Gnamma Holes
A gnamma hole is a naturally occurring rock hole found on most granite rock outcrops across Australia.
There are two types of Gnamma Holes:
1. The more common type are pan-gnamma’s. They have flat bottoms and steep, sometimes overhanging walls. A large one could be 8 m across with a depth of 20 cm. This type is generally found on the flattish upper parts of the rock dome.
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2. The rarer type are pit-gnamma’s. They are typically hemispherical and have a large depth relative to their surface area – perhaps 55 cm deep and 1 m in diameter. These are often found on the lower flanks of the rock outcrop.
Gnamma hole formation is a three-stage process: - 1. initiation of a depression; 2. breakup of the rock, and 3. evacuation of the debris.
1. Initial depressions have many causes ranging from insolation causing flaking, to the breakdown of crystalline irregularities, to lichen attack and to subaerial weathering by the attack of acidic groundwater on the bedrock granite.
2. The breakup of the rock includes the continual role of insolation and the direct action of wind and running water, continual wetting and drying of the bedrock granite.
3. Excavation of weathered material is by wind or water and by human interference. These processes form a basic shape of a roundish, hemispherical basin, normally deeper than wide.
Gnamma holes act as a water collection and storage tank – even a cooling tank. Water follows the contours of the bare rock into a gnamma during rain events. As described, gnamma holes can be of varying depth and size but their small surface area to water volume reduces losses from evaporation. As water in a gnamma is surrounded by dense crystalline rock, it is usually kept cool. In some gammas the water is replenished from stores in underground decomposed rock.
Please note that this is an Earthcache and has conditions for logging it. You cannot log a “Find” without answering the logging requirements set out below.
To log this Earthcache, answer the following questions:
1. At the GZ is a large gnamma hole, what type do you think it is? Why?
2. Approximate the size of the hole – Length, Width & Depth.
3. From your observations around the GZ, describe any weathering that is present and are any new gnamma holes forming?
4. Finally, take of photo of yourself or your GPS to show a view of GZ. Include it in your message, but please do not post it in your log, it might give away too much.
When you have your answers to the above, please message us, using the link at the top of the page underneath the name of the Cache.
Additionally, only one find claim per message. Each Geocacher claiming a find must submit an individual response. One team cannot lodge a response on behalf of a group of people.
Good Luck & Happy Caching!