The earthcache is located on the grounds of the Tidbinbilla
Nature Reserve. Permission has been received for this earthcache
from the Reserve.
Please note that the opening hours of the Reserve are 9am-6pm
during Eastern Standard Time, and 9am - 8pm during Daylight Savings
Time. You cannot enter the reserve outside of these hours.
During the Ordovician period of between 500 and 440 million
years ago, the Tidbinbilla Valley, like most of the eastern half of
Australia lay deep at the bottom of an ocean. Layers of mud, rock
and silt built up sedimentary rocks, which dominate most of the
current landscape.
Over time these layers were lifted, and by the Silurian period
(440 to 400 million years ago) formed the foundation of a shallow
seabed. The layers that formed during this period consisted of
sandstone, breccia and small amounts of limestone and shale.
Eventually, the land was forced above sea level - most likely by
the force of the magma below the earth's crust.
In some places, molten magma forced its way into cracks that
appears within the sedimentary rocks to form granite. The granite
that formed in this region is part of the Murrumbidgee Batholith.
As the granite cooled and cracked, intrusions of quartz and aplite
appeared.
Over several millions of years, erosion slowly wore down the
sedimentary layers formed during the Silurian and Ordovician
periods to reveal the harder granite intrusions peaking up above
the surface.
Eventually these intrusions have also eroded, forming rounded
peaks, known as tors, which stand on the top of many hills in the
Tidbinbilla region.
The granite tors that form Gibraltar rocks are constantly being
shaped by rain, wind, frost and fire. After the 2003 bushfire, some
of these granite boulders shed layers, like skin coming off an
onion.
Intense heat from the fires expanded the rocks. But as they
cooled, the outer layers contracted faster than the middle,
resulting in cracks and later shedding of layers.
The coordinates will take you to the steps at the base of this
granite tor.
To claim this EarthCache you will need to email me the answers
to the following questions:
- Look closely at a piece of freshly broken granite from a bolder
and tell me the three (3) colours you can see, and what minerals
each one is (email me the answer, don't include it here)?
- Take a photograph of your group at the location and include
with your post.
You can log your visit along with your find immediately,
however, you must email me with the answers to the questions above
and include your photo with your log. Failure to comply with these
instructions will result in your log being deleted.
Your log should indicate how many people were in your party and
any additional information or experiences you gained from this
Earthcache.