into the valleys below form a 'covering' upon the softer coal measures below, and perhaps partially protecting them from more rapid weathering. However, this is only a temporary effect, as creeks on the valley floors 'sweep away' materials, and help to keep slopes steep (with active erosional potential).