Latitude -33.714955, Longitude 150.311407
Located in the Sydney Surrounds Region of NSW, nearest town Katoomba
The Blue Mountains has it all geomorphologically speaking: uplifts, igneous activity, erosion, sedimentary deposition and metamorphic changes. These have combined to produce an amazing landscape well deserving its World Heritage Listing.
The oldest rocks in the Blue Mountains comprising the region's basement, are faulted, folded, intruded and lightly metamorphosed rocks formed mainly from marine sediments deposited 470-330 million years ago (m.y.) ago during the Silurian and Devonian periods. Carbonate reefs formed in the shallow seas of that time that have now been transformed into the limestone of the Jenolan Caves and other karst areas within the region. Volcanic rocks, such as the Bindook Porphyry Complex in the southern part of the Blue Mountains National Park, also date from this time (Fergusson, 1980).
Following deformation of the Silurian-Devonian age rocks, shallow marine and terrestrial sediments were laid down during the Late Devonian period. Subsequently, during the Carboniferous, there was a major uplift and a period of erosion before the deposition of the Sydney Basin Sequence.
A movement in the earth meant that the quartzite landscape was flooded by a shallow sea from the east. Streams flowing into this sea carried huge amounts of sediment, which were deposited in horizontal layers. Later, these layers formed rock beds of shales, siltstones and mudstones. In swampy areas around the margins of the sea, piles of dead vegetation were buried under the sediment. They would eventually become seams of coal. All in all, about 500 metres of marine sediments were laid down at this time - between 250 and 280 m.y. ago.
A new phase began with the Triassic Period, 250 m.y. ago. Large rivers began dumping vast quantities of sand on top of the shales, burying them. Throughout this burial process, the weight of the accumulating sediments caused the layers to sink, creating a basin.
Sand collected in the basin, which continued to subside. As the deeper beds were buried, they were forged into hard rock by heat and pressure. Above them, the first layers of sand formed the Narrabeen sandstones (about 200m thick). The sands that followed formed the Hawkesbury sandstones. They were about 300m thick in the central part of the Sydney Basin.
About 170 m.y. ago, the sands stopped being deposited. Forces in the earth started pushing the rock strata upwards. The hard rock layers on the bottom bent and flexed, and the sandstone above them fractured into a series of vertical cracks called joints.
Eventually, the rock layers rose into a broad plateau (and they may still be rising). The plateau was highest on its western margin, reaching elevations of over 1,000m, and sloped down to an abrupt downturn at its eastern edge. You can see this today, in the low escarpment just west of Penrith and the Nepean River where a sudden rise in elevation occurs along the Lapstone Monocline.
The uplift wasn't necessarily a calm, gradual affair. It featured some dramatic volcanic activity, probably starting around 150 m.y. ago. A number of volcanic necks, called diatremes, flowed up through the cracks in the sandstone and shale. Then, more recently, basalt lava poured from vents and spread over the landscape. By analysing the radioactive minerals in this basalt rock, geologists have found that some of these flows are around 17 m.y. old. The uppermost rock layer consists of olivine basalt remnants, formed from cooling lava and now confined to tops of higher peaks of the western Blue Mountains like Mt Hay, Mt Wilson, Mt Irvine, Mt Banks and Mt Tomah which is capped with a layer of 14 m.y. old basalt that is up to 140m thick. A wall at Mt Tomah Botanic Garden is built from sections of basalt pillars and there are other samples of hexagonal columns in the gardens.
When you look at the Blue Mountains, you're looking at the plateau created by the uplift 170 m.y. ago. The reason they look like mountains is that the plateau has been dissected. Deep valleys and gorges have been cut into it, leaving stubborn peaks behind.
So what did the dissecting? Weather was partly responsible - the effects of wind, rain, and heating and cooling. But gravity takes the most credit for this spectacular landscape. Water must always travel downhill, forming rivers as it goes. These rivers, which look absurdly small from the cliff-top lookouts, have carved out the magnificent valleys below. But this hasn't simply been a case of water gradually wearing its way down through the layers of rock - otherwise the mountains would be rounded, not chiselled. Sandstone is relatively resistant to erosion, but the shales and coals underneath it are much softer. These lower layers wear back relatively easily, undermining the sandstone.
Weakened by vertical joints and the softer layers of shale within them, massive blocks of sandstone break off and topple down the slopes. Sheer cliffs are left behind, gradually retreating with each dislodged block.
The photo shown well illustrates how running water has probably altered the landscape. The creek has formed a gully approaching the plateau edge. This down-cutting, through hydraulic and abrasive action, has taken place over various parts of the plateau, forming in places deep gorges that are subsequently enlarged by widening (and cliff retreat). The creek plunges over the edge, gathering speed (and increasing in power) on the way down. It is capable of carving a deep pool and undercutting the cliff at the plateau base. When sufficiently undercut, part of the cliff will collapse, the debris will be removed by the stream, and the process of cliff retreat will continue. 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 Measures) 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.
