Box Vale - Geologic Time EarthCache

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. Mobile phone reception should be available on this part of the trail.

Geologic Time

Geologic time is the history of the earth as determined by the age of rocks.

The geologic time scale marks divisions within geologic time. This is something like minutes and hours of a clock, or months and years on a calendar. But instead of these small units of measurement, the geologic time scale is separated by major events in the earth's development. For example, before and after there was life on the planet, or before and after the dinosaurs, etc. Easy, right? The geologic time scale is where we hear words like Precambrian, Mesozoic, and Jurassic. Each word representing a significant period in the earth's history.

But how old is a rock, really? Well, geologists use two methods for determining a rock's age.

1. Absolute time. This is where we can say a rock is two-million and ten years old, and will be two-million and eleven a week from next Tuesday. The methods for determining the absolute age of a rock are very complex and draw on the knowledge and workshops of physicists. Some catch-words you'll hear or read about when talking about absolute time are radiometric dating, alpha and beta particles, atom decay, and half-life. Fortunately, most geocachers don't have access to physicists' tools, equipment, and laboratories, so we can leave this subject behind in deference to resources we do have.

2. Relative time. This is where we can say one rock is older or younger than another rock. Sounds simple, right? Don't you believe it for a second. Some remarkably insightful conclusions have been drawn from this basic idea. Let's review a few principles of relative time:

• Uniformitarianism. Leave it to a scientist to use big words where small ones will do. Uniformitarianism means uniformity or consistency throughout time. That is, the processes we witness today probably worked the same way long ago. For example, erosion carries sediments to low lying areas today. If things are uniform, erosion probably acted in the same way millions of years ago too.
• Original horizontality. Sediment settles horizontally when it's originally deposited. If we see sedimentary rock today that is not horizontal, we can assume it has been deformed by movements of the earth's crust.
• Superposition. This is where we find layers and layers of sedimentary rock. The rock on the bottom is oldest. The rock in the top-most layer is the youngest.
• Cross-cutting. This is where a shift in the earth has taken place that disrupts original horizontality. We can conclude that the cross-cut has occurred 'after' the rocks were formed. This process can get complicated. Suppose hot lava forced its way into sedimentary layers of rock, then later, after it cooled, some rock eroded, then the earth shifted, creating a cross-cut in the lava, and then more sedimentary rock was formed on top of the whole thing. By examining the horizontal layers and the disturbances, geologists can determine the sequence of events that contributed to the formation of the rocks in that locality.

To claim this cache.

Visit the posted coordinates which will take you about half-way along the Box Vale Walking Track to the Boulder Cutting. You will find evidence of sedimentary rock formations that do "not" conform to the principle of original horizontality.

1. How far off horizontal is the sedimentary rock? I mean, at what angle does the rock now lie (eg. 10 degrees, 45 degrees, 90 degrees). Do you think the rock has been completely over turned (upside down). Is the oldest rock at the top or the bottom?

2. Have a look around GZ. Can you see any rock that you cannot determine its relative age? Why or why not?

3. If you give an age for a rock in years, is it called relative time or absolute time or accurate time?

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.