Earth Cache
Technically speaking, this earth cache is not 'exactly' on the The Box Vale Tramway Walking Track. However it is very close to it and it's only a short stroll from the Box Vale parking area along a fire trail. For additional information about the area please refer to other nearby caches. You may also obtain information and maps from the Visitors Information Centre in Mittagong. Mobile phone reception should be available on this part of the trail.
Rock Pools
Rock pools (or potholes) are cylindrical holes, drilled into the bed of a river, that vary in depth and diameter from a few centimeters to several meters. They are generally found in the upper course of a river where its flow is turbulent and it has enough potential energy to erode vertically. This is a much different phenomenon than a plunge hole which is located at the base of a waterfall in the lower course of a river. Also, rock pools are different from tide pools found in coastal areas. The term 'rock pool' is used when the hole is within the river and contains water; the term 'pothole' is used when the flow of water has altered and the hole is dry.
What makes a rock pool? The list of ingredients is pretty short. Bedrock. Turbulent water. And the abrasive mixture that geologists call suspended sediment. Start with bedrock. Without it, potholes won't form. Why? Anything less rock-solid will simply be washed away. "Washed away"? That's the key to understanding Ingredient #2. Potholes are carved from rock, and moving water provides the energy. This leaves only the suspended sediment. It provides the cutting tools for the job. Three ingredients: rock, water, sediment. Three roles: medium, muscle, tool. Nature as sculptor.
A 'rock pool' is produced by force of water and abrasion. It is formed and enlarged by the abrading action of pebbles and cobbles that are carried by eddies, or circular water currents that move against the main current of a stream. When the force of water and the sediment it carries is greater than the resistance of the riverbed it begins to wear away the rock surface. Once the process has begun, the rock in that location continues to erode until it forms a rock pool. 'Potholes' in a rock outcrop indicate the former site of a rapid or cataract.
To log this cache:
Take an easy stroll along the fire trail to the posted coordinates. You will observe one large rock pool in Nattai Creek and a few smaller ones - some in the creek, some on the side. The rock pools in the creek are still developing and you can see stones in the holes that contribute to the abrasion of the sides. Now answer the following:
1. List the three ingredients that are required to make a rock pool.
2. Measure or estimate the diameter and depth of 'one' of the rock pools. Note. If you wade in the shallows, mind your step as the creek leaves a slippery residue that may cause you to lose your footing.
3. You will notice that at most times the flow of Nattai Creek is not very turbulent at all. Can you explain how such a gentle stream created rock pools? Keep in mind the creek crossing is situated between two weirs, both of which are above the Forty Foot Falls.
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.
WARNING. Do not drink the water.
DANGER. Do not attempt this cache during or immediately following heavy rainfall.
IMPORTANT. This cache does not require swimming or wading.
** Congrats to Throsbyonchurch for FTF! **