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, 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).
The rifting process that commenced approximately 92 m.y. 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.
Another factor of landscape sculpting in the Blue Mountains is fire which increases rates of erosion, directly through the breakdown of sandstone surfaces, and indirectly through the loss of vegetation and soil in post fire rain. Surprisingly, biological agents like wombats, rabbits, brush turkeys etc can influence weathering, erosion, soil formation and it's down slope movement. For example, a study of lyrebirds has shown that they move more than 60 tonnes of material a year. Mining is a major contributor to cliff face collapse and development and road works also affect erosion rates and downstream sedimentation.
You'll learn a lot about the Blue Mountains geology from one of the lookouts in the Upper Mountains - Visit Govetts Leap, Echo Point or Wentworth Falls. You can see the valleys that have been carved out by streams, and trace the sequence of rock types.
Start by looking down. Far below, in the deepest part of the valley where the river flows, there may be some of the ancient quartzite basement rocks. Above them are the softer shales and coals, worn away into gentle slopes that are covered with trees. Then, rising abruptly out of these dense forests, you'll see the isolated sandstone remnants of the plateau. In the bald cliffs of Mount Solitary or Narrow Neck you can see the different layers of sediment that were buried and pressed into rock, hundreds of millions of years ago.
Once you've viewed the layered rock from afar, try studying it up close on the National Pass track. (See www.nationalpass.com.au for details). The track travels beside a shale layer, winding its way around the cliff face on a fairly narrow ledge. If it wasn't for the shale, the track wouldn't exist - shale is softer than sandstone, and over time it has been worn away. The underlying sandstone layers remain, forming the ledge.
To see erosion in action, you could follow a river into one of the park's dramatic canyons and gorges. The Grand Canyon Track (off Neate's Glen Bush Carpark at Blackheath allow three and a half hours to walk the circuit, not counting stops) is probably the best way to do this if you don't have specialised canyoning equipment. You'll see how Greaves Creek worked its way down through joint planes, slicing through the rock like a giant (and very slow) knife. At one point, the canyon is 30m deep and only two metres wide. Canyons in the Blue Mountains are typically narrow, steep sided and deep. It is considered that the main cause of these valleys is the upstream migration of a location in a river or channel where there is a sharp change in channel slope, such as a waterfall resulting from differential rates of erosion above and below this point known as a knickpoint. These knickpoints develop on soft claystone layers and follow a vertical joint. Swirl holes and rock mills are common in the canyons and indicate rapid stream flow and rotary abrasion.
Visit Euroka or Murphys Glen, both near Glenbrook. These popular picnic areas and camping spots stand on the remains of volcanic necks, or diatremes. The soft volcanic rock has eroded into saucer-shaped depressions with fertile soil. If you take the Old Oberon-Colong Stock Route to Yerranderie, you'll find more volcanic rocks.
Basalt caps the tops of Mount Banks, Mount Hay and Mount Bell, near Mount Wilson. These rounded peaks are all that remain of volcanic activity from around 17 m.y. ago, when lava flowed from vents and spread out over the plateau. You'll find rich plant communities growing on the fertile volcanic soil here.
You can see spectacular gouged formations in shallow caves like: Wind-eroded Cave, near Anvil Rock at Blackheath; Walls Cave, near Blackheath; Kings Cave near Linden and Lyrebird Dell near Leura. These caves are the result of salt weathering and water moving through the sandstone rock. The water and salt removed iron and clay from the sandstone, leaving the intricate shapes you can see today.
Pagodas are also affected by vertical and horizontal joints, with their shapes being related to the presence of relatively hard bands or iron oxide which has been dissolved from overlaying rocks and travelling downwards to accumulate along bedding plains and joints. Ironstone ledges are probably best preserved where destructive fires are relatively absent.
Pulpit Rock Lookout takes its name from the pinnacle of rock that stands out from the main cliffline. There are actually several lookouts here, with sweeping views of Mount Banks, the Grose Valley, and the cliffs of Govetts Leap.
With cave systems such as those of Jenolan and Wombeyan, the Greater Blue Mountains World Heritage Area reveals another important landscape type. A karst landscape is one in which the rock, usually limestone or similar, is highly soluble in natural waters. The result is an environment of caves, blind valleys, springs and gorges.
There are a number of karst landscapes in this part of New South Wales including Wombeyan, Jenolan, Colong and Tuglow caves that have formed in limestone that began as coral reefs and carbonate accumulations in the shallow seas 420 m.y. ago. There is evidence that some of the guided tour caves at Jenolan formed 339 m.y. ago making them the oldest known caves in the world.
Wombeyan is unusual as a karst landscape in that the limestone from which it was formed has been metamorphosed into marble as a result of the heat and pressure from an intrusion of granite some 380 m.y. ago.