Earth Cache
You will be walking along The Box Vale Tramway. It was built in 1888 to carry coal mined in the Nattai Gorge to the main southern line at Mittagong. The line passes through several cuttings, a tunnel, and it ends at the "incline" by which coal was brought up from the gorge. The Colliery was closed in 1896 and the tramway was abandoned until 1996 when it was opened as a four-and-a-half kilometer bush walking track. Additional information and maps are available from the Visitors Information Centre in Mittagong.
You will probably lose Mobile phone reception as you get closer to the lookout.
Background
Around 300-million years ago a large part of eastern Australia was still part of the ocean. Then forces deep below the surface of the earth - faulting and folding as well as volcanic activity - pushed upward lifting mountains out of the sea. These mountains formed a vast range that extends from the south end of Victoria to the north end of Queensland. Since their initial formation these mountains have experienced significant erosion and eventually came to form the Great Dividing Range we know today. The Southern Highlands - the site of this earthcache - is part of the Great Dividing Range.
It is important to note that there are no active volcanos in Australia today, and that Mount Gibraltar and other volcanos of the Southern Highlands have not been active for 50-million years.
The geology of the Great Dividing Range and the Southern Highlands consists largely of limestone, sandstone, quartzite, schist and dolomite.
The sharp rise between the coastal lowlands and the eastern uplands have revealed an impressive gorge country. Gorges can be formed in different ways. We'll talk about two of them.
Geological Rifts
A geological rift is a crack in the surface of the earth. Rifts can be inches deep and wide, or they can divide a whole continent. But no matter what size it is, geological rifts are formed in the same basic way.
Deep below the surface of the earth molten rock (magma) forces its way toward the surface. If it reaches the surface then it forms a volcano. But even when it does not reach the surface it creates tremendous pressure. After all, the magma is pushing through rock that is already there. And as the magma forces its way upward the rock that is already there has to go some place. This situation creates pressure from side to side. If the magma is strong enough, it will actually move the existing rock out of the way.
When the magma cools, it forms intrusive igneous rock. In the process, however, it can create a rift. And some rifts are quite large and can form the basis of a gorge.
Erosion
Erosion is a process where water - in this case - carries particles that have an abrasive effect on existing rock so that the rock slowly breaks down and wears away. In this way, a small stream, for example, can have a radical effect over the course of millions of years. If a stream flows over consistently hard rock then the water might cut a deep slot gorge, as pictured here:
Of course, our world is anything but consistent, and a stream that is eroding an area is just as apt to encounter a softer layer of - perhaps sedimentary - rock. In this case, the water might erode the edges of the softer rock before it continues to cut downward. When that happens, the rock above loses the support of the lower layer and the wall of the gorge might collapse.
From here, any number of things might happen. Depending on the size of the wall, it might crumble into boulders, boulders and rock debris might form a sump where sedimentation could occur, water coursing over boulders might create rapids in the stream or river, the collapse might backfill part of the eroded gorge and in that way create a water fall between the new and old parts of the gorge, and in almost all cases we would have to portage with our kayaks and canoes around the area.
Another effect we might see that results from erosion is in an area of long-standing sedimentation. In this case, there might be many many layers of different types of rock with varying degrees of hardness. Should this happen, the speed of the erosion would be evidenced in terraces or stair-step formations as the process sped up or slowed down depending on the rock's susceptibility to be broken-down and carried away.
Finally, there is the case where a stream flows over an area made up entirely of softer rocks or dispersive soils. When this happens, the water carries away particles from the surrounding area very rapidly and thus forms a valley in the shape of a letter V.
Heavy rains and flood waters often assist a stream or river in this process and if allowed to continue will eventually form a canyon with a broad floor or base.
An intermediate stage of this process might see escarpments in the high parts of the valley while there are still parts of the V visible down in the lower parts of the valley.
The posted coordinates will take you to the Box Vale lookout where you will see two of the formations just described - a valley and a gorge.
Cast your gaze at a bearing of approximately 335°. That's North by Northwest or Maestro-Tramontana depending on your compass. Witness the Nattai Valley within the context of the above description of erosion.
Now turn your attention North by East or 10° or Quarto di Tramontana verso Greco depending on your compass. You will see the Nattai Gorge wall immediately across from the lookout. This might be best viewed past mid-day because the morning light will place the wall in shadows. An overcast day may reduce the effect of backlighting but increase the chance of getting rained on. Take note of the north wall, particularly in comparison to the Nattai Valley you just viewed. Assume that the south wall - which is out of sight just below the lookout - resembles the north wall.
To claim this cache.
1. Based on your observations of the Nattai Valley as well as the descriptions of erosion above, what shape is the valley and what does the shape tell us about the rocks that make up the valley?
2. Based on your observations of the Gorge wall directly across from the lookout, and by way of contrast to your observations of the Nattai Valley for part one, do you believe this part of the gorge was formed by a geological rift or by erosion? Please include a few words of explanation.
Optional.
3. How do you account for the different shape or structure of these two locations, especially given that they are adjacent to one another on the same river?
Please send your answers to me via the geocaching website - e-mail or message. The answers to these questions should not be included in your log, nor should your log include spoiler photos. You do not need to wait for a reply from me to log your find.
Lookout for cliff edges and drop offs. Please be careful